What eye movement impairments are caused by a lesion of cranial nerve (CN) 3, the oculomotor nerve?

Cranial Nerve III Lesion: Eye Movement Impairments

A lesion of cranial nerve III (oculomotor nerve) causes the affected eye to rest in a "down and out" position (abducted and infraducted) with inability to move the eye upward, downward, or inward (medially), accompanied by ptosis and potentially pupillary dilation. 1

Primary Eye Movement Deficits

The oculomotor nerve innervates four of the six extraocular muscles, and its dysfunction produces specific, predictable movement impairments:

Muscles Paralyzed by CN III Lesion

• Superior rectus - loss of elevation 1
• Inferior rectus - loss of depression 1
• Medial rectus - loss of adduction 1
• Inferior oblique - loss of elevation and extorsion 1
• Levator palpebrae superioris - causes ptosis (eyelid drooping) 1

Resulting Eye Position

The eye assumes a characteristic "down and out" position because the two muscles NOT innervated by CN III remain functional and unopposed: 1, 2

• Lateral rectus (innervated by CN VI) - pulls the eye outward (abduction) 1, 2
• Superior oblique (innervated by CN IV) - pulls the eye downward and causes intorsion 1, 2

Clinical Presentation

Cardinal Features

• Ptosis - partial or complete drooping of the upper eyelid, which may actually reduce diplopia complaints by occluding the visual axis 1
• Diplopia - double vision with both horizontal and vertical components due to misalignment 1
• Incomitant deviation - the degree of misalignment varies with direction of gaze 1
• Accommodative deficiency - difficulty reading due to impaired parasympathetic function 1

Pupillary Involvement: Critical Diagnostic Distinction

The presence or absence of pupillary involvement is crucial for determining etiology and urgency: 1

• Pupil-involving palsy - suggests vascular compression (especially posterior communicating artery aneurysm) and requires urgent neuroimaging with CTA or MRA 1, 3
• Pupil-sparing palsy - suggests vasculopathic etiology (microvascular ischemia from diabetes, hypertension, hyperlipidemia) 1, 3

Anatomic Localization and Variants

Complete vs. Divisional Palsies

The oculomotor nerve divides into superior and inferior divisions within the cavernous sinus, and lesions at different locations produce distinct patterns: 1

• Superior division - affects levator palpebrae superioris and superior rectus only 1
• Inferior division - affects medial rectus, inferior rectus, inferior oblique, and pupillary sphincter 1
• Divisional palsies typically localize the lesion to the cavernous sinus or orbit 1

Nuclear vs. Fascicular Lesions

If complete CN III palsy is accompanied by contralateral superior rectus weakness, the lesion is nuclear (due to the unpaired central caudal nucleus innervating both levator muscles). 1

Common Clinical Pitfalls

Key Distinctions to Avoid Misdiagnosis

• Do not confuse CN III palsy with CN VI palsy - CN VI palsy causes isolated abduction deficit with esotropia ("eye turned in"), but NO ptosis, NO vertical movement deficits, and NO pupillary abnormalities 2
• Recognize aberrant regeneration - more common after trauma or compressive lesions, can present with paradoxical eyelid elevation on downward gaze or adduction 1, 4, 5
• Assess for multiple cranial nerve involvement - simultaneous CN III, IV, and VI palsies suggest cavernous sinus or orbital apex pathology 1

Urgent Evaluation Required

Any pupil-involving CN III palsy requires emergent vascular imaging (CTA or MRA) to exclude posterior communicating artery aneurysm, which can be life-threatening. 1, 3