What are the signs of a midbrain stroke?

Signs of Midbrain Stroke

The most common signs of midbrain stroke include ipsilateral oculomotor nerve palsy (presenting as ptosis and medial gaze palsy) combined with contralateral hemiparesis, known as Weber's syndrome, along with eye movement disorders, cerebellar dysfunction, and occasionally vertical gaze abnormalities.

Clinical Manifestations

Oculomotor Abnormalities

• Ipsilateral third nerve (oculomotor) palsy, characterized by ptosis and impaired medial gaze of the affected eye 1
• Internuclear ophthalmoplegia (INO) with impaired adduction of the affected eye and abduction nystagmus of the contralateral eye 2
• Vertical gaze paresis may occur in some cases 3
• Diplopia is frequently encountered 4
• Isolated bilateral ptosis can be a rare presenting sign that may be mistaken for myasthenia gravis 5

Motor Symptoms

• Contralateral hemiparesis (weakness on the opposite side of the body from the oculomotor palsy) in Weber's syndrome 1
• Pure motor hemiparesis can occur in some cases 3
• Hypesthetic ataxic hemiparesis (combination of sensory loss and ataxia with weakness) 3

Cerebellar Dysfunction

• Bilateral cerebellar dysfunction is common, especially in caudal paramedian midbrain infarctions 4
• Dysarthric speech (slurred speech) 4
• Truncal or gait ataxia (unsteady walking) 4
• Four-limb ataxia (incoordination in all extremities), which can occur even with unilateral lesions 3, 4

Cognitive and Consciousness Changes

• Cognitive dysfunction may be present in some cases 1
• Progressive impairment of consciousness can occur if there is significant swelling causing compression of adjacent structures 6

Radiological Findings

• Diffusion-weighted MRI is highly sensitive for detecting acute midbrain infarcts 1
• Bilateral caudal paramedian midbrain infarctions often present with a characteristic "V-shaped" appearance on axial MRI 4
• Sagittal imaging may show a backward oblique sign in the lower level of the midbrain 4
• CT scan may show hypodensity in the midbrain region, but is less sensitive than MRI for detecting small brainstem infarcts 6

Anatomical Considerations

• Middle midbrain involvement is most common, primarily in the paramedian territory supplied by the basilar artery 3
• Infarcts in the mesencephalic territory of the posterior cerebral artery are less common 3
• Superior cerebellar artery territory infarcts of the midbrain are extremely rare 3
• The midbrain has a complex vascular supply including branches from the basilar, superior cerebellar, posterior cerebral, posterior communicating, anterior choroidal, and posterior choroidal arteries 2

Prognostic Factors

• Despite being a brainstem stroke, midbrain infarcts often carry a favorable prognosis if treated early with appropriate management of risk factors 1
• Significant clinical improvement can occur within weeks of medical treatment 1
• Monitoring for deterioration with frequent assessment of level of arousal and pupillary function is recommended 7
• Progressive swelling can lead to brainstem compression and worsening outcomes if not properly managed 6

Diagnostic Pitfalls

• Isolated midbrain strokes may be misdiagnosed as other conditions like myasthenia gravis, especially when presenting with isolated ptosis 5
• The clinical picture may be subtle or atypical, particularly with isolated upper or lower midbrain infarcts 3
• CT scans may miss small midbrain infarcts, making MRI the preferred imaging modality 6