BCSC Strabismus Flashcards
What does strabismus mean?
From the Greek work strabismos, meaning “to squint or look obliquely,” strabismus refers to ocular misalignment of any cause
What is orthophoria?
the ideal condition of ocular alignment under binocular conditions
What is heterophoria?
an ocular deviation kept latent by the fusional mechanism (latent strabismus)
What is orthotropia?
correct direction or position of the eyes (although a less stringent condition than orthophoria)
What is heterotropia?
a deviation that is manifest and not kept under control by the fusional mechanism (manifest strabismus)
Eso-
Rotation of the eye such that (s.t.) the cornea –> nasally, fovea –> temporally (–> indicates direction of deviation) (convergent strabismus)
Exo-
Rotation of the eye s.t. the cornea –> nasally, fovea –> temporally (divergent strabismus)
Hyper-
Rotation of eye s.t. cornea –> superiorly, fovea –> inferiorly (a form of vertical strabismus)
Hypo-
Rotation of eye s.t. cornea –> inferiorly, fovea –> superiorly (a form of vertical strabismus)
Incyclo-
Rotation of eye s.t. superior pole –> nasally, inferior pole –> temporally (intorsional strabismus)
Excyclo-
Rotation of eye s.t. superior pole –> temporally, inferior pole –> nasally (extorsional strabismus)
-phoria
A latent deviation that is controlled by the fusional mechanism, so that the eyes remain aligned under normal binocular conditions
-tropia
A manifest deviation that exceeds the control of the fusional mechanism, so that the eyes are not aligned under binocular conditions
Phoria (n.)
A “latent” deviation in which fusional control is ALWAYS present
Intermittent tropia (n.)
A deviation in which fusional control is SOMETIMES present
Tropia (n.)
A “manifest” deviation in which fusional control is NEVER present
Comitant (adj.)
The same (or not varying by more than a few prism diopters) in all directions of gaze and fixation with either eye
Incomitant
Varying in size with direction of gaze or with the eye used for fixating (usually due to paralytic or restrictive cause)
Alternating fixation
Fixation moving from one eye to the other spontaneously
Monocular fixation
Definite preference for fixation with one eye
Congenital/Infantile strabismus
Terms used interchangeably for deviation documented before the age of 6 months
Acquired strabismus
Term used for deviation with onset after 6 months, after a period of apparently normal visual development
Abbreviations: E, X, RH, LH
Esophoria, exophoria, right hyperphoria, left hyperphoria
Abbreviations: ET, XT, RHT, LHT
(Constant) esotropia, exotropia, right hypertropia, left hypertropia
Abbreviations: E(T), X(T), RH(T), LH(T)
Intermittent esotropia, exotropia, right hypertropia, left hypertropia
Abbreviations: RHoT, LHoT
Right hypotropia, left hypotropia
Abbreviations: O, EX = 0
Orthophoria (orthotropia)
What does the addition of a prime to the abbreviations for a deviation indicate?
That the deviation occurs at near fixation (e.g., E’ indicates esophoria at near)
Optical axis
The best fit line for the optical centers of three parts of the eye anatomy: 1) cornea, 2) lens, 3) fovea
Pupillary axis
The corneal surface normal that passes through the center of the pupil (since the radius of curvature of the cornea is not the same as the distance between the pupil center and cornea, there should theoretically be only one such line)
Visual axis
Line connecting fixation point and the foveal center
What are the 7 extraocular muscles?
4 rectus muscles, 2 oblique muscles, and the levator palpebrae superioris
Which EOMs does the upper division of CN III supply?
Levator, SR
Which EOMs does the lower division of CN III supply?
MR, IR, IO
What is primary position?
Positioning such that the eyes are directed straight ahead and the head is straight
What is the primary action of a muscle?
The predominant effect on the position of the eye (component of force vector with largest magnitude) when contracting with the eye in primary position
What do the secondary and tertiary actions of a muscle refer to?
The second and third largest magnitude components of the force vector generated by muscle contraction with the eye in primary position
Approximately how many degrees can each globe be rotated in either direction when in primary position?
50 degrees
What is the secondary action of the SR muscle (in primary position)?
Intorsion
What is the secondary action of the IR muscle (in primary position)?
Extorsion
From where do all the rectus muscles arise?
The annulus of Zinn
From where does the SO arise?
The obital apex
What angle does the vertical plane passing through the two vertical rectus muscles make with the vertical plane including the visual axis?
23 degrees
What angle does the SO form with the vertical plane containing the visual axis in primary position?
51 degrees
In what directions is the SO redirected at the trochlea?
Inferior, posterior, and lateral
Into which quadrant of the globe does the SO tendon insert?
posterosuperior
What are the primary and secondary actions of the SO in primary position?
1) intorsion, 2) depression
From where does the IO originate?
the periosteum of the maxillary bone, just posterior to the orbital rim and lateral to the orifice of the lacrimal fossa
What angle does the IO form with the vertical plane containing the visual axis in primary position?
51 degrees
What are the primary and secondary actions of the IO in primary position?
1) extorsion, 2) elevation
From where does the levator arise?
The apex of the orbit from the lesser wing of the sphenoid just superior to the annulus of Zinn
The fascial sheath of the levator is connected with that of which EOM?
SR
What are the distances of the rectus muscle insertions from the limbus in the order MR, IR, LR, SR?
5.5mm, 6.5mm, 6.9mm, 7.7mm
Which EOMs are supplied by the lateral muscular branch of the opthalmic artery?
LR, SR, SO, levator
Which EOMs are supplied by the medial muscular branch of the ophthalmic artery?
IR, MR, IO
Which veins empty the EOMs?
Superior and inferior orbital veins and the vortex veins
What histologic type of muscle is EOM?
voluntary striated muscle
Is the ratio of nerve fibers to muscle fibers in EOMs greater than in skeletal muscle?
Yes, 10x greater in EOMs
What are the two layers of EOMs?
Orbital layer (acting on pulley system) and Global layer (inserting on globe)
Are singly innervated fibers in the EOMs slow twitch or fast twitch?
Fast twitch (80% of orbital layer, 90% of global layer)
What are the 3 subgroups of singly innervated muscle fibers in the EOMs?
red (most mitochondria and most fatigue resistant), intermediate, white
Do each of the rectus muscles have a capsule extending from origin to insertion?
Yes
Of what does the muscle cone consist?
EOMs, EOM sheaths, intermuscular membrane
What is the Tenon capsule??
The principal orbital fascia, which forms the envelope within which the globe moves.
With what does the Tenon capsule fuse posteriorly?
Optic nerve sheath
With what does the Tenon capsule fuse anteriorly?
Intermuscular septum, approximately 3mm from limbus
Where do the global layers of the 4 rectus muscles penetrate the Tenon capsule?
10mm posterior to their insertions (posterior to the equator)
Where do the oblique muscles penetrate the Tenon capsule
anterior to the equator
Which EOMs have fibroelastic pulleys consisting of collagen, elastin, and smooth muscle?
The 4 rectus muscles and IO
What do pulleys serve to do for the EOMs?
They serve as functional origins for their respective EOMs
From which two EOM pulleys does the Lockwood ligament arise?
IO and IR pulleys
Are the lower lid retractors coupled to the conjoined IR/IO pulleys?
Yes, which allows coordination of lower eyelid positioning with vertical eye positioning during vertical gaze shift
What is the distance from the rectus muscle insertion of the nerves to the rectus muscles?
26mm (the nerves are located 26mm and further posterior to the rectus muscle insertions), which is relevant for performing muscle surgeries.
Does recession strengthen or weaken a muscle?
Weaken
Does resection strengthen or weaken a muscle?
Strengthen
Why should care be taken to avoid peentration of the Tenon capsule 10mm or more posterior to the limbus?
To avoid prolapse of fatty tissue from within the Tenon capsule and formation of a restrictive adhesion to muscle, intermuscular membrane, or conjunctiva. Such an adhesion would limit ocular motility
Where is the sclera the thinnest?
Just posterior to the 4 rectus muscle insertions
What are the orientations of the x, y, and z axes according to Fick?
x is medial-lateral (passing through the equator), y is anterior-posterior (passing through the pupil), z is rostral-caudal
What is Listing’s equatorial plane?
the plane of the globe’s equator. It contains both the x and z axes of Fick
What is primary position?
the position of the eyes when fixating straight ahead
What are the secondary positions?
Gaze straight 1) up, 2) down, 3) right, 4) left
What are tertiary positions?
The four oblique positions of gaze: up and right, up and left, down and right, down and left
What are the cardinal positions?
There are six cardinal positions: up and right, up and left, right, left, down and right, down and left
What is the arc of contact?
The length of muscle actually in contact with the globe
What is the secondary action of the superior rectus primary position?
Intorsion
What is the secondary action of the inferior rectus in primary position?
Extorsion
What is the secondary action of the inferior oblique in primary position?
Elevation
What is the secondary action of the superior oblique in primary position?
Depression
What is the tertiary action of the superior rectus in primary position?
A D duction
What is the tertiary action the inferior rectus in primary position?
A D duction
What is the tertiary action of the inferior oblique in primary position?
Abduction
What is the tertiary action of the superior oblique in primary position?
Abduction
What are the two, quite distinct, definitions of the term field of action?
1) to indicate the direction of rotation of the eye from primary position if the muscle were the only one to contract, 2) to refer to the gaze position in which the effect of the muscle is most readily observed
Electromyography can be helpful in evaluating which conditions?
Paralysis, recovery from paralysis, abnormalities of innervation in myasthenia gravis, and muscle atrophy
What are ductions?
Monocular rotations of an eye
What does the term agonist mean with regard to eye movements?
the primary muscle moving the eye in a given direction
What does the term synergist mean with regard to eye movements?
the muscle in the same eye as the agonist that acts with the agonist to produce a given movement
What does the term antagonist mean with regard to eye movements?
The muscle in the same eye as the agonist that acts in the direction opposite to that of the agonist
What is Sherrington’s law of reciprocal innervation?
Sherrington’s law postulates that increased innervation (meaning neuronal firing, not actual innervation) and contraction of a given EOM are accompanied by a reciprocal decrease in innervation and contraction of its antagonist
What are versions?
binocular eye movements with the eyes moving in the SAME direction
What are vergences?
vinocular eye movements with the eyes moving in OPPOSITE directions
What does the term yoke muscles mean?
Yoke muscles are 2 muscles (one in each eye) that are the primary movers of their respective eyes into a given position of gaze (e.g., RLR and LMR for right gaze)
What is Hering’s law of motor correspondence?
Hering’s law posits that equal and simultaneous innervation (i.e. neuronal signaling) flows to yoke muscles concerned with the desired direction of gaze
What is “primary deviation?”
The amount of misalignment that occurs when the normal eye is fixating
What is “secondary deviation?”
The amount of misalignment that occurs when the abnormal (e.g. paretic or restricted) eye is fixating
Which is generally larger in the setting of a restriction or paresis, the primary or secondary deviation?
The secondary deviation (relates to Hering’s law)
What is the alignment of the eyes under complete EOM paralysis?
Divergent
What is the AC/A ratio?
The ratio of accommodative convergence (amount eyes turn inward) to accommodation (number of diopters of lens accommodation)
What is proximal (instrument) convergence?
An induced convergence movement caused by a psychological awareness of near; this is particularly apparent when a person looks through an instrument such as a binocular microscope
What is fusional convergence?
A movement to converge and position the eyes s.t. similar retinal images project on corresponding retinal areas. This is accomplished without changing the refractive state of the eyes and is prompted by biTEMPORAL retinal image disparity
What is fusional divergence?
An optomotor reflex to diverge and align the eyes s.t. similar retinal images appear on corresponding retinal areas. It is acomplished without changing the refractive state of the eyes and is prompted by biNASAL retinal image disparity
What is the maximum speed of saccadic movements?
500 degrees/sec
What are corresponding points on the retinas of each eye?
Points such that their simultaneous stimulation results in the subjective sensation that the stimulating target or targets come from the same direction in space
What is anomalous retinal correspondence?
Dissimilar spatial relationships between corresponding retinal areas and their respective foveas
What is a horopter?
The 3d subspace (or perhaps more specifically, a curved 2d manifold) within which all points are seen singly for a given fixation point
What is Panum’s area of single binocular vision?
A superspace of the empirical horopter in which single binocular vision can be achieved through fusion of disparate images (outside the horopter) or by corresponding images (within the horopter)
What is fusion?
The cortical unification of visual objects into a single percept that is made possible by the simultaneous stimulation of corresponding retinal areas
What is sensory fusion?
An artificial classification that refers to fusion aided by innate retinotopic organization of visual cortex
What is motor fusion?
An artificial classification that refers to fusion aided by a vergence movement causing similar retinal images to fall and be maintained on corresponding retinal areas even though natural or artificial causes may otherwise induce disparities
In what situation does stereopsis occur?
In situations s.t. retinal disparity is too great to permit the simple superimposition or fusion of the 2 visual directions, but is not great enough to elicit diplopia
Which feature of binocular vision allows a subjective ordering of visual objects in depth, or 3 dimensions?
Stereopsis
What are the 6 monocular layers of the LGN?
4 dorsal parvocellular layers and 2 ventral magnocellular layers
Which retinal ganglion cells project to the magnocellular layers of the LGN?
Parasol retinal ganglion cells (P->M)
Which retinal ganglion cells project to the parvocellular layers of the LGN?
Midget retinal ganglion cells (M->P)
Which retinal ganglion cells are more highly concentrated in the central retina (midget or parasol)?
midget retinal ganglion cells
What is the peak number of retinal ganglion cells in the human retina and when is it reached?
2.2-2.5 million, reached at week 18 of gestation. Number drops to final count of 1.5 million by first few months of life
When does refinement of synaptic connections in striate cortex level off?
by age 10
What is the benefit of invasion of a deprived eye’s relinquished striate cortex territory by the axonal arbors from the non-deprived eye?
Unclear, as visual acuity does not improve beyond normal in the non-deprived eye
Does astigmatic amblyopia develop in the first year of life?
No, it appears that astigmatic amblyopia does not develop until age 3
When does the critical period for development of strabismic amblyopia appear to begin?
4 months of age
What is visual confusion?
simultaneous perception of 2 different objects projected onto corresponding retinal areas (does not happen in the fovea)
What is central fusional disruption?
an intractable diplopia involving 1) avoidance of bifoveal stimulation, 2) absence of suppression, 3) loss of fusional amplitudes
What is suppression?
alteration of visual sensation involving inhibition or prevention of images from one eye from reaching consciousness during binocular visual activity
What is amblyopia?
A unilateral or bilateral reduction of best-corrected visual acuity that cannot be attributed directly to the effect of any structural abnormality of the eye or posterior visual pathways.
What is the cause of amblyopia?
Abnormal visual experience in early life. Generally, such abnormal visual experiences are due to one or more of: 1) strabismus, 2) anisometropia or high bilateral refractive errors, 3) stimulus deprivation
What is the prevalence of amblyopia in North America?
2% to 4%
Are the critical periods for the development of amblyopia due to various causes the same?
No, there are different critical periods for each cause of amblyopia
What is the crowding phenomenon?
The finding that Snellen letters or equivalent symbols of a given size become more difficult to recognize if they are closely surrounded by similar forms, such as a full line or field of letters
Strabismus having which characteristics is most likely to cause significant amblyopia?
1) constant, 2) non-alternating, 3) esodeviation
Which is usually reduced more in strabismic amblyopia, grating acuity or Snellen acuity?
grating acuity (e.g. Teller cards)
What is eccentric fixation?
The consistent use of a nonfoveal region of the retina for monocular viewing by an amblyopic eye
Below what degree of myopic anisometropia is anisometropic amblyopia rarely seen?
below 3D
What is ametropic amblyopia?
Bilateral amblyopia, usually mild, resulting from large, approximately equal, uncorrected refractive errors in both eyes of a young child
What magnitudes of bilateral refractive error are generally required for development of ametropic amblyopia?
5D in hyperopia, 6D in myopia
What is the most common cause of deprivational amblyopia?
congenital or early acquired cataract
Beyond what threshold diameter is a cataract in a child under the age of 6 considered amblyogenic?
3mm when centrally located
What are the 3 general steps in the treatment of amblyopia?
1) elimination (if necessary) of any obstacle to vision (e.g. cataract), 2) correction of significant refractive error, 3) force use of weaker eye by limiting use of stronger eye
When should removal of visually significant congenital lens opacities occur for optimal recovery?
Within the first 4-6 weeks of life
The interval for operations on the first and second eyes in a patient with bilateral congenital cataracts should be no longer than what?
1-2 weeks (amblyopia of different mechanism can develop due to disparity)
When should a traumatic cataract be removed in a child under the age of 6 years?
Within a “few” weeks of injury
Why is refraction performed with cycloplegia in children?
Accommodation can occur in an uncontrolled manner in amblyopic eyes. If the amblyopic eye is hyperopic, accommodation may or may not cause underestimation of the refractive error. On the other hand, if the eye were hyperopic but not amblyopic, accommodation would likely be used to compensate, and the refractive error would be underestimated. Since no “accommodative correction factor” can be applied to amblyopic eyes, elimination of the accommodation variable is needed to accurately assess true refractive error.
Can strabismus develop during patching?
Yes (etiology unclear)
How many hours per day of patching is usually prescribed for moderate to severe amblyopia?
> = 6 hours/day
What is a Bangerter foil?
A neutral density filter (a filter that reduces the intensity of all wavelengths of light equally, giving no changes in color rendition) that can be used for “penalization” of the better eye in amblyopia
What are common forms of penalization?
Pharmacologic (atropine 1% or homatropine 5%), fogging (excessive plus-power lenses), diffusing filters, neutral density filters (Bangerter foil)
What are techniques that can reduce the likelihood of (or treat) iatrogenic amblyopia?
1) avoid full-time patching (use part-time patching or penalization), 2) alternate occlusion, 3) pause therapy for 1-2 weeks if concern raised
What is the CSM method of reporting visual acuity?
C – corneal light reflex position, S – steadiness of fixation, M – maintenance of fixation with 1 and both eyes
What is the induced tropia test?
Evaluation of ability of patient to maintain fixation with either eye under binocular conditions with vertical deviation (usually created with a base-vertical prism)
What are the 4 basic types of ocular alignment tests?
1) cover tests, 2) corneal light reflex tests, 3) dissimilar image tests, 4) dissimilar target tests
What are the 3 types of cover tests?
1) cover-uncover, 2) alternate cover, 3) simultaneous prism and cover
What is the most important test for detecting the presence of a manifest strabismus?
monocular cover-uncover test
In performing a cover-uncover test on a patient with a phoria, when will the patient’s eyes move?
only when uncovering
In performing a cover-uncover test on a patient with a tropia, when will the patient’s eyes move?
1) both when covering and uncovering OR 2) only when covering (if fixation alternates)
Which test is primarily used to identify a phoria?
alternate cover test – the eyes are not allowed to fixate simultaneously, disrupting fusion and unmasking phorias
Measuring with a prism during which test allows assessment of the total deviation due to both phoria and tropia?
Alternate cover test
Measuring with a prism during which test allows assessment of the manifest deviation only?
Cover-uncover test
When measuring a manifest deviation with a prism, which eye is covered and in front of which eye is a prism placed?
The fixating eye is covered and uncovered while the prism is placed in front of the deviating eye
What are the main types of corneal light reflex tests for ocular alignment?
Hirschberg, modified Krimsky, Bruckner, major amblyoscope
What is the conversion of 1mm decentration of corneal light reflex to degrees and prism diopters for the Hirschberg test?
1mm = 7 degrees = 15 prism diopters
What is angle kappa?
The angle between the visual axis and the pupillary axis of the eye.
What is positive angle kappa?
The fovea is usually temporal to the pupillary axis, meaning the corneal light reflex will be slightly nasal to the center of the cornea. This situation is referred to as positive angle kappa, and simulates exodeviation. Negative angle kappa simulates esodeviation.
Why is the Maddox rod test not a satisfactory test for quantifying horizontal deviations?
Accommodative convergence cannot be controlled with the Maddox rod test
What test category do the Maddox rod, double Maddox rod, and red glass tests fall under?
Dissimilar image tests
What test category do the Lancaster red-green projection test, Hess screen test, and major amblyoscope test fall under?
Dissimilar target tests
At what distance is near alignment usually tested?
13 inches (33cm)
How is the near point of convergence determined?
By placing a fixation object at 40cm and progressively moving toward the patient until 1 eye loses fixation and turns out. The point at which 1 eye loses fixation is the near point of convergence. (The eye maintaining fixation is the dominant eye)
What is the normal near point of convergence?
8-10cm
What are the 2 methods of measuring AC/A ratio?
1) Gradient method, 2) Heterophoria method
What can be used to measure fusional vergences?
Haploscopic device, rotary prism, bar prism
What is the 3-Step Test?
An algorithm that can be used to help identify the paretic cyclovertically acting muscle responsible for a hyperdeviation
What are the three steps of the 3-Step Test?
1) Identify which eye is higher, 2) Identify whether worse in left or right gaze, 3) Identify whether worse with right or left head tilt
What did the Prism Adaptation Study demonstrate?
A smaller undercorrection rate (about 10% less) when surgery was based on multiple rounds of prism adaptation than standard techniques
What are the two classes of tests commonly used to assess sensory binocularity?
1) Worth 4-dot testing, 2) Stereoacuity testing
What are the two types of stereopsis tests?
1) contour tests, 2) random dot tests
What do contour stereopsis tests involve?
Actual horizontal separation of the targets presented to each eye (with polarized or red-green glasses). Monocular depth cues may be present at lower stereoacuity levels.
What do random-dot stereopsis tests involve?
embedding of stereo figures in a background of random dots
Why is tropicamide not used alone for cycloplegia in children?
It does not provide sufficient cycloplegia at standard doses in children
Which cycloplegic agent has the highest incidence of hypersensitivity reactions?
Atropine
What percent of ocular deviations in the pediatric populations are esodeviations?
more than 50%
Esotropia must be documented by what age in order to classified as congenital (now referred to as infantile)?
by 6 months
IO overaction and dissociated vertical deviation occur in what percent of patients with infantile esotropia?
50%
What are the 6 major types of esodeviation?
1) Pseudoesotropia, 2) infantile, 3) Nystagmus and esotropia, 4) accommodative, 5) non-accommodative acquired, 6) incomitant
Is persistent asymmetry of monocular horizontal smooth pursuit common in patients with infantile esotropia?
yes, it often persists beyond 6 months, with nasal-to-temporal smooth pursuit impaired
When is the preferred time to perform surgery to correct infantile esotropia?
Between 4 and 6 months to achieve optimal binocular vision (including stereopsis)
What magnitude of deviation after surgery is considered a successful result of surgery to correct infantile esotropia?
Within 8 prism diopters of orthotropia, with development of monofixation syndrome, is considered successful (even though bifoveal fusion would not be achieved)
What is accommodative esotropia?
A convergent deviation of the eyes associated with activation of the accommodative reflex.
What is the usual age of onset for accommodative esotropia?
between 6 nonths and 7 years
What are the 3 factors in the development of refractive accommodative esotropia?
1) uncorrected hyperopia, 2) accommodative convergence, 3) insufficient fusional divergence
What is the primary treatment for refractive accommodative esotropia?
Correction of the full amount of hyperopia, as determined under cycloplegia
What are the 4 approaches to management of high AC/A esotropia?
1) Bifocals, 2) long-acting cholinesterase inhibitors, 3) surgery, 4) observation
Why are bifocals helpful in high AC/A esotropia?
Greater accommodation is required at near, making high AC/A esotropia worse at near. Reduction in the amount of accommodation required at near with +3D bifocals in turn reduces the amount of accommodative convergence that occurs, thereby improving the esotropia.
What is a common cause of acute esotropia?
Artificial disruption of binocular vision due to ocular injury
What is the characteristic movement seen with spasm of the near synkinetic reflex?
Substitution of a convergence movement for a gaze movement on horizontal versions
What are 3 causes of pseudoexotropia?
1) wide interpupillary distance, 2) positive angle kappa, 3) ocular abnormalities such as temporal dragging of macula (as may occur in ROP)
What is the most common type of exodeviation?
intermittent exotropia (latent at times, manifest at others)
What is the usual age of onset for intermittent exotropia?
Before age 5 years
What are common situations in which intermittent exotropia is manifest?
1) end of the day, 2) fatigue, 3) illness, 4) drowsiness
What is the progression of intermittent exotropia in most patients?
Toward constant exotropia
What is tenacious proximal fusion?
A proximal vergence aftereffect that occurs in some patients with intermittent exotropia. The aftereffect is due to a slow-to-dissipate fusion mechanism that prevents intermittent exotropia from manifesting at near fixation with a brief cover test
What are the 3 primary classifications of intermittent exotropia?
1) Basic type (same at near and far), 2) Divergence excess (worse at distance), 3) convergence insufficiency (worse at near)
What is a common criterion used for deciding to perform surgery on a patient with intermittent exotropia?
Manifest deviation more than 50% of the time
What is the most common surgical procedure for intermittent exotropia?
Bilateral LR recession
When must exotropia present in order to be considered infantile?
before 6 months of age
What are usual characteristics of convergence insufficiency?
1) asthenopia, 2) blurred near vision, 3) reading problems in the presence of poor near fusional convergence amplitudes, 4) remote near point of convergence
What is one disease in which convergence insufficiency is seen more often than age-matched controls?
Parkinson disease
What is convergence paralysis?
Normal adduction and accommodation, accompanied by exotropia and diplopia on attempted near fixation only
What are possible sites of lesions leading to convergence paralysis?
1) corpora quadrigemina, 2) nucleus of CN III
What is an A-pattern deviation?
A horizontal deviation that is more convergent or less divergent in upgaze than in downgaze
What is a V-pattern deviation?
A horizontal deviatino that is more convergent or less divergent in downgaze than in upgaze
In what percent of horizontal strabismus cases is an A or V pattern found?
15%-25%
What magnitude of difference between upgaze and downgaze is required for an A-pattern to be considered clinically significant?
> = 10 prism diopters
What magnitude of difference between upgaze and downgaze is required for a V-pattern to be considered clinically significant?
> = 15 prism diopters
Is IO overaction associated with A or V patterns?
V patterns
Is SO overaction associated with A or V patterns?
A patterns
In which eye and which gaze can ipsilateral SO function be assessed (for over- or under- action)?
Look for ability to depress (or limit elevation) in adduction. Therefore, look at 1) L eye in Right-Down gaze and 2) R eye in Left-Down gaze. If L SO is underacting, L eye will be higher than normal in Right-Down gaze. If R SO is underacting, R eye will be higher than normal in Left-Down gaze. If L SO is overacting, L eye will be lower than normal in Right-Up gaze. If R SO is overacting, R eye will be lower than normal in Left-Up gaze.
In which eye and which gaze can ipsilateral IO function be assessed (for over- or under- action)?
Look for ability to elevate (or limit depression) in adduction. Therefore, look at 1) L eye in Right-Up gaze and 2) R eye in Left-Up gaze. If L IO is underacting, L eye will be lower than normal in Right-Up gaze. If R IO is underacting, R eye will be lower than normal in Left-Up gaze. If L IO is overacting, L eye will be higher than normal in Right-Down gaze. If R IO is overacting, R eye will be higher than normal in Left-Down gaze.
How much correction should be performed on the horizontal rectus muscles for an A or V pattern strabismus with IO overaction?
Same amount as without an A or V pattern
When is upward or downward displacement of the horizontal recti performed for A or V pattern strabismus?
If there is no oblique muscle dysfunction
In what direction are the horizontal rectus muscle insertions moved (upward or downward) when addressing an A or V pattern strabismus without oblique muscle dysfunction?
LR insertion toward base of pattern, MR insertion apex of pattern. Moving in a particular direction weakens the muscle in that direction (and strengthens it in the opposite direction). LR needs to be weaker at base, while MR needs to be stronger at base in order to bring eyes inward.
What are the 2 components of surgical treatment for V-pattern strabismus (esotropia or exotropia)?
1) horizontal rectus muscle procedure in usual amounts, 2) weakening of IOs if overacting OR horizontal rectus insertion vertical displacement if IOs not overacting
What are the 2 components of surgical treatment for A-pattern esotropia?
1) horizontal rectus muscle procedure in usual amounts, 2) horizontal rectus insertion vertical displacement (whether or not SOs are overacting)
What are the 2 components of surgical treatment for A-pattern exotropia?
1) horizontal rectus muscle procedure in usual amounts, 2) weakening of SOs if large A-pattern OR horizontal rectus insertion vertical displacement if small A-pattern
Are most vertical deviations comitant or incomitant?
incomitant
What is primary IO overaction?
IO overaction without associated SO or SR paralysis or paresis
What is secondary IO overaction?
IO overaction accompanied by paresis or palsy of SO or SR
Primary IO overaction develops between what ages?
1 year and 6 years
What proportion of patients with infantile strabismus will develop primary IO overaction?
2/3
In which cases of isolated primary IO overaction is a weakening procedure on the IO indicated?
All but the mildest cases of isolated primary IO overaction
What is the most prominent finding in IO overaction?
Overelevation in adduction
What is the most prominent finding in SO overaction?
Overdepression in adduction
What are the clinical features of dissociated vertical deviation (DVD)?
Either eye slowly drifts upward and outward with simultaneous extorsion during periods of visual inattention or occlusion
Is DVD usually unilateral or bilateral?
Usually bilateral, but frequently asymmetric
Why is establishing the chronicity of a 4th nerve palsy important?
Recent onset may be due to intracranial lesion and thus requires neurologic investigation
Which eye is preferred for fixation in a 4th nerve palsy?
Either eye (affected or unaffected)
Do unilateral 4th nerve palsies usually show a V pattern?
No, but bilateral 4th nerve palsies usually do show a V pattern
What are the clinical characteristics of a 4th nerve palsy?
1) ipsilateral hypertropia, 2) worse on contralateral gaze (overelevation in adduction), 3) worse on ipsilateral head tilt
In a patient with 4th nerve palsy with hyperdeviation <= 15 prism diopters in primary, what is the first approach to surgical treatment?
Weakening of ipsilateral IO (recession of contralateral IR can be added if > 15 prism diopters)
1mm of vertical rectus muscle recession results in what amount of vertical correction in primary gaze?
approximately 3 prism diopters
What are 2 options for surgical treatment of bilateral 4th nerve palsy?
1) Bilateral IO recession or 2) Harada-Ito procedure
What is the Harada-Ito procedure?
A procedure used to treat bilateral 4th nerve palsy that involves displacement of the anterior portions of the superior oblique tendons to a location adjacent to the upper edge of the lateral rectus muscle, about 8 mm posterior to that muscle’s insertion. It improves deficient intorsion, but not the vertical deviation.
What can be used to differentiate IO paralysis from skew deviation?
Hypotropic eye should be intorted in IO paralysis, but extorted in skew devation
What 3 features can be used to differentiate IO paralysis from Brown Syndrome?
1) Forced ductions (negative in IO vs. positive in Brown), 2) strabismus pattern (A vs. V), 3) SO overaction (present vs. absent)
What are the clinical features of monocular elevation deficiency (formerly called double elevator palsy)?
1) hypotropia of the involved eye increasing in upgaze, 2) chin-up position, 3) fusion in downgaze, 4) ptosis or pseudoptosis
What are the 3 types of monocular elevation deficiency?
1) restriction, 2) elevator muscle weakness, 3) combination
What are indications for treatment of monocular elevation deficiency?
1) large vertical deviation in primary position, 2) abnormal chin-up position
What is a Knapp procedure?
Transposition of the MR and LR upward toward the SR. Can be used in the treatment of a monocular elevation deficiency
Hypoesthesia in what distribution, especially after local swelling has subsided, is a sign of orbital fracture?
the cutaneous distribution of the infraorbital nerve
What is an indication for urgent surgical intervention in orbital floor fracture?
evidence of restriction (which may be due to IR entrapment)
What is fixation duress?
Recession of the contralateral IR with or without the use of a posterior fixation suture to limit the contralateral eye in downgaze in a patient with orbital floor fracture.
What is the key feature of Duane syndrome?
Anomalous co-contraction of the MR and LR muscles, producing retraction of the globe in actual or attempted adduction.
Duane syndrome belongs what class of disorders?
Congenital cranial dysinnervation disorders
What are the 3 primary types of Duane syndrome?
1) poor ABduction with esotropia in primary, 2) poor ADduction with exotropia in primary, 3) poor ABduction and ADduction with or without horizontal deviation in primary.
What 3 points can be used to distinguish a (Type 1) Duane syndrome with poor abduction from a 6th nerve palsy?
1) globe retraction on adduction, 2) lack of correspondence between small esotropia in primary and large abduction deficit, 3) small-angle exotropia is found in contralateral gaze in Type 1 Duane, but not 6th nerve palsy
What surgical management can be used to minimize deviations and globe retraction in primary gaze in Duane syndrome?
Type 1: MR recession, Type 2: LR recession, Type 3: recession of both MR and LR
What is the presumed etiology of Brown syndrome?
abnormalities of the SO tendon-trochlea complex
What is a possible cause of acute-onset Brown syndrome?
sinusitis (may warrant CT in unexplained acute cases)
What are the primary clinical features of Brown syndrome?
1) deficient elevation in adduction that improves in abduction
What testing and result are required for the diagnosis of Brown syndrome?
positive forced duction testing demonstrating restricted passive elevation in adduction
What is appropriate management of mild Brown syndrome?
observation
When is surgery indicated for 6th nerve palsy?
if spontaneous resolution does not occur within 6 months of onset
What is the usual first operation attempted for correction of 6th nerve palsy?
MR recession and LR resection (if there is some residual LR function)
Which muscles are affected by thyroid eye disease (TED) in order of decreasing severity and frequency?
IR, MR, SR, LR
Is TED usually bilateral or unilateral?
Usually bilateral and asymmetric
What are the most common deviations seen in TED?
1) hypotropia, 2) esotropia
Should strabismus surgery be performed before or after orbital decompression in a patient with TED?
If orbital decompression will be needed, it should be performed before strabismus surgery
What is the natural history of CPEO?
onset in childhood with ptosis; slow progression to total paralysis of eyelids and EOMs
What percent of cases of myasthenia gravis have both ptosis and limited ocular rotations?
90%
After how long does ptosis usually worsen in a patient with MG asked to look upward?
after 30 seconds
Eyelid closure for what period of time will lead to temporary resolution of ptosis in a patient with MG?
20-30 minutes
Application of ice to the eyelids for how long will lead to improved levator and EOM function in a patient with MG?
2-5 minutes
What are three features distinguishing CPEO from MG?
1) Fatigability (seen in MG), 2) Response to Tensilon (seen in MG), 3) Diplopia (seen in MG)
What is Congenital Fibrosis Syndrome?
A group of congenital disorders in which EOM restriction is present and fibrous tissue replaces the EOMs
How can congenital fibrosis syndrome be distinguished from TED?
congenital onset in congenital fibrosis syndrome
What is Mobius syndrome?
A rare condition characterized by both 6th and 7th nerve palsies, resulting in abduction deficits and masklike facies, respectively.
How can esotropia results from high myopia?
High axial length can cause stretching and slippage of the lateral rectus pulley and supporting tissues, leading to esotropia
What can be performed to eliminate diplopia due to exotropia in INO?
MR resection and contralateral LR recession
What are potential etiologies for superior oblique myokymia?
aberrant regeneration of the 4th nerve or vascular compression of the 4th nerve
What are the clinical features of SO myokymia?
1) abnormal torsional movements causing diplopia and monocular oscillopsia
What is asthenopia?
A sense of ocular fatigue
What is a “hang-back” recession?
Suspension of a recessed tendon by sutures that pass through the thick stump of the original insertion rather than sutures passing through thin sclera.
What are the two primary means for “strengthening” a muscle?
1) resecting a portion of the muscle and putting it back together, 2) advancing the insertion of the muscle toward the limbus
What does the use of adjustable sutures involve?
Externalized sutures with slip knots, so that the position of a muscle can be altered during the early postoperative period
What are “Stay” or “Pull-Over” sutures?
Temporary (10-14 days) sutures attached to the sclera (at the limbus or rectus muscle insertion) brought out through the eyelids and secured to periocular skin to fix the eye in a selected position during postop healing.
What is a myotomy?
cutting across a muscle. It is used by some to weaken the IO.
What is a myectomy?
Removal of a portion of a muscle
What is a marginal myotomy?
Cutting partway across a muscle. It is usually used to weaken a rectus muscle further after a maximal recession.
What is a tenotomy?
Cutting across a tendon
What is a tenectomy?
Removing a portion of a tendon
For what purpose is a tenotomy or tenectomy usually used?
To weaken the SO
What is a muscle recession?
removal and reattachment of a muscle so that its insertion is closer to its origin. It is the standard weakening procedure for rectus muscles.
What is denervation and extirpation?
The ablation of the entire portion of the muscle within the Tenon capsule, including the muscle’s nerve supply
For what purpose is denervation and extirpation used?
To weaken severely or recurrently overacting inferior oblique muscles
What is “recession and anteriorization?”
Movement of a muscle’s insertion anterior to its original position.
For what purpose is “recession and anteriorization” used?
Used only on the IO to reduce its elevation function. Particularly useful in the presence of both IO overaction and DVD.
What is a posterior fixation suture?
An attachment of a rectus muscle to the sclera 11-18mm posterior to the insertion using a nonabsorbable suture; fixation to the muscle’s pulley may be safer and equally effective.
For what purpose is a posterior fixation suture used?
To weaken a muscle by decreasing its mechanical advantage; often used in conjunction with recession; sometimes used in DVD, nystagmus, high AC/A esotropia and incomitant strabismus
What are the procedures that can be used to weaken the SO?
1) tenotomy, 2) tenectomy, 3) Z-lengthening, 4) functional lengthening (with silicone spacer, fascia lata, suture loops), 5) recession
Simultaneous surgery on how many rectus muscles can result in anterior segment ischemia?
2 or more, through disturbance of the anterior ciliary vessels running with the rectus muscles
What are the two primary types of incisions used in strabismus surgery?
1) Fornix incision (actually on the bulbar conj, but close to the fornix), 2) Limbal or peritomy incision
How long is a traditional fornix incision?
8-10mm
How long is a traditional peritomy incision?
120 degrees at the limbus
Induced astigmatism after strabismus surgery occurs most commonly in what situation?
surgery on 2 rectus muscles of 1 eye
How can successful strabismus surgery still result in diplopia?
Movement of the image of the fixation object in the formerly deviating eye out of a suppression scotoma
What is generally done if sclera is perforated and vitreous escapes through the perforation site during strabismus surgery?
Immediate application of local cryotherapy or laser therapy
What are the earliest signs of anterior segment ischemia due to anterior ciliary artery disruption in strabismus surgery?
AC cell and flare; more severe cases can be associated with corneal epithelial edmea, Descemet folds, irregular pupil
Which rectus muscle is most difficult to recover when it is lost during strabismus surgery?
MR
Which eyelid (superior or inferior) is more likely to be retracted or advanced during a vertical rectus muscle procedure?
Inferior
For what procedures is topical anesthesia not sufficient in EOM surgery?
pulling on a restricted muscle or attempting to expose a difficult to expose muscle
What form of topical anesthetic can be used for most EOM surgeries in cooperative patients?
tetracaine 0.5% or proparacaine 0.5%
What are the disadvantages of peribulbar or retrobulbar injections for EOM surgery?
these injected anesthetics may affect ocular alignment in the first few hours after surgery, which forces the surgeon to wait at least half a day to adjust adjustable sutures.
What is the oculocardiac reflex?
A slowing of the heart rate caused by traction on the extraocular muscles, particularly the MR
Can the oculocardiac reflex produce asystole?
Yes, in its most severe form
What is botox?
purified botulinum toxin type A
What is the mechanism of action of botox?
blockade of the release of acetylcholine at the neuromuscular junction, thereby paralyzing muscle
What is the time to onset of botulinum toxin action?
2-4 days
What is the duration of action of botulinum toxin in EOM and orbicularis?
5-8 weeks in EOMs, 3 months in orbicularis
In which situations is botulinum most effective in treating strabismus?
1) small to moderate angle esotropia and exotropia (<40 prism diopters), 2) postop residual strabismus, 3) acute paralytic strabismus (esp. 6th nerve palsy) to eliminate diplopia during recovery from palsy, 4) cyclic esotropia, 5) active TED, 6) supplement to MR recessions in large-angle infantile esotropia
