Ataxic Hemiparesis

Contents

  1. Definition and Clinical Features
  2. Pathophysiology and Anatomical Localization
  3. Localizing Clues
  4. References

Definition and Clinical Features

Ataxic hemiparesis is one of the classic "lacunar syndromes" originally described by C. Miller Fisher. It is characterized by the combination of unilateral weakness (hemiparesis) and cerebellar-type incoordination (hemiataxia) occurring on the same side of the body.

Typically, the hemiparesis is predominantly crural, meaning the leg is more severely affected than the arm or face. The ataxia is out of proportion to the degree of weakness, causing significant clumsiness and unsteadiness. In addition to the core motor and coordination deficits, patients may also present with dysarthria, nystagmus, paresthesias, and occasionally, focal pain.

MRI showing a lacunar infarct in the pons causing ataxic hemiparesis

Ataxic hemiparesis is a classic lacunar stroke syndrome where a single, small, deep brain lesion simultaneously disrupts both descending motor tracts and cerebellar coordinating pathways.

Pathophysiology and Anatomical Localization

The syndrome occurs when a single, small, deep cerebral infarction (a lacune) disrupts both the pyramidal (corticospinal) tract and the adjacent corticopontocerebellar fibers. Because these tracts run closely together in the deep white matter and brainstem, a single small lesion can impair both strength and coordination simultaneously.

The clinical signs (ataxia and paresis) manifest contralateral to the brain lesion. Recognized anatomical localizations include:

  • Basis Pons: The classic location described by Fisher, typically at the junction of the upper third and lower two-thirds of the contralateral basis pons.
  • Internal Capsule: Specifically the posterior limb, often involving the territory of the anterior choroidal artery or lenticulostriate penetrating arteries.
  • Thalamocapsular Region: Involving the thalamus and adjacent internal capsule.
  • Corona Radiata: Disrupting the converging fibers before they reach the internal capsule.
  • Other sites: Red nucleus and the paracentral region (in the anterior cerebral artery territory).

Localizing Clues

While the core syndrome of ataxic hemiparesis can be caused by lesions in several different locations, subtle accompanying signs can help the clinician pinpoint the exact site of the infarction:

  • Sensory Loss: The presence of objective sensory loss strongly suggests involvement of the internal capsule or thalamus (thalamocapsular lesion), as the purely pontine lesions classically spare sensory pathways.
  • Pain: Pain occurring in the absence of other sensory features strongly points to thalamic involvement.

 

References

Bogousslavsky J, Regli F, Ghika J, Feldmeyer JJ. Painful ataxic hemiparesis. Archives of Neurology 1984; 41: 892-893

Fisher CM. Ataxic hemiparesis. A pathologic study. Archives of Neurology 1978; 35: 126-128

Gorman MJ, Dafer R, Levine SR. Ataxic hemiparesis: critical appraisal of a lacunar syndrome. Stroke 1998; 29: 2549-2555

 

Cross References

Ataxia; Hemiataxia; Hemiparesis; Pseudochoreoathetosis