Caudal paramedian midbrain syndrome

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By Douglas J Lanska MD MS MSPH

Article under review: Mossuto-Agatiello L. Caudal paramedian midbrain syndrome. Neurology 2006;66:1668-71.

Background: The paramedian midbrain is supplied by penetrating vessels from the tip of the basilar artery (rostral and middle) and from paramedian branches of the basilar artery (caudal) (Bogousslavsky et al 1994).

The superior cerebellar peduncle (brachium conjunctivum) is a massive fiber bundle, arising in the deep cerebellar nuclei, decussating in the tegmentum of the caudal midbrain, and then either terminating in the red nucleus or forming the "capsule of the red nucleus" and proceeding rostrally to terminate in the ventrolateral nucleus of the thalamus.

Caudal midbrain lesions involving the decussation of the superior cerebellar peduncles have been named the "syndrome of the commissure of Wernekink." Knowledge of this syndrome has been based largely on the clinicopathological observations of single cases by Lhermitte and colleagues and Garcin and colleagues (Lhermitte et al 1941; Garcin et al 1971). Similar cases have been recognized based on clinical and radiologic observations (Kim and Kim 2005).

Purpose: To characterize the clinical and radiologic features of patients with unilateral lower midbrain lesions involving the decussation of the brachium conjunctivum.

Design: Case series.

Methods: Five patients were studied with unilateral paramedian caudal midbrain infarction evident on MRI. These patients were studied at a stroke rehabilitation center after the acute stage.

Results: Patients ranged from 34 to 64 years (median 42 years). Four were men. Etiologies identified or suspected included embolism, penetrating branch small vessel disease, intracranial vertebral artery dissection, and ruptured basilar tip aneurysm. Other studies have attributed similar lesions to small vessel disease when lesions were deep and localized, and to proximal posterior cerebral artery atherothrombosis when extending to the medial surface of the midbrain (Kim and Kim 2005).

Bilateral cerebellar dysfunction: All patients had bilateral asymmetric cerebellar dysfunction characterized by gaze-evoked nystagmus, scanning dysarthria, limb dysmetria affecting all 4 limbs, dysdiadockokinesis, and truncal and gait ataxia. The identified lesions probably affected the decussation of the superior cerebellar peduncles, so that a single unilateral lesion could disrupt bilateral dentatorubrothalamic pathways and produce bilateral cerebellar signs.

Palatal myoclonus (variably present): Palatal myoclonus associated with bilateral increased signal intensity and enlargement of the inferior olivary nuclei on MRI, due to lesions in the so-called Guillain-Mollaret triangle is variably present. The vertices of the Guillain-Mollaret triangle are the dentate nucleus, the contralateral red nucleus, and the contralateral inferior olivary nucleus. The dentate nucleus sends efferents through the superior cerebellar peduncle (brachium conjunctivum), to a decussation in the tegmentum of the caudal midbrain, and then projects to the red nucleus and the ventrolateral nucleus of the thalamus. The red nucleus and the basal ganglia send projections through the central tegmental tract to the homolateral inferior olivary nucleus. The central tegmental tract is located dorsally in the causal midbrain tegmentum, ventrolateral to the medial longitudinal fasciculus and dorsolateral to the brachium conjunctivum. The inferior olivary nucleus sends projections to the contralateral dentate nucleus through the inferior cerebellar peduncle, thus completing the sides of the Guillain-Mollaret triangle.

Internuclear ophthalmoparesis (variably present): Internuclear ophthalmoparesis is due to disruption of the medial longitudinal fasciculus, which is located just ventral to the periaqueductal grey matter and dorsal to the brachium conjunctivum in the caudal midbrain tegmentum. The medial longitudinal fasciculus conveys fibers from the contralateral paramedian pontine reticular formation, allowing a yoking of the lateral rectus muscle ipsilateral to the paramedian pontine reticular formation with the medial rectus muscle on the opposite side. Disruption of the medial longitudinal fasciculus produces impaired adduction in the ipsilateral eye with attempted lateral gaze to the opposite side (from the medial longitudinal fasciculus).

Conclusions and commentary: The caudal paramedian midbrain syndrome is defined by:

  • Acute onset
  • Bilateral asymmetric cerebellar dysfunction
  • Internuclear ophthalmoparesis (variably present)
  • Delayed onset of palatal myoclonus (variably present)
  • Absent, minor, or transient weakness
  • Preserved sensation

Radiologic features include:

  • Caudal paramedian midbrain infarct
  • Delayed appearance of bilateral olivary degeneration after at least 1 month

The author concluded that this neurologic picture is "not distinctive enough to warrant its recognition as a specific vascular syndrome," because bilateral ataxia is the only consistent clinical manifestation (Mossuto-Agatiello 2006).

Caudal paramedian midbrain infarction must be distinguished from other unilateral anteromedial midbrain lesions, which often involve the third nerve fascicles or nucleus (rostral midbrain), the red nucleus (rostral midbrain), and the median portion of the cerebral peduncle (ventrolateral midbrain). Such syndromes include Claude syndrome (caused by a lesion of the superior cerebellar peduncle projecting to the red nucleus and the thalamus and the adjacent third nerve nucleus or fascicle, causing ipsilateral third nerve palsy and contralateral ataxia) (Broadley et al 2001; Seo et al 2001; Fong 2005), Weber syndrome (caused by a lesion of the cerebral peduncle and third nerve fascicle, causing ipsilateral third nerve palsy and contralateral hemiparesis), isolated nuclear third nerve paresis (Bogousslavsky et al 1994), pure motor hemiparesis (Bogousslavsky et al 1994), ataxic hemiparesis (Bogousslavsky et al 1994; Kumral et al 2002; Kim and Kim 2005) in some cases with cheiro-pedal-oral (hand-foot-mouth) hypesthesias due to involvement of the more laterally placed medial lemniscus (Bogousslavsky et al 1994), or other lacunar syndromes (Kumral et al 2002).