Diencephalon
Lesions

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1. VASCULAR LESIONS - THALAMIC SYNDROME

A Sample Case: A 60 year old man was noticed by his son as suddenly acting confused and having slurred speech.

The patient's history included high blood pressure which was confirmed upon examination. Otherwise he was in generally good health. There was no papilledema.

The neurologic exam indicated disorientation with respect to time, place and personal details. He had poor memory of recent events. He confused the right and left sides of his body and perseverated his poorly articulated speech which included many inappropriate words.

Ocular movements were all intact but he had a right homonymous hemianopsia. Pupils were symmetric and normally responsive to light. Hearing was normal. Corneal, gag, uvula and jaw-jerk reflexes were normal. The tongue behaved and appeared normal. When asked to smile his facial muscles responded symmetrically; when smiling in response to a joke only the left side smiled.

There was marked weakness of the right arm and leg with increased deep tendon reflexes and a Babinski sign. Pain and temperature sensation were normal on the right side but discriminitive touch and proprioception were reduced.

Explanation -

Disorientation, confusion and impaired speech indicates a problem at diencephalic or cortical levels. The right visual field defect indicates a lesion behind the chiasm on the left, e.g., optic tract, lateral geniculate, optic radiation or visual cortex. Volitional facial motor response was normal but the loss of full emotional smiling also indicates a lesion above the 7th nucleus; the basal ganglia participates with the frontal cortex and thalamus in emotional expression - a lesion on the left would result in a deficit on the right. Other cranial nerve functions are normal.

Upper motor neuron signs of the right arm and leg indicate a corticospinal tract injury on the left - perhaps in the internal capsule or above. Motor cortex injury would require a very large injury along the lateral aspect of the precentral gyrus and would probably have spared the foot which is represented on the medial side. A lesion in the posterior limb of the internal capsule would give the described loss of motor function. The homonymous hemianopsia is consistent with a focal lesion of the internal capsule lesion, otherwise a multifocal lesion is required to explain findings which include the frontal motor cortex and the occipital visual cortex. (A vascular lesion of the visual cortex is likely to result in a quadrantic defect and/or macular sparing.) Sensory findings where pain and temperature sensation on the right were normal but proprioception was not, is not readily explained by an internal capsule lesion but rather suggests a lesion of the nearby medial lemniscus on the left.

The sudden onset suggests a vascular lesion involving the thalamus and posterior limb of the internal capsule including optic radiations. The blood supply to the thalamus comes from penetrating branches of the circle of Willis, basilar, posterior cerebral and middle cerebral arteries.