What are the possible causes of an inability to adduct the left eye?

Inability to Move Left Eye Inward (Adduction Deficit)

The inability to adduct the left eye is most commonly caused by a left third nerve (oculomotor) palsy affecting the medial rectus muscle, though internuclear ophthalmoplegia (INO), restrictive disorders (thyroid eye disease, orbital trauma), myasthenia gravis, and mechanical restrictions must also be considered based on associated findings.

Primary Differential Diagnosis

The key is to distinguish between neurological versus mechanical/restrictive causes through specific examination findings:

Third Nerve (Oculomotor) Palsy

• Most common cranial neuropathy causing adduction failure 1
• Presents with both horizontal AND vertical misalignment (not isolated adduction deficit)
• Associated findings that confirm diagnosis:
  • Ptosis (partial or complete) from levator dysfunction
  • Eye positioned in abduction and infraduction at rest
  • Pupil involvement may or may not be present
  • Critical distinction: Pupil-involving third nerve palsy requires urgent neuroimaging (MRA/CTA) to rule out posterior communicating artery aneurysm 1

Risk stratification for workup 1:

• Vasculopathic risk factors (diabetes, hypertension, hyperlipidemia) in elderly: May observe initially if pupil-sparing
• Young patients, pupil involvement, or other cranial neuropathies: Immediate neuroimaging mandatory
• Giant cell arteritis symptoms (temporal tenderness, jaw claudication): Urgent ESR/CRP and temporal artery biopsy

Internuclear Ophthalmoplegia (INO)

• Lesion in the medial longitudinal fasciculus (MLF) 2
• Classic presentation:
  • Failure to adduct affected eye on lateral gaze
  • Nystagmus in the abducting (contralateral) eye - this is pathognomonic
  • Convergence typically preserved (distinguishes from third nerve palsy) 3
• Most common causes: Multiple sclerosis (younger patients) and brainstem stroke (older patients) 2
• MRI of brainstem is diagnostic imaging of choice 2

Restrictive/Mechanical Causes

Thyroid Eye Disease (TED):

• Medial rectus is commonly involved, but causes restriction of abduction more than adduction failure
• Look for proptosis, lid retraction, inflammatory signs 1
• Recessions of restricted muscles are surgical mainstay 1

Orbital trauma/iatrogenic:

• Prior eye surgery (scleral buckle, glaucoma hardware, orbital fracture repair) 1
• Direct muscle damage or scarring
• Forced duction testing differentiates restriction from paresis

Sagging Eye Syndrome (in elderly myopes):

• Causes esotropia (convergent strabismus), not isolated adduction deficit
• Presents with diplopia at distance, age 60-80 years 1
• Lateral rectus sags inferiorly, medial rectus relatively overacts

Myasthenia Gravis

• Fatigable weakness - key distinguishing feature 4
• Variable diplopia that worsens with sustained gaze
• Positive Cogan lid twitch sign 4
• Can mimic any cranial nerve palsy pattern
• Confirm with anti-acetylcholine receptor antibodies
• Ice pack test improves ptosis/motility temporarily

Critical Examination Maneuvers

1. Pupil examination: Involved pupil = surgical emergency until proven otherwise
2. Convergence testing: Preserved in INO, impaired in third nerve palsy
3. Abducting eye nystagmus: Present in INO, absent in third nerve palsy
4. Forced duction testing: Distinguishes mechanical restriction from neurogenic palsy
5. Fatigability testing: Sustained upgaze for 60 seconds to unmask myasthenia
6. Associated vertical deviation: Third nerve palsy has hypotropia; isolated adduction deficit without vertical component suggests INO or restriction

Immediate Management Algorithm

Step 1 - Pupil Assessment:

• Pupil involved → Emergency MRA/CTA for aneurysm 1
• Pupil spared → Proceed to Step 2

Step 2 - Pattern Recognition:

• Adduction deficit + abducting nystagmus + preserved convergence → INO → Brain MRI
• Adduction deficit + ptosis + vertical misalignment → Third nerve palsy → Risk-stratified imaging
• Adduction deficit + fatigability + variable symptoms → Myasthenia → Serologic testing, ice pack test
• Adduction deficit + proptosis/restriction → Orbital imaging (CT/MRI orbits)

Step 3 - Neuroimaging Decision 1:

• Mandatory imaging: Young patients, pupil involvement, other neurologic signs, no vasculopathic risk factors
• Observation acceptable initially: Elderly with vasculopathic risk factors, pupil-sparing, isolated finding
• If no resolution in 3 months: Image regardless of initial risk stratification

Common Pitfalls

• Assuming isolated adduction deficit = third nerve palsy: INO is frequently missed without checking for contralateral abducting nystagmus 2
• Delaying aneurysm workup: Pupil-involving third nerve palsy requires same-day vascular imaging 1
• Missing myasthenia: Always test for fatigability; can present as "pseudo-INO" or mimic any pattern 4
• Overlooking mechanical causes: Prior ocular surgery history is critical - check for scleral buckles, glaucoma devices 1

Referral to neuro-ophthalmology or strabismus specialist is warranted when diagnosis is uncertain, multiple cranial nerves involved, or lack of improvement over 3-6 months 1.