Diencephalon
Lesions

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1. VASCULAR LESIONS - THALAMIC PAIN SYNDROME (of DEJERINE-ROUSSY)

A Sample Case: A progressive numbness of his arm and left leg, which over time resolved, next proceeded to great discomfort and led a man of 40 years of age to seek medical assistance. He had already noticed a mild weakness on his left side.

The patient was alert and well oriented with good long-term memory but less good short-term memory functions. His general physical exam was normal. Proprioception and postural sensibility were reduced on the left. Application of a painful stimulus elicited a stabbing, knife-like or burning, agonizing sensation; even light touch such as the pressure of clothing was extremely uncomfortable. The face showed no sensory alterations. The discomfort was not relieved by analgesics.

A left-sided spastic hemiparesis was found and there were involuntary choreiform movements noted. Voluntary movements of the facial muscles were normal on both sides but emotional responses were asymmetric due to lack of involvement of the left side of the face.

Eye movements were normal but visual field examination showed a left homonymous lower quadranopsia. Hearing was normal. Jaw jerk and corneal reflexes were normal. Tongue and palate responses were normal.

Explanation -

The unusual facet of this case, which makes it recognizable, is the description of the pain. The agonizing pain on the left ("stabbing, crushing, burning, etc") is actually a dysthesia resulting from irritation of the thalamus (on the right side), which can be brought about by the most trivial of cutaneous stimuli. The thalamic nucleus most often involved is the ventroposterolateral nucleus (and results in the loss of proprioception). Even changes in ambient temperature, barometric pressure, music, fright, or argument can aggravate the pain. Often the syndrome begins with numbness, which may spare the face, but then evolves into the entire painful syndrome. The involvement of the arms, trunk and legs argues against a cortical lesion.

Spastic hemiparesis of the left arm and leg suggests injury to the right corticospinal tract, probably in the posterior limb of the internal capsule, which lies just lateral to the thalamus and may be injured directly or by swelling of the affected thalamus. The involuntary choreoathetoid movements could also result from the nearby ventrolateral nucleus of the thalamus or subthalamic nucleus containing efferents of the basal ganglia. This injury to the basal ganglia, its efferent pathways, or the related thalamic nuclei, result in loss of emotional facial responses.

General cortical functions are preserved but there is a deficit in short-term memory which may require a relay in the thalamus prior to its registration.

There is no cranial nerve involvement except for the optic nerve where there is a left lower quadrantic anopsia which indicates a post-chiasmatic lesion on the right. Only the lower visual field is missing so the medial portion of the lateral geniculate nucleus, the upper fibers of the optic radiation, or the cuneus of the visual cortex in the occipital lobe may be considered. The smallest lesion likely would be in the posterior thalamus and would exclude the visual cortex.

The lesion appears to be unilateral and localized to the posterior portion of the thalamus. If the lesion is due to a vascular occlusion then the vessels involved would be the thalamogeniculate branches from the proximal portion of the posterior cerebral artery.