Day 11 (1): Anatomy and Physiology of the Extraocular Muscles Flashcards
What are extraocular muscles?
- striated muscles with specific functions and a unique anatomy
- motor unit of the eye
Function: Responsible for the correct alignment of the eyes in order for the sensory unit to function properly
1. Wide visual field
- eyes able to move freely in all directions
2. Foveal fixation
- fovea: responsible for acute of 20/20 vision
- for the clearest possible vision, image must be centered on the fovea of BOTH eyes
3. Single binocular vision
- each point on the retina corresponds to a specific point in the visual field
- the corresponding points in both L and R eyes must focus on the same image
- pathology: diplopia
What are the two types of EOM fibers present in the eye?
Ratio of nerve fibers to EOM fibers is higher than in other skeletal muscles
- 1 nerve fiber = 3 - 5 EOM fibers
- reason why ocular movements are rapid
- Felderstruktur: unique to EOMSs
- afibrillar and slow
- tonic and stamina-oriented
- en grappe (grape-like) nerve endings - Fibrillenstruktur: similar to other muscles
- fibrillar and fast
- phasic
- en plaque (plate-like) nerve endings
What are the 7 extraocular muscles?
- Rectus Muscles (4)
- 2 Horizontal: Medial Rectus, Lateral Rectus
- 2 Vertical: Superior Rectus, Inferior Rectus - Oblique (2)
- Superior Oblique
- Inferior Oblique - Levator Palpebrae Superioris (LPS)
- acts on the superior eyelids
What is the Annulus of Zinn?
Ring of fibrous tissue at the orbital apex surrounding the optic nerve at its entry point in the orbit
What is the Spiral of Tillaux?
- an imaginary line/circle connecting all the insertions of the rectus muscles
- NOT equidistant to the limbus in all quadrants
What are the origin and insertion points of the EOMs?
ALL Rectus muscles
O: Annulus of Zinn
I: Spiral of Tillaux (ANTERIOR to the equator)
Superior Oblique
O (anat): Lesser wing of the sphenoid at the orbital apex SUPEROMEDIAL to the AoZ and MEDIAL to the origin of the LPS
O (func): Trochlea at superomedial orbital wall
I: SUPERIOR sclera POSTERIOR to the equator and INFERIOR to the Superior Rectus
Inferior Oblique
O: Maxillary bone POSTEROLATERAL to the lacrimal fossa
I: Macular area POSTERIOR to the equator
- passes posterolaterally under the IR
Levator palpebrae superioris
O: Orbital apex ABOVE the AoZ
I: Superior eyelids
What is the clinical significance of the insertion points of the EOMs?
- Landmarks
- serve as a reference points when measuring amounts of muscle for resection or recession
- serve as landmarks to determine where to transfer the muscles - Thinnest point of the muscle
- mostly tendinous - Thinnest points of the sclera
- most prone to lacerations in trauma cases
- cases of scleral perforating injuries: always check under the insertion of the muscle nearest the lacerations
What innervates the EOMs?
SO4 LR6
CN 4 (Trochlear Nerve) - with trochlea
- Superior Oblique
CN 6 (Abducens Nerve) - abducts
- Lateral Rectus
CN 3 (Oculomotor Nerve)
Superior Branch: all with SUPERIOR
- Superior Rectus
- Levator Palpebrae Superioris
Inferior Branch: OTHERS
- Inferior Rectus
- Inferior Oblique
- Medial Rectus
What is the blood supply and drainage of the EOMs?
ARTERIAL
Muscular arteries (2) - LLS MMI
- LATERAL branch: lateral and superior m.
- LR: with Lacrimal Artery
- LPS
- SR
- SO - MEDIAL branch: medial and inferior m.
- MR
- IR: with Infraorbital Artery
- IO: with Infraorbital Artery - Anterior Ciliary Arteries (7):
- may arise from terminal branches of the ophthalmic artery or from muscular arteries
- 2 branches for each rectus m. except LR
- + Long PCA = Major Arterial Circle of Iris
VENOUS: parallels the arterial system
- Superior and Inferior Orbital Veins
- Vortex Veins (4 - 8)
- posterior to the equator
- near the N and T margins of the SR and IR
- also the primary drainage of the uvea
What is the clinical significance of knowing the blood supply and drainage of the EOMs?
- Anterior Segment Ischemia
- when simultaneous surgery on more than 2 rectus muscles is performed
- because ACA:
+ provides branches to the episclera, sclera, limbus and the conjunctiva
+ supplies the Major Arterial Circle of Iris - Profuse bleeding
- due to injury to the vortex veins in close proximity to the margins of the rectus muscles
Discuss the Medial Rectus.
- ONLY rectus muscle NOT adjacent to an oblique muscle
- strabismus surgery: HIGHEST risk of being dropped or lost
- inserts CLOSEST to the limbus
O: Annulus of Zinn
I: Spiral of Tillaux 5.5 mm from limbus
A: Adduction
N: CN3/Oculomotor - Inferior Branch
S: Muscular Artery - Medial Branch, ACA (2)
Discuss the Lateral Rectus.
O: Annulus of Zinn
I: Spiral of Tillaux 6.9 mm from limbus
A: Abduction
N: CN6/Abducens
S: Muscular Artery - Lateral Branch, ACA (1), Lacrimal Artery
Discuss the Superior Rectus.
- forms a 23-degree angle with the visual axis in primary position
- inserts FARTHEST from the limbus
O: Annulus of Zinn
I: Spiral of Tillaux 7.7 mm from limbus
A: Elevation, Intorsion, Adduction
N: CN3/Oculomotor - Superior Branch
S: Muscular Artery - Lateral Branch, ACA (2)
Discuss the Inferior Rectus.
- forms a 23-degree angle with the visual axis in primary position
O: Annulus of Zinn
I: Spiral of Tillaux 6.5 mm from limbus
A: Depression, Extorsion, Adduction
N: CN3/Oculomotor - Inferior Branch
S: Muscular Artery - Medial Branch, ACA (2), Infraorbital Artery
Discuss the Superior Oblique.
- longest EOM
- becomes tendinous before passing through the trochlea
- forms a 51-degree angle with the visual axis in primary position
O:
Anatomic - Lesser wing of the sphenoid at the orbital apex SUPEROMEDIAL to the AoZ and MEDIAL to the origin of the LPS
Functional - Trochlea at superomedial orbital wall
I:
SUPERIOR sclera POSTERIOR to the equator and INFERIOR to the Superior Rectus
- penetrates the Tenon’s capsule 2 mm NASAL and 5 mm POSTERIOR to the nasal insertion of the SR
- passes under the SR and fans out as it inserts in the SUPERIOR sclera POSTERIOR to the equator
A: Intorsion, Depression, Abduction
N: CN4/Trochlear
S: Muscular Artery - Lateral Branch
Discuss the Inferior Oblique.
- shortest EOM (37 mm long)
O: Maxillary bone POSTEROLATERAL to the lacrimal fossa
I: Macular area POSTERIOR to the equator
- passes posterolaterally under the IR
A: Extorsion, Elevation, Abduction
N: CN3/Oculomotor - Inferior Branch
M: Muscular Artery - Medial Branch, Infraorbital artery
What are the different fascial components of the EOMs?
- Tenon’s Capsule
- forms a sleeve within which the globe moves
- likened to a glove of the globe
- fuses with:
+ anterior: Intermuscular Septum + Bulbar Conjunctiva
+ posterior: Optic Nerve Sheath
- begins fused with the IM septum and bulbar conjunctiva 3 mm from the limbus then sleeves over the EOMs and fuses with the ON sheath
Clinical correlate:
- needs to be penetrated during surgery to access the EOMs
- Intermuscular Septum/Membrane
- fascial sheath connecting the muscle sheaths of ALL rectus muscles
- fuses with the Tenon’s capsule and bulbar conjunctiva 3 mm posterior to the limbus
Clinical correlate:
- needs to be dissected to isolate EOM action and achieve maximum surgical effect
- can aid in finding lost muscles in surgery
- Muscle Sheaths
- fascial capsule of each rectus muscle
- SR sheath adherent to undersurface of LPS sheath
+ recession: upper lid retraction
+ resection: upper lid ptosis
- IR sheath extends to the lower lid retractors
+ recession: lower lid retraction
+ resection: lower lid elevation (reverse ptosis)
What constitutes the muscle cone?
- All EOMs
- Muscle sheaths of each EOM
- Intermuscular septum/membrane
- Landmark to determine whether structure in the orbit is EXTRAconal or INTRAconal
What is the clinical significance of adipose tissue within the Tenon’s capsule?
- Located 10 mm from the limbus within the Tenon’s capsule
- Avoid penetration of the capsule 10 mm or more posterior to limbus to avoid prolapse and adhesions
What is the Lockwood Ligament?
- IO muscle sheath
- IR muscle sheath
- continuous with the lower lid retractors
- functions like a hammock that supports the globe inferiorly
- IO passes beneath the IR 12 - 14 mm posterior to the inferior limbus
Differentiate Muscle Resection from Recession.
Resection/Plication
- STRENGTHENING
- SHORTENING or FOLDING over the muscle
- reattached at ORIGINAL or more DISTAL site
- INCREASES muscle tension
Recession
- WEAKENING
- REMOVED from original insertion site
- reattached at more PROXIMAL site
- DECREASES muscle tension
What is the difference between optokinetic and optostatic action of the EOM?
OptoKINETIC
- contraction to move the eye towards the direction of the action
OptoSTATIC
- contraction even when the eye is not moving to maintain muscle tension and fixation
What is the normal field of view of the eye?
Isolated eye movement: 15 - 20 degrees
- if greater: head movement is necessary
Total: 50 degrees in each direction
What are the different axes of EOM movement?
Axes of Fick
X Axis (ELEVATION or DEPRESSION)
- transverse/horizontal axis
- passes through center of eye at equator
- perpendicular to Z axis
Z Axis (ABDUCTION or ADDUCTION)
- vertical axis
- passes through center of eye at equator
- perpendicular to X axis
Y Axis (INTORSION or EXTORSION)
- sagittal axis
- passes through the pupil
What is the Listing’s Plane?
Plane formed by the X and Y axes that passes through the EQUATOR of the globe.
- divides the eye into ANTERIOR and POSTERIOR portions
Summarize the actions of the EOMs.
MR: Adduction
LR: Abduction
SR: Elevation, Intorsion, Adduction
IR: Depression, Extorsion, Adduction
SO: Intorsion, Depression, Abduction
IO: Extorsion, Elevation, Abduction
Adductors: MR, SR, IR
Abductors: LR, SO, IO
Elevators: SR, IO
Depressors: IR, SO
External Rotators: IO, IR
Internal Rotators: SO, SR
Remember:
1. Inferiors = Extorts
2. Superiors = Intorts
3. Rectus = ADducts (anterior to equator)
4. Obliques = ABducts (posterior to equator)
What are the cardinal positions of gaze?
Primary: straight gaze
Secondary: up, down, left and right
Tertiary/Oblique: right upgaze, left upgaze, left downgaze, right downgaze
What muscles are working in the different CARDINAL positions of gaze?
- take advantage of the anatomic side at which the cyclovertically acting muscles have their PUREST vertical action
+ VERTICAL muscles: purest vertical action when ABducted 23 degrees from midline
+ OBLIQUE muscles: purest vertical action when ADducted 51 degrees from midline - torsional/rotatory function are NOT evaluated
- up/downgaze: two pairs of muscles contract in each eye
- each gaze position produced by contraction of a pair of YOLK muscles from each eye.
Secondary Positions:
Upgaze: LSR + LIO, RSR + RIO
Downgaze: LIR + LSO, RIR + RSO
Left gaze: LLR, RMR
Right gaze: LMR, RLR
Tertiary/Oblique Positions:
Left Upgaze: LSR, RIO
Right Upgaze: LIO, RSR
Left Downgaze: LIR, RSO
Right Downgaze: LSO, RIR
What is a muscle’s field of action?
- Direction of the eye when a muscle contracts
- Gaze position where the effect of a muscle is best observed
Fields of Action:
Left Eye
- LLR: left gaze
- LMR: right gaze
- LSR: left upgaze
- LIR: left downgaze
- LSO: right downgaze
- LIO: right upgaze
Right Eye
- RLR: right gaze
- RMR: left gaze
- RSR: right upgaze
- RIR: right downgaze
- RSO: left downgaze
- RIO: left upgaze
What are the two classifications of eye movements?
DUCTION: MONOcular eye movements
- tested one eye at a time with the untested eye covered
- Adducted: towards midline (nasally)
- Abducted: away from midline (temporally)
- Elevation: upwards
- Depression: downwards
- Intorsion: internal rotation
- Extorsion: external rotation
VERSION: BInocular eye movements
- both eyes tested at the same time
- Levoversion: left gaze
- Dextroversion: right gaze
- Supraversion: upward gaze
- Infraversion: downward gaze
- Levocycloversion: rotation to the left
- Dextrocycloversion: rotation to the right
In the clinics:
- Versions done first
- Ductions done after if abnormality is noted
What are the muscle pairings involved in duction (monocular) eye movements?
Muscles are found in the SAME eye
AGONIST
- primary muscle moving the eye in a given direction
- e.g. RMR in R eye adduction or LLR in L eye abduction
SYNERGIST
- secondary muscle in the SAME eye that acts WITH the agonist to move the eye in a given direction
- involves muscles with secondary or tertiary actions
- e.g. RSR and RIO in R eye elevation or LIR and LSO in L eye depression
ANTAGONIST
- secondary muscle in the SAME eye that acts AGAINST a muscle to move the eye in a given direction
- e.g. RLR in R eye adduction (antagonistic to RMR)
What is Sherrington’s Law of Reciprocal Innervation?
- Governs muscle innervation in MONOcular movements
- Describes the relationship between the agonist and antagonist muscles in DUCTION
- INCREASED innervation in a contracting muscle (AGONIST) towards a given direction is accompanied by a RECIPROCAL DECREASE in innervation of its ANTAGONIST muscle
E.g. R eye adduction
- Agonist: RMR contracts - increased innervation
- Antagonist: RLR relaxes - decreased innervation
What are the two types of binocular eye movements?
Versions
- conjugate eye movement
- BOTH eyes move in the SAME direction
- Left gaze
- Right gaze
- Up gaze
- Right gaze
- Oblique gazes
- Cycloversions: dextro and levo
Vergences
- disconjugate eye movement
- BOTH eyes move in OPPOSITE directions
- Convergence: eyes move TOWARDS midline
- Divergence: eyes move AWAY from midline
What are the muscle pairs involved in version (binocular) eye movements?
Yolk Muscles
- primary muscles IN EACH EYE that accomplish a given version
E.g.
Horizontal gaze: MR and LR
Upgaze: SR and IO
Downgaze: IR and SO
What is Hering’s Law of Motor Correspondence?
- Governs muscle innervation in BInocular movements
- Describes the relationship between yolk muscles in VERSION
- There is EQUAL and SIMULTANEOUS innervation received by yolk muscles acting in a given gaze direction
Give an example of agonistic, antagonistic and yolk muscles in a given gaze direction.
Example
Right gaze of the R eye: RLR
Synergist: RSO, RIO
- INCREASED innervation (Sherrington’s Law of Reciprocal Innervation)
Antagonist: RMR
- DECREASED innervation (Sherrington’s Law of Reciprocal Innervation)
Yolk: LMR
- EQUAL innervation (Hering’s Law of Motor Correspondence)
Eye deviation due to muscle UNDERaction are due to what causes?
- TRUE muscle paresis
- Mechanical restriction
- Inhibitional palsy
What are the clinically significant vergence mechanism?
- Tonic Convergence
- constant innervational tone to maintain straight eyes despite divergent orbits - Accommodative Convergence
- UNCONSCIOUS synkinetic near reflex when focusing on near objects (reading or focused work) - Voluntary Convergence
- conscious application of synkinetic near reflex (convergence exercises) - Proximal/Instrument Convergence
- induced convergence at near (looking through microscope) - Fusional Vergence
- positions eyes so that similar retinal images correspond to respective retinal areas
Convergence:
- prompted by BITEMPORAL retinal image disparity
Divergence
- prompted by BINASAL retinal image disparity
