Quick Case Recap

What I covered in the study guide above:

🔴 LEVEL 1 — Basic Anatomy & Physiology

Floor of the 4th ventricle, in the dorsal pons, beneath the facial colliculus (facial nerve fibers loop around it - forming the "facial colliculus" on the floor of the 4th ventricle).

The ipsilateral lateral rectus muscle - responsible for abduction of the eye.

(1) Motor neurons - whose axons form CN VI and innervate the ipsilateral lateral rectus. (2) Interneurons - whose axons cross the midline, ascend in the contralateral MLF, and synapse on the contralateral medial rectus subnucleus of CN III - coordinating conjugate horizontal gaze.

Because the nucleus contains both the motor neurons (for LR) AND the interneurons that travel via the contralateral MLF to drive the medial rectus. A nuclear lesion destroys both, producing a complete ipsilateral conjugate horizontal gaze palsy. A fascicular/nerve lesion only affects the motor axons to LR.

Pure abduction (outward movement) of the eye in the horizontal plane. It is the primary abductor; the only muscle innervated by CN VI.

🟠 LEVEL 2 — Clinical Presentation

Binocular, horizontal diplopia - worse at distance than near, and maximal in the direction of the paretic lateral rectus (ipsilateral gaze). Covering either eye eliminates it (distinguishes from monocular diplopia which is a refractive/corneal issue).

Esotropia - the affected eye deviates medially because the medial rectus is unopposed with loss of lateral rectus function.

The patient turns the face toward the side of the palsy (ipsilateral face turn). This places the eyes in a position where the paretic lateral rectus is not being demanded, reducing or eliminating diplopia.

At distance fixation, both eyes need to maintain slight divergence. The paretic lateral rectus cannot generate sufficient force, so the deviation is more pronounced. At near, convergence demand partially compensates.

Left eye 5° esotropia - the light reflex falls nasal to the corneal center of the left eye, indicating that the left eye is deviating medially (esotropia) due to left lateral rectus weakness.

🟡 LEVEL 3 — Localization & Syndromes

CN VI has the longest intracranial course of any cranial nerve. In raised ICP, the brainstem is displaced downward (caudally), stretching CN VI over the petroclinoid ligament or compressing it against the clivus. The nerve is damaged at a point distant from the primary lesion - so CN VI palsy does NOT indicate a pontine lesion when raised ICP is present.

A triad of: (1) CN VI palsy, (2) retroorbital facial pain (CN V1 trigeminal), (3) ipsilateral deafness (CN VIII). Caused by inflammation/infection at the petrous apex (Dorello canal), classically from complicated otitis media. The canal is a narrow bony tunnel under the petroclinoid ligament where CN VI is most vulnerable.

Lesion at the PPRF or CN VI nucleus + ipsilateral MLF. Result: ipsilateral horizontal gaze palsy (one full gaze direction gone) + ipsilateral INO (failure of adduction on the other side). The only remaining horizontal eye movement is abduction of the contralateral eye. Caused by demyelination, stroke, or hemorrhage in the paramedian pontine tegmentum.

A cavernous sinus lesion can cause CN VI palsy BUT typically also involves CN III, CN IV, and CN V1/V2 (pain/sensory loss in forehead and cheek), plus Horner syndrome (sympathetic fibers travel with the ICA through the sinus). Proptosis and chemosis may occur with cavernous sinus thrombosis. An isolated CN VI with no other cranial nerve signs does NOT localize to the cavernous sinus.

🟢 LEVEL 4 — Differentials & Investigation

1. CN VI palsy (neurogenic) - negative forced duction
2. Thyroid eye disease - restrictive, positive forced duction, proptosis, lid lag
3. Myasthenia gravis - variable/fatigable, positive ice test
4. Duane syndrome Type 1 - congenital, globe retraction on adduction
5. Orbital blowout fracture with medial rectus entrapment - positive forced duction, history of trauma
6. Convergence spasm - miotic pupils, intermittent, ductions full
7. Idiopathic orbital inflammatory syndrome - pain, proptosis

A drop of topical anesthetic is instilled, the conjunctiva is grasped with forceps at the limbus, and the eye is mechanically rotated in the direction of limited movement. Positive (resistance felt) = restrictive cause (thyroid eye disease, entrapment, fibrosis). Negative (eye moves freely) = neurogenic or myogenic cause (CN VI palsy, MG).

MG: symptoms fluctuate during the day (worse in evening), variable ptosis, fatigability on sustained upgaze (Cogan lid twitch), positive ice test (ptosis improves with ice), positive edrophonium test, positive acetylcholine receptor antibodies. CN VI palsy: constant, no ptosis, no fatigability, no diurnal variation.

Given young age and no classical vascular risk factors:

• MRI/MRA brain and brainstem (done - showed pontomedullary infarction)
• Echocardiography + bubble contrast study (rule out PFO - patent foramen ovale, present in ~25% of young stroke patients)
• Hypercoagulable screen: antiphospholipid antibodies, Factor V Leiden, Protein C/S, antithrombin III
• OCP/hormonal contraceptive history
• ANA, ANCA, vasculitis screen
• Fasting glucose, HbA1c, lipids (baseline)
• ECG + Holter (rule out paroxysmal AF as embolic source)

• Young patient without vascular risk factors (as here)
• Any additional neurological sign or symptom
• Bilateral CN VI palsy
• Papilledema on fundus examination
• Palsy not resolving within 3-6 months
• Increasing abduction deficit on follow-up

🔵 LEVEL 5 — Management & Advanced

1. Neurology consult - acute ischemic stroke protocol
2. Antiplatelet therapy (aspirin ± clopidogrel depending on stroke workup)
3. Risk factor modification
4. Investigate underlying cause (PFO, coagulopathy in this young patient)
5. Symptomatic - occlusion patch or fogging tape for diplopia
6. Prism glasses if angle is stable
7. Monitor every 6 weeks
8. Botulinum toxin injection (medial rectus) if palsy persists beyond 3-6 weeks
9. Strabismus surgery if no recovery at 6 months with stable deviation

BoNT-A is injected into the ipsilateral medial rectus - this prevents medial rectus contracture while the lateral rectus recovers, and also reduces the angle of deviation. Best used in the acute phase (within 3-6 weeks).

2024 Meta-Analysis (Khalili et al., 38 studies, 643 patients):

• Success rate in acute palsy: 79%, chronic palsy: 33%
• Symptomatic response: 84%, functional response: 64%
• OR 2.67 vs expectant management in acute palsy
• Diabetic etiology has highest success rate

Surgery is indicated when the deviation is stable and there is no recovery at 6 months. For complete palsy: vertical rectus transposition procedures are used since there is no functional lateral rectus to resect:

• Hummelsheim procedure (split superior and inferior rectus transposed laterally + medial rectus recession)
• Jensen procedure (partial tendon transposition)
• Full tendon transposition augmented with BoNT

For partial palsy with some recovery: medial rectus recession ± lateral rectus resection

• Vasculopathic/microvascular causes: ~70-80% recover spontaneously within 3-6 months
• Idiopathic: similar good prognosis
• Trauma, inflammation: generally good
• Neoplasm: poor prognosis without treating the primary
• This patient (brainstem infarct): depends on extent of infarction and stroke management; partial recovery is common with early rehabilitation

Young stroke (age <45) has different risk factors from elderly stroke:

• Patent foramen ovale (PFO) - paradoxical embolism
• Oral contraceptive use - prothrombotic
• Antiphospholipid syndrome
• Cervical artery dissection (vertebral artery dissection → posterior circulation stroke)
• Hypercoagulable states
• Vasculitis (SLE, ANCA)
• Cardiac embolism (structural defects, paroxysmal AF)
• Migraine with aura (small increased risk) This mandates a full young stroke workup beyond simply checking glucose and BP.

⚫ BONUS — Examiner Trap Questions

Not necessarily. This is the pattern of divergence insufficiency, which can be benign (primary divergence insufficiency) OR can indicate bilateral subtle CN VI palsy from raised ICP. You must examine for papilledema, obtain full history, and consider MRI.

Gradenigo syndrome - petrous apicitis. The triad is CN VI palsy + retroorbital pain + ipsilateral deafness. Requires urgent imaging (CT temporal bone + MRI brain) and IV antibiotics; may need surgical drainage.

Yes - and this is an important sign. Bilateral CN VI palsy without any other explanation is a raised ICP sign until proven otherwise. Get urgent fundoscopy (papilledema?), CT/MRI, and investigate for intracranial hypertension (pseudotumor cerebri, mass lesion, CSF drainage obstruction).

A fibro-osseous canal at the petrous apex of the temporal bone, through which CN VI passes along with the inferior petrosal sinus. The canal is bounded above by the petroclinoid ligament (Gruber's ligament). CN VI is particularly vulnerable here because it makes a sharp angle as it enters the canal, making it susceptible to compression in raised ICP, petrous apicitis, and trauma.

Recurrent CN VI palsy is a RED FLAG. This is NOT vasculopathic behavior (those are single episodes that resolve). Differentials include:

• Neurovascular compression (vascular loop at root entry zone - emerging cause)
• Recurrent demyelination (MS)
• Nasopharyngeal carcinoma / skull base tumor (most dreaded)
• Meningeal carcinomatosis
• Recurrent post-viral (in children, COVID-19 reported) Requires urgent repeat MRI with contrast and ENT evaluation.

Diplopia Charting - Complete Explanation from the Very Beginning

Part 1 - The Fundamental Concept: Why Does Diplopia Happen?

Normal Situation

When a Muscle is Weak (e.g., Left Lateral Rectus Palsy)

• The right eye moves fully left - its image falls on the fovea (true image, correctly localized)
• The left eye lags behind - the object's image falls on a point nasal to the fovea (extrafoveal, incorrectly localized)

• A nasal retinal image is projected temporally (outward)
• So the left eye's image is projected too far to the left - this becomes the false/peripheral image

The Two Golden Rules of Diplopia Analysis

1. The direction in which the images are most widely separated = the direction of action of the paretic muscle
2. The more peripheral (outer/further displaced) image belongs to the eye with the paretic muscle

Part 2 - Crossed vs. Uncrossed Diplopia

Uncrossed (Homonymous) Diplopia

• Occurs in esotropia (eye turns IN - e.g., CN VI palsy)
• The deviated eye's image is on the same side as the deviated eye
• Example: Left CN VI palsy → left eye turns in → left eye's false image appears on the LEFT side
• The images do NOT cross over

Crossed (Heteronymous) Diplopia

• Occurs in exotropia (eye turns OUT - e.g., CN III palsy, medial rectus palsy)
• The deviated eye's image is on the opposite side from the deviated eye
• The images cross over each other

Memory trick: In esotropia (eye turns IN) = UNcrossed. In exotropia (eye turns OUT) = CROSSED.

Part 3 - Methods of Diplopia Charting

Method 1: Simple Torch Test (Bedside - No Equipment)

1. Sit the patient 1 meter in front of you
2. Hold a small torch/penlight at eye level
3. Ask the patient to look at the light
4. Move the light through the 9 positions of gaze (see below)
5. Ask: "Where do you see two lights? Where are they farthest apart?"

Up-Left    |    Up    |    Up-Right
Left       |  Primary |    Right
Down-Left  |   Down   |  Down-Right

• "Do you see one light or two?"
• "Are they side by side (horizontal) or one above the other (vertical)?"
• "Which position makes them farthest apart?"

Method 2: Red Glass Test (Bedside - Essential to Know)

1. Dim the room lights slightly
2. Place the red glass over the right eye
3. Hold a penlight 1 meter away from the patient
4. Move through all 9 positions of gaze
5. Patient reports: position of the red image vs. white image

• Red image = Right eye
• White image = Left eye

The image that is farther away from the center (more peripheral) belongs to the paretic eye

• Draw a 3x3 grid representing the 9 gaze positions
• In each position, mark two dots: one red (RE image), one white (LE image), as the patient reports them
• The position where the dots are farthest apart = direction of the paretic muscle

Applying Red Glass to This Case (Left CN VI Palsy)

• Maximum separation in left gaze → paretic muscle acts in left gaze
• The white image (left eye) is the peripheral one → left eye is paretic
• Left eye's paretic muscle acting in left gaze = Left Lateral Rectus
• Diagnosis confirmed: Left CN VI palsy

Method 3: Maddox Rod Test

• Cylinders horizontal → patient sees a vertical red line (tests horizontal deviation)
• Cylinders vertical → patient sees a horizontal red line (tests vertical deviation)
• Cylinders oblique → tests for torsional diplopia (cyclotorsion)

• Red line to the RIGHT of the white dot → uncrossed → the right eye (with rod) is deviating IN = right esotropia = right CN VI palsy
• Red line to the LEFT of the white dot → crossed → exotropia

Method 4: Hess Chart (Formal Charting)

• Red lens over the fixating eye - it can only see red lights
• Green lens over the non-fixating eye - it can only see the green pointer
• Since each eye sees only its assigned color, the eyes cannot fuse - they are dissociated

1. Patient sits 50 cm from the Hess screen
2. Red-green goggles placed on the patient (red lens over the right eye first)
3. Red lights on the screen are illuminated one at a time at each of the 9 positions (inner and outer fields)
4. Patient holds a green pointer and is asked to superimpose the green light (from the pointer) over each red light on the screen
5. The examiner marks on the Hess chart grid where the patient places the green pointer
6. This is done for all 9 positions → plots the right eye chart (right eye is fixating through the red lens)
7. Goggles are reversed (green lens now over right eye, red over left)
8. The entire procedure is repeated → plots the left eye chart

The smaller chart = the eye with the paretic muscle The larger chart = the eye with the overacting yoke muscle

• When the right eye (paretic) fixates and tries to point the green pointer at a red dot, it cannot move far enough in the direction of the paretic muscle
• So the green pointer lands short → the plotted chart is contracted (smaller) in the direction of the palsy

• When the left eye fixates, extra innervation drives the yoke muscle to overact
• The green pointer overshoots → the chart is expanded in the direction of the yoke muscle

Part 4 - Reading the Hess Chart in CN VI Palsy

In Right CN VI Palsy (Right Lateral Rectus Palsy):

• Right chart (paretic eye) - SMALLER:
  • Marked underaction of the lateral rectus corner (right side of chart is pulled inward)
  • Slight overaction of the medial rectus (opposite corner bulges slightly)
  • Fixation spot deviated nasally (esotropia in primary position)
• Left chart (yoke eye) - LARGER:
  • Marked overaction of the medial rectus (left medial rectus is the yoke of the right lateral rectus by Hering's Law)
  • The left medial rectus corner of the chart expands outward
• Primary angle (deviation when the RIGHT paretic eye fixes): ~15°
• Secondary angle (deviation when the LEFT normal eye fixes): ~20°
• Secondary angle > Primary angle = confirms the paretic eye is the one that is fixing

In this case (LEFT CN VI palsy): Mirror image of the above. Left chart is smaller with underaction at the lateral rectus corner. Right chart is larger with overaction of the right medial rectus (yoke of left LR).

Part 5 - Yoke Muscles (Hering's Law) - Critical for Reading Hess Chart

Part 6 - Changes in Hess Chart Over Time

• Paretic eye chart: small, with underaction in direction of paretic muscle
• Yoke eye chart: large, with overaction of yoke muscle
• Charts are very unequal - easy to diagnose

• Secondary contracture develops in the ipsilateral antagonist of the paretic muscle (e.g., in right LR palsy, the right medial rectus contracts/shortens)
• This shows up as apparent overaction of the right MR on the right chart
• This causes inhibitional palsy of the antagonist of the yoke (left LR appears to underact)
• Now you might wrongly suspect the left LR is primarily paretic!

• Both charts become progressively more concomitant (equal)
• The incomitance disappears
• It becomes impossible to tell which muscle was the original culprit from the chart alone
• This is why serial Hess charts from the beginning are valuable

Clinical pearl: If a patient presents late with a comitant deviation and you have no early Hess chart, you cannot determine the primary paretic muscle from the Hess chart alone. This is why early charting matters.

Part 7 - Lees Screen (Alternative to Hess Screen)

Part 8 - Summary: Which Test to Use When

Part 9 - Applying to This Case (Left CN VI Palsy)

• Left chart: SMALLER - restricted at the lateral gaze corner (LR direction), fixation spot deviated nasally
• Right chart: LARGER - overaction of right medial rectus corner (yoke of left LR)
• Confirms: Left lateral rectus underaction = Left CN VI palsy