What is the most likely diagnosis for a 55-year-old gentleman with binocular double vision, ptosis, mid-dilated pupil, and limited extraocular motility except for abduction in his left eye, with no comorbidities or neurological deficits, and normal visual acuity (VA) of 6/6 in both eyes?

Left Third Nerve (Oculomotor) Palsy

This patient has a left third nerve (oculomotor) palsy, evidenced by the classic triad of ptosis, mid-dilated pupil, and limited extraocular motility with preserved abduction. 1

Most Likely Diagnosis (0.5 marks)

Left oculomotor (third cranial nerve) palsy with pupillary involvement. 1

The combination of ptosis, mid-dilated pupil, and ophthalmoplegia sparing only abduction (lateral rectus function) is pathognomonic for complete third nerve palsy. 1

Extraocular Muscles Innervated by Cranial Nerve III (2 marks)

The oculomotor nerve innervates the following extraocular muscles: 1

• Superior rectus - elevates and intorts the eye
• Inferior rectus - depresses and extorts the eye
• Medial rectus - adducts the eye
• Inferior oblique - elevates and extorts the eye
• Levator palpebrae superioris - elevates the upper eyelid (causing ptosis when paralyzed)

Additionally, the oculomotor nerve carries parasympathetic fibers to the pupillary sphincter (causing mydriasis when affected) and ciliary muscle (affecting accommodation). 1

Expected Eyeball Position (1 mark)

The affected left eye will be positioned "down and out" (abducted and infraducted) with incyclotorsion. 1

This occurs because the only functioning extraocular muscles are: 1

• Lateral rectus (cranial nerve VI) - pulls the eye laterally (abduction)
• Superior oblique (cranial nerve IV) - depresses and intorts the eye

The unopposed action of these two preserved muscles creates the characteristic "down and out" position. 1

Management (1.5 marks)

Immediate/Urgent Phase

This patient requires urgent neuroimaging because pupillary involvement indicates potential compressive lesion, particularly posterior communicating artery aneurysm, which is immediately life-threatening. 2, 3

• Obtain MRI with gadolinium and MR angiography or CT angiography immediately to rule out aneurysm or other compressive lesions (sensitivity 98%, specificity 99%). 2, 3
• Neurosurgical consultation should be obtained immediately if imaging reveals aneurysm. 2
• Maintain blood pressure control to prevent aneurysm rerupture if present. 2

Critical distinction: While this 55-year-old has no stated comorbidities, pupillary involvement mandates urgent imaging regardless of age or vascular risk factors. 2, 3 Pupil-sparing third nerve palsy in older patients with diabetes/hypertension can be observed, but pupil-involving cases require immediate workup. 1

Observation Phase (if imaging negative)

• Monitor for spontaneous resolution over 3-6 months, as many cases (particularly microvascular) resolve spontaneously. 1
• Prism glasses or occlusion therapy to manage diplopia during recovery period. 1
• Referral to neuro-ophthalmology for ongoing management and to confirm diagnosis if uncertain. 1

Definitive Treatment (if no recovery after 6 months)

Surgical management is symptom-directed with goals to eliminate diplopia in primary position and create functional binocular vision. 1

Surgical options include: 1

• Medial rectus recession (to address the unopposed lateral rectus)
• Vertical rectus transposition procedures for larger deviations
• Ptosis repair (levator resection or frontalis sling) once alignment is stable

Important caveat: Patients should be counseled that complete restoration is unlikely, and diplopia in extreme gaze positions will likely persist despite surgery. 1