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Ophthalmology > Day 11 (1): Anatomy and Physiology of the Extraocular Muscles > Flashcards

Day 11 (1): Anatomy and Physiology of the Extraocular Muscles Flashcards

1
Q

What are extraocular muscles?

A
  • 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

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2
Q

What are the two types of EOM fibers present in the eye?

A

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

  1. Felderstruktur: unique to EOMSs
    - afibrillar and slow
    - tonic and stamina-oriented
    - en grappe (grape-like) nerve endings
  2. Fibrillenstruktur: similar to other muscles
    - fibrillar and fast
    - phasic
    - en plaque (plate-like) nerve endings
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3
Q

What are the 7 extraocular muscles?

A
  1. Rectus Muscles (4)
    - 2 Horizontal: Medial Rectus, Lateral Rectus
    - 2 Vertical: Superior Rectus, Inferior Rectus
  2. Oblique (2)
    - Superior Oblique
    - Inferior Oblique
  3. Levator Palpebrae Superioris (LPS)
    - acts on the superior eyelids
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4
Q

What is the Annulus of Zinn?

A

Ring of fibrous tissue at the orbital apex surrounding the optic nerve at its entry point in the orbit

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5
Q

What is the Spiral of Tillaux?

A
  • an imaginary line/circle connecting all the insertions of the rectus muscles
  • NOT equidistant to the limbus in all quadrants
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6
Q

What are the origin and insertion points of the EOMs?

A

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

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7
Q

What is the clinical significance of the insertion points of the EOMs?

A
  1. 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
  2. Thinnest point of the muscle
    - mostly tendinous
  3. 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
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8
Q

What innervates the EOMs?

A

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

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9
Q

What is the blood supply and drainage of the EOMs?

A

ARTERIAL

Muscular arteries (2) - LLS MMI

  1. LATERAL branch: lateral and superior m.
    - LR: with Lacrimal Artery
    - LPS
    - SR
    - SO
  2. MEDIAL branch: medial and inferior m.
    - MR
    - IR: with Infraorbital Artery
    - IO: with Infraorbital Artery
  3. 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

  1. Superior and Inferior Orbital Veins
  2. 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
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10
Q

What is the clinical significance of knowing the blood supply and drainage of the EOMs?

A
  1. 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
  2. Profuse bleeding
    - due to injury to the vortex veins in close proximity to the margins of the rectus muscles
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11
Q

Discuss the Medial Rectus.

A
  • 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)

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12
Q

Discuss the Lateral Rectus.

A

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

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13
Q

Discuss the Superior Rectus.

A
  • 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)

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14
Q

Discuss the Inferior Rectus.

A
  • 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

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15
Q

Discuss the Superior Oblique.

A
  • 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

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16
Q

Discuss the Inferior Oblique.

A
  • 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

17
Q

What are the different fascial components of the EOMs?

A
  1. 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

  1. 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

  1. 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)
18
Q

What constitutes the muscle cone?

A
  1. All EOMs
  2. Muscle sheaths of each EOM
  3. Intermuscular septum/membrane
  • Landmark to determine whether structure in the orbit is EXTRAconal or INTRAconal
19
Q

What is the clinical significance of adipose tissue within the Tenon’s capsule?

A
  • 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
20
Q

What is the Lockwood Ligament?

A
  1. IO muscle sheath
  2. 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
21
Q

Differentiate Muscle Resection from Recession.

A

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

22
Q

What is the difference between optokinetic and optostatic action of the EOM?

A

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

23
Q

What is the normal field of view of the eye?

A

Isolated eye movement: 15 - 20 degrees
- if greater: head movement is necessary

Total: 50 degrees in each direction

24
Q

What are the different axes of EOM movement?

A

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

25
Q

What is the Listing’s Plane?

A

Plane formed by the X and Y axes that passes through the EQUATOR of the globe.
- divides the eye into ANTERIOR and POSTERIOR portions

26
Q

Summarize the actions of the EOMs.

A

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)

27
Q

What are the cardinal positions of gaze?

A

Primary: straight gaze
Secondary: up, down, left and right
Tertiary/Oblique: right upgaze, left upgaze, left downgaze, right downgaze

28
Q

What muscles are working in the different CARDINAL positions of gaze?

A
  • 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

29
Q

What is a muscle’s field of action?

A
  • 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

30
Q

What are the two classifications of eye movements?

A

DUCTION: MONOcular eye movements
- tested one eye at a time with the untested eye covered

  1. Adducted: towards midline (nasally)
  2. Abducted: away from midline (temporally)
  3. Elevation: upwards
  4. Depression: downwards
  5. Intorsion: internal rotation
  6. Extorsion: external rotation

VERSION: BInocular eye movements
- both eyes tested at the same time

  1. Levoversion: left gaze
  2. Dextroversion: right gaze
  3. Supraversion: upward gaze
  4. Infraversion: downward gaze
  5. Levocycloversion: rotation to the left
  6. Dextrocycloversion: rotation to the right

In the clinics:
- Versions done first
- Ductions done after if abnormality is noted

31
Q

What are the muscle pairings involved in duction (monocular) eye movements?

A

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)

32
Q

What is Sherrington’s Law of Reciprocal Innervation?

A
  • 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

33
Q

What are the two types of binocular eye movements?

A

Versions
- conjugate eye movement
- BOTH eyes move in the SAME direction

  1. Left gaze
  2. Right gaze
  3. Up gaze
  4. Right gaze
  5. Oblique gazes
  6. Cycloversions: dextro and levo

Vergences
- disconjugate eye movement
- BOTH eyes move in OPPOSITE directions

  1. Convergence: eyes move TOWARDS midline
  2. Divergence: eyes move AWAY from midline
34
Q

What are the muscle pairs involved in version (binocular) eye movements?

A

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

35
Q

What is Hering’s Law of Motor Correspondence?

A
  • 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
36
Q

Give an example of agonistic, antagonistic and yolk muscles in a given gaze direction.

A

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)

37
Q

Eye deviation due to muscle UNDERaction are due to what causes?

A
  1. TRUE muscle paresis
  2. Mechanical restriction
  3. Inhibitional palsy
38
Q

What are the clinically significant vergence mechanism?

A
  1. Tonic Convergence
    - constant innervational tone to maintain straight eyes despite divergent orbits
  2. Accommodative Convergence
    - UNCONSCIOUS synkinetic near reflex when focusing on near objects (reading or focused work)
  3. Voluntary Convergence
    - conscious application of synkinetic near reflex (convergence exercises)
  4. Proximal/Instrument Convergence
    - induced convergence at near (looking through microscope)
  5. 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

Ophthalmology (44 decks)

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