Why does a left Middle Cerebral Artery (MCA) stroke cause left gaze?

Why Left MCA Stroke Causes Ipsilateral (Left) Gaze Deviation

A left MCA stroke causes conjugate eye deviation to the LEFT (ipsilateral to the lesion) because the left frontal eye field, which normally drives gaze to the contralateral (right) side, is damaged—leaving the intact right frontal eye field unopposed, which then drives the eyes toward the left side of the lesion.

Neuroanatomical Mechanism

The frontal eye fields (FEF) in the frontal cortex control voluntary horizontal eye movements, with each hemisphere driving gaze to the contralateral side 1. When the left MCA territory is infarcted:

• The left frontal eye field is destroyed, eliminating its ability to drive rightward gaze
• The right frontal eye field remains intact and unopposed, continuing to drive leftward gaze
• The result is conjugate deviation of the eyes toward the side of the lesion (left in this case) 2, 3

This phenomenon is known as Prévost's sign and represents an imbalance in the cortical control of horizontal gaze 2.

Clinical Presentation and Time Course

Conjugate eye deviation is typically a transient finding in acute stroke:

• In most patients (57%), the deviation resolves within 48 hours 3
• In 90% of cases, it disappears within 5 days 3
• Early resolution of eye deviation is an isolated phenomenon that precedes improvement in other neurological deficits like hemiparesis 3

The rapid recovery occurs because the contralateral (right) frontal eye field compensates for the damaged left hemisphere, restoring balanced horizontal gaze control 3.

Association with Stroke Severity

Conjugate eye deviation is a marker of severe stroke and correlates with specific clinical features:

• It appears as part of the constellation of severe MCA territory deficits including contralateral hemiplegia, sensory loss, and higher cortical dysfunction 1, 4
• Gaze deviation is specifically associated with spatial neglect in right hemisphere strokes 2
• In patients with spatial neglect and right MCA stroke, the average spontaneous gaze position was 46 degrees to the right (ipsilesional), while patients without neglect had near-midline gaze 2

Prognostic Implications

The presence of conjugate eye deviation indicates higher stroke severity and risk of malignant edema:

• Gaze deviation is one of the early severe neurological symptoms that may predict progression to malignant MCA infarction 5
• It appears alongside other ominous signs including hemiparesis, higher cortical dysfunction, nausea, and vomiting 1, 5
• These clinical predictors help identify patients at risk for space-occupying brain edema requiring consideration of decompressive hemicraniectomy 1, 4

Important Clinical Caveats

Prolonged eye deviation (>5 days) suggests bilateral pathology:

• When conjugate deviation persists beyond the typical 5-day window (lasting 13-43+ days), this indicates preexisting damage to the contralateral frontal lobe 3
• The delayed recovery occurs because the compensatory mechanism from the opposite hemisphere is impaired by prior stroke 3

The deviation is ipsilesional, not contralesional:

• A common pitfall is confusing the direction—the eyes deviate toward the lesion (left MCA stroke → left gaze), not away from it 2, 3
• This differs from pontine lesions, where eyes deviate away from the lesion due to different anatomical pathways 6