Extraocular Muscles (EOM) & Squint

Extraocular Muscles (EOM)😊

• 6 EOMs act on the eyeball, and

• 1 EOM acts on the eyelid (Levator palpabrae superioris).

• 6 cardinal movements of eye

Nerve supply:

• LR6: Lateral rectus supplied by Abducent N. (CN 6).

• SO4: Superior oblique supplied by Trochlear N. (CN 4).

• Others: CN 3.

• Note:
• Superior oblique function confirmed by asking patient to look at the tip of their nose.

1. Muscle Actions:

• Mnemonic: SIN → Superior always intorsion

2. Clinical Action:

• Opp of RAD

• MOLR (molar)
• Medial (Adduction) → Oblique → (Inferior-elevate; superior-depress)
• Lateral (Abduction) → Rectus → (Superior-elevate; Inferior-depress)

Terminology related to EOM

• Antagonist:
• Opposite action in the same eye (e.g., Rt. LR & Rt. MR).

• Agonists:
• Same action in the same eye (e.g., Rt. SR & Rt. IO).

• Yoke muscles/Contralateral synergists:
• One muscle from each eye that helps to look in the same direction
• Version movement
• (e.g., Rt. MR & Lt. LR).
• Mnemonic: Flip the name for corners OR use MOLR
• Eg : Levoelevation → RIO → LSR, etc

Mechanisms of binocular vision

• Hering’s law:
• Equal innervation to yoke muscles.
• Mnemonic: He always Enables

• Sherrington’s law:
• Reciprocal inhibition to antagonist (relaxes)
• Mnemonic: She always inhibits

• A defect in these mechanisms leads to squint.

Types of Squint

• Pseudo strabismus:
• Appears deviated, but no squint present.

• True strabismus:
• Phoria:
• Latent squint, only manifests under stress.
• Tropia (Manifest squint):
• Comitant:
• Constant deviation
• Incomitant:
• Variable deviation
• 2° > 1°deviation
• Paralytic:
• Due to CN palsy
• FDT -ve
• Restrictive:
• Due to spasm of EOM
• FDT +ve

Directions of deviation (all deviations of the right eye)::

Investigations

• Double Maddox rod test
• For Incyclotropia and excyclotropia

• Ocular movement examination:
• In all directions of gaze.

• Calculate refractory error:
• Esotropia seen in Hypermetropia

Measure fusional vergence: RAF

• When Countries tries to fuse () → Army (RAF) → Extra Power (for hypertropia in Esophoria)

• Extra power needed to prevent Esophoria

• Diagnosis: 
• RAF ruler → Royal Armed Force

Measure ocular deviation:

a. Hirschberg’s test:

• Hirschberg → Iceburg → deviates
• DOOR
• Direction Opposite Of Reflection
• Light Nasally → Exo deviation

CR at	Degree	PD
Pupillary centre	0	0
Pupillary margin	15	30
Between Pupillary margin and limbus	30	60
Limbus	45	90

• 1° ⇒ 2 PD
• 1 D (1 letter) → 2 PD (2 letter)

• 7° ⇒ 1 mm deviation
• 7 Days → 1 week ()

In a 3-year-old child who has recently developed 15-degree accommodative esotropia, what is the most effective treatment approach?

b. Cover-Uncover test:

c. Maddox rod:

• Uses:
1. Diagnose phoria at far fixation,
2. Macular function test,
3. cyclotropia measurement with a variation → double maddox rod test.

• Beat with Mad rod
• those Cycling (Cyclotropia) with Mac (Macular fn test) for far distance (far fixation)

• A mad () person can only look at mac () and cycle () from far () away

• When the axis of the cylinders is kept horizontal
• Normal (orthophoria): Vertical red line through the white light
• Esophoria: Red line will be to the right of the bright spot
• Exophoria: Red line will be to the left of the bright spot
• Mnemonic: Esho is Right → Ex Left me

• When the axis of the cylinders is kept vertical:
• Normal: Horizontal red line will pass through the white light
• Right hyperphoria: Horizontal red line perceived below the white light
• Left hyperphoria: Horizontal red line perceived above the white light

d. Prism Bar Cover Test (PBCT): 

• Most accurate test to measure degree of tropia

• Place different prisms to measure degree of deviation → Then Perform cover and uncover test → If power of prism is correct, then eye will not be able to move

• Mnemonic: DOOB → Base of prism placed opposite to direction of deviation.

Measurement of diplopia:

e. Hess chart (Most common).

• Hess → 2 H curved → Diplopia

 

• All concave lines

• Do not confuse with Amsler’s grid
• Straight lines → Not concave
• For Macular disorders
• Patient asked to look at central dot.
• Wavy lines indicate macular disorder.
• Straight lines indicate normal.
• Metamorphism

• Lee’s screen.

Tests for sensory anomalies

f. Worth 4 Dot Test:

• Mnemonic: Worth only if you have sensation

• Equipment: 4 dots (1 Red, 2 Green, 1 White).
• Interpretation:
• Harmonious ARC → Normal binocular single vision
• Non-harmonious ARC
• Diplopia
• Right eye suppression
• Left eye suppression

Tests for Stereopsis (Depth Perception)

Purpose of Stereopsis Tests

• Assess depth perception

• Check bilateral single vision integrity.

• Indirect indicator of squint.

1. Two Pencil Test

• Method:
• Examiner holds one pencil.
• Patient tries to touch its tip with their own pencil.

2. Titmus Fly Test

• Uses Polaroid vectographic images

• Requires polarized glasses

• Steriopsis

3. Random Dot Stereogram Tests

• Best test for stereopsis.

• Advantage:
• Removes monocular depth clues.
• Relies solely on binocular vision

• Common Types:
• Lang Test
• Frisby Test
• Frisby-Davis Distance Stereotest

Paralytic Squint

• Clinical features:
• Gaze to Opposite to muscle paralysed
• Head tilt to Same side of muscle involved

3rd nerve palsy:

• Muscles affected:
• LPS, SR, IR, MR, Sphincter pupillae, Ciliary muscle.

• Action lost:
• Elevation of lid, Elevation, Depression, Adduction, Miosis, Accommodation.

• C/F:
• Ptosis (LPS)
• Hypotropia,
• Exotropia → X → Crossed diplopia
• Mydriasis (Loss of sphinter pupillae),
• Loss of accommodation (Ciliary muscle loss), 
• Down & out pupil

• Management:
• Wait and watch (CN III regenerates → covered by neurilemma).

4th nerve palsy:

• Muscle affected: Superior oblique

• Action lost:
• Intorsion (1°),
• Depression (2°),
• Depression in adduction.

• C/F:
• Excyclotropia
• Hypertropia
• Chin depression
• C/L head tilt & I/L gaze

• Confirmation of Dx: 
• Park’s 3-step test
• Applicable only to Left side
• If Right side given, convert to left side
• IGI & ITO
• I/L gaze worsen → Inferior
• I/L Tilt worsen → Oblique
• IGI and ITO Left the chat

Which muscle is paralyzed in a patient who has left-sided head tilt resulting in right hypotropia that worsens with right head tilt and dextroversion?

• Right hypotropia → Left hypertropia (Dont work always)
• worsens with Right (C/L) head tilt → ITO → so Rectus
• worsens with dextroversion (right gaze → C/L) → IGI → Superior
• = Right Superior Rectus

• Right hypotropia (simple always working technique)
• either RIO or RSR is involved
• worse with R gaze ⇒ worse on looking lateral ⇒ Rectus (MOLR)

6th nerve palsy:

• Muscle affected: 
• Lateral rectus.

• Action lost:
• Abduction.

• C/F:
• Esotropia → Uncrossed diplopia,
• Face turn same side,
• Absent LE abduction on left gaze.

Horizontal Gaze

R → Fucking (FEF) Milf (MLF) 3 some

L → 6 (6th CN) Policeman (PPRF)

Pathway of left gaze:

• Initiation from
• C/L FEF → I/L PPRF Centre →
• 6th Nucleus →
• I/L Lateral Rectus &
• C/L MLF → C/L 3rd Nucleus → C/L Medial Rectus.

• How to remember
• For Left gaze
• Right side
• FEF, MLF (alone gaze defect, rest all palsy)
• Contralateral lesions
• Left side
• PPLF, 6th CN
• Ipsilateral lesions

Lesions:

• I/L gaze palsy:
• Lesion at I/L PPRF OR
• Lesion at C/L FEF.

• Internuclear ophthalmoplegia → I/L gaze defect 
• Lesion at Right (C/L) MLF lesion → Left (I/L) gaze defect 
• → I/L adduction (-) & C/L abduction (+)

• In MLF → oru kannu vacch hus nem oru kannu vach kamukaneyum nokkum

• Randu perkkum idayil (Internuclear)

• She has a defect

Vertical gaze

• Rostral interstitial nucleus of the MLF → RiMLF

• Rostral Interstitial Nucleus of Cajal (riMLF)

Restrictive Squint

Duane’s retraction syndrome:

• Diagnosis: Forced duction test (FDT)
• Limitation of movement
• Palpebral fissure closure
• Globe retraction

• Mnemonic:
• Type 1 → Limited movement when abducted
• Type 2 → Add 2 → adduction
• Type 3 → both

Concomitant Squint

Esotropia/Crossed-eyes:

• Most common presentation of concomitant squint.

1. Accommodative:

• Refractive:
• Cause: Hypermetropia (SO HYPER).
• Esotropia distant > near.
• Rx: Convex spectacles.

• Non-refractive:
• Cause: High accommodative convergence/accommodation ratio.
• Due to ↑↑ AC/A ratio
• D/t ↑ convergence or ↓ accommodation
• (AC: Accomodative convergence; A → Accomodation)
• 1 D accommodation = 2 degree of convergence
• Esotropia near > distant.
• Rx:
• Bifocal spectacles,
• Miotics (Ecothiophate).
• Surgery
1. Bimedial recession (weakening of B/L medial recti)
2. Faden surgery

2. Non-accommodative:

• Role of accommodation is absent

• AKA Essential infantile esotropia.

• Age: 4-6 months.

• MoA:
• Large angle squint (>15°) leading to 2 eyes focusing on different objects → Lead to suppression of deviated eye → Lead to ambylopia (Lazy eye) of deviated eye → Loss of vision without organic cause.

• No treatment.

• Prevention:
• Occlusion therapy (until 6 yrs of age):
• Patch normal eye for ‘X’ days (X = Age of child)
• followed by patching deviated eye for 1 day.
• Should be corrected before
• Strabismic ambylopia
• 8 years
• Anisometric ambylopia
• Teenage

What should be the subsequent course of action for a patient presenting with one eye displaying convergent squint and visual acuity of 6/60, while the other eye has no squint but also displays visual acuity of 6/60?
A. MRI
B. Squint surgery
C. Botulinum toxin
D. Refraction

Myasthenia Gravis

• Defective function of EOM.

• Clinical Features:
• Asymmetrical ptosis: 
• Earliest manifestation
• B/L, worsens as day progresses (due to ↓ ACh, fatigue).
• Diplopia.
• NOTE: Ptosis +
• Normal Light reflex → MG
• MIOSIS → Horner’s
• MYDRIASIS → CN 3 palsy
• Cogan lid twitch sign:
• Upper eyelid shoots up on switching from downward gaze to upward gaze.

Tensilon test:

• Edrophonium injection
• ↓↓ ptosis

• Not done now
• Trigger cholingeric crisis, hypersalivation
• Patient may not be able to swallow → aspiration

• Mnemonic: Edada phone → tension kuraykan

ICE PACK test

• Now used

• Put ice pack over eyes → Ptosis ↓↓

• Sensitivity of tensilon and ice pack tests equal