What type of deficit would a stroke affecting the perforating arteries that supply the posterior limb of the internal capsule most likely result in?

Stroke Affecting the Posterior Limb of the Internal Capsule Results in Pure Motor Hemiplegia

A stroke affecting the perforating arteries that supply the posterior limb of the internal capsule most typically results in pure motor hemiplegia, characterized by contralateral weakness affecting the face, arm, and leg without sensory, visual, or cognitive deficits.

Anatomical Basis

The posterior limb of the internal capsule contains critical motor fibers:

• It houses the descending corticospinal tract fibers that originate from the primary motor cortex
• These fibers are densely packed as they pass through this region
• The perforating arteries supplying this area are terminal branches (lenticulostriate arteries) from the middle cerebral artery 1
• These are end arteries without collateral circulation, making this region particularly vulnerable to ischemia

Clinical Presentation

When a stroke affects the posterior limb of the internal capsule, the following clinical features are typically observed:

• Motor deficit: Contralateral hemiparesis/hemiplegia affecting face, arm, and leg 2
• Preserved sensory function
• Absence of visual field defects
• Absence of cognitive or language deficits (when in dominant hemisphere)
• Absence of ataxia

This constellation of symptoms is known as "pure motor hemiplegia" and represents one of the classic lacunar syndromes 2.

Evidence Supporting Motor Deficit Correlation

Research has demonstrated a strong correlation between axonal injury in the posterior limb of the internal capsule and motor impairment after stroke. A magnetic resonance spectroscopy study showed a strong relationship between reduction in N-acetylaspartate (NAA) in the internal capsule and contralateral motor deficit (r²=0.9, p<0.001) 3.

The probability of recovery of isolated upper limb movement decreases progressively with lesion location, with internal capsule lesions having the poorest prognosis compared to cortical or corona radiata lesions 4. This is consistent with our understanding of the convergence of corticofugal motor efferents as they pass through the corona radiata to the posterior limb of the internal capsule 4.

Pathophysiology

The mechanism of stroke in this region typically involves:

• Small vessel disease (lipohyalinosis) associated with hypertension
• Embolism from cardiac sources
• Less commonly, atherosclerosis of larger vessels with occlusion of perforating arteries

The lenticulostriate arteries supplying the posterior limb of the internal capsule are terminal arteries without anastomoses, making them particularly vulnerable to ischemia 1.

Clinical Implications

Understanding the relationship between internal capsule lesions and motor deficits is crucial for:

1. Localizing the stroke based on clinical presentation
2. Predicting functional outcomes
3. Planning appropriate rehabilitation strategies

Pure motor hemiplegia resulting from posterior limb internal capsule lesions may have a better prognosis than more extensive strokes, but worse than purely cortical lesions affecting motor areas 4.

Conclusion

The dense concentration of descending motor fibers in the posterior limb of the internal capsule explains why strokes affecting the perforating arteries in this region characteristically result in pure motor hemiplegia without other neurological deficits.