What are the causes of medial rectus palsy?

Causes of Medial Rectus Palsy

Medial rectus palsy results from oculomotor (third) nerve dysfunction, with causes including vascular ischemia (particularly in elderly patients with diabetes, hypertension, or hyperlipidemia), intracranial aneurysms (especially posterior communicating artery), tumors (meningioma, schwannoma, metastases), trauma, subarachnoid hemorrhage, viral illnesses including COVID-19, demyelinating disease, leptomeningeal disorders, and infectious etiologies such as syphilis and Lyme disease. 1

Vascular/Microvascular Causes

• Ischemic microvascular disease is the most common etiology in elderly patients with vasculopathic risk factors including hypertension, diabetes mellitus, and hyperlipidemia 1
• These patients typically present without pupillary involvement (pupil-sparing third nerve palsy) 1
• Giant cell arteritis must be considered urgently in elderly patients presenting with scalp tenderness, jaw claudication, or temporal pain—immediate ESR and C-reactive protein testing with temporal artery biopsy is required 1

Compressive/Structural Lesions

• Intracranial aneurysms, particularly posterior communicating artery aneurysms, are critical to identify as they typically present with pupillary involvement (dilated pupil) 1
• Tumors including meningiomas, schwannomas, and metastatic lesions can compress the third nerve along its course 1
• Increased intracranial pressure with papilledema or meningeal signs suggests space-occupying lesions or hydrocephalus 1

Traumatic Causes

• Head trauma can cause direct injury to the third nerve, particularly at the tentorial edge or orbital apex 1
• Subarachnoid hemorrhage may result in third nerve dysfunction through direct compression or vasospasm 1

Infectious and Inflammatory Causes

• Viral illnesses including COVID-19 have been associated with third nerve palsies 1
• Demyelinating diseases such as multiple sclerosis can affect the third nerve nucleus or fascicle 1
• Leptomeningeal disorders including infectious meningitis (bacterial, viral, fungal), inflammatory meningitis, or carcinomatous meningitis 1
• Syphilis and Lyme disease are specific infectious causes requiring serologic testing 1

Diagnostic Algorithm by Patient Population

Elderly Patients with Vasculopathic Risk Factors

• If hypertension, hyperlipidemia, or diabetes present WITHOUT temporal tenderness, jaw claudication, or scalp pain: check blood pressure, serum glucose, and hemoglobin A1c 1
• Observe for spontaneous resolution over 4-6 weeks 1
• If no improvement by 4-6 weeks, proceed to MRI brain with and without contrast 1

Elderly Patients with Red Flag Symptoms

• If scalp tenderness, jaw claudication, or pain present: immediately check ESR and C-reactive protein, perform temporal artery biopsy if elevated 1

Young Patients or Those Without Vasculopathic Risk Factors

• Neuroimaging should be performed immediately in all young patients or those without compelling vasculopathic risk factors 1
• MRI brain with and without contrast, with specific attention to the third nerve pathway 1
• If aneurysm suspected despite normal MRA or CTA, proceed to catheter angiogram 1

Patients with Signs of Increased Intracranial Pressure

• Papilledema, bilateral sixth nerve palsy, or meningeal signs (stiff neck with headache) require lumbar puncture following neuroimaging 1
• Lumbar puncture should measure opening pressure and analyze CSF for glucose, protein, cell count, cytology, and culture 1
• Systemic serology must include Lyme and syphilis testing 1

Patients with Multiple Cranial Neuropathies

• Presence of other cranial nerve deficits mandates immediate neuroimaging regardless of age or risk factors 1
• Consider leptomeningeal processes, brainstem lesions, or cavernous sinus pathology 1

Critical Pitfalls to Avoid

• Never delay evaluation for giant cell arteritis in elderly patients with temporal symptoms—permanent vision loss can occur without immediate corticosteroid treatment 1
• Pupil-involving third nerve palsy requires urgent aneurysm evaluation with MRA/CTA or catheter angiography, as this represents a neurosurgical emergency 1
• Do not assume microvascular etiology in young patients—these cases require comprehensive neuroimaging from the outset 1
• Bilateral third nerve palsies are never microvascular—always indicate serious intracranial pathology requiring immediate investigation 2