What type of nerve palsy would limit left eye movement from the midline medially?

Left Eye Medial Movement Limitation: Third Nerve (Oculomotor) Palsy

A left third nerve (oculomotor) palsy is responsible for limiting left eye movement from midline medially (adduction), as the medial rectus muscle—which moves the eye inward—is innervated by cranial nerve III. 1

Anatomical Basis

The oculomotor nerve (cranial nerve III) innervates four of the six extraocular muscles, including the medial rectus muscle, which is solely responsible for adducting the eye (moving it medially/nasally from midline). 1 When this nerve is damaged:

• Paresis of the medial rectus prevents the eye from moving inward past midline 1
• The eye typically rests in an abducted and infraducted position due to unopposed action of the preserved lateral rectus (CN VI) and superior oblique (CN IV) muscles 1
• Additional muscles affected include the superior rectus, inferior rectus, and inferior oblique 1

Key Clinical Features to Distinguish Third Nerve Palsy

Ptosis (eyelid drooping) is a hallmark finding that helps differentiate third nerve palsy from other cranial nerve palsies:

• The levator palpebrae superioris muscle, which elevates the upper eyelid, is innervated by CN III 2
• Ptosis may be partial or complete, and can actually mask diplopia complaints because the drooping lid occludes the visual axis 1
• Absence of ptosis argues strongly against third nerve palsy 2

Pupillary involvement is a critical diagnostic feature:

• The pupil may be dilated (mydriatic) and unreactive if parasympathetic fibers are affected 1
• Pupil-involving third nerve palsy raises concern for compressive lesions like aneurysm and requires urgent neuroimaging 3

Differential Diagnosis: Why Not Sixth Nerve Palsy?

Sixth nerve (abducens) palsy causes the opposite problem—it limits lateral (outward) eye movement, not medial movement:

• CN VI solely innervates the lateral rectus muscle, responsible for eye abduction 2
• Sixth nerve palsy presents with esotropia (eye turned inward) and inability to move the eye outward from midline 2
• No ptosis occurs with isolated sixth nerve palsy 2

Diagnostic Approach

Neuroimaging is essential in most cases of third nerve palsy:

• MRI with gadolinium should be performed urgently, particularly if the pupil is involved, to exclude aneurysm or compressive lesions 3
• In young patients or those with other cranial neuropathies, neurologic changes, or signs of elevated intracranial pressure, neuroimaging should be obtained regardless of vasculopathic risk factors 1
• In elderly patients with vasculopathic risk factors (diabetes, hypertension, hyperlipidemia), observation may be reasonable initially, but lack of resolution warrants imaging 1

Localization matters for etiology:

• If complete third nerve palsy is accompanied by contralateral superior rectus weakness, the lesion is nuclear (midbrain) 1
• Associated findings help localize: cerebellar ataxia suggests superior cerebellar peduncle involvement; hemiplegia suggests cerebral peduncle involvement 1, 4

Management Priorities

Treatment focuses on eliminating diplopia in primary position and creating a functional field of binocular vision:

• Observation for 3-6 months is appropriate initially, as spontaneous recovery may occur, particularly in vasculopathic cases 1
• Temporary measures include prisms (press-on or ground-in), occlusion, or botulinum toxin injection of antagonist muscles 1
• Surgical intervention is complex and depends on residual function; options range from recession-resection procedures for partial palsies to more extensive transposition procedures for complete palsies 1, 5

Critical caveat: Patients should be counseled that even after successful treatment, diplopia will likely persist in extreme gaze positions due to the underlying neurological deficit and difficulty balancing ductions. 1