Dementias Lacking Distinctive Histologic Features

This is a heterogenous group of sporadic and familial dementias affecting individuals in their fifth and sixth decades. Primary progressive aphasia, cognitive decline, and behavioral changes occur in various combinations. The pathology, confined to the frontotemporal lobes, consists of neuronal losses, spongiosis, and astrogliosis, particularly in the outer cortical layers. Conventional and sensitive silver stains and currently applied immu-nohistologic stains fail to reveal neuronal or glial inclusions.

TABLE 5.7.

Motor Neuron Diseases

Amyotrophic Lateral Sclerosis Spinal muscular atrophy Bulbospinal muscular atrophy

.UMN

Primary lateral sclerosis Hereditary spastic paraparesis

LMN, lower motor neuron; UMN, upper motor neuron.

cases. Some familial cases are associated with mutations in a gene on chromosome 21 that encodes the free radical-scavenging enzyme copper/zinc superoxide dismutase-1 (SOD1). SOD1 converts cytotoxic oxygen radicals to hydrogen superoxide. A rare juvenile variant with autosomal recessive or x-linked inheritance is associated with mutation in a gene (alsin) on chromosome 2. In the etiology of sporadic cases, oxidative stress and the excitotoxic effect of the neurotransmitter glutamate are considered.

The clinical presentation is characterized by a concurrence of symptoms and signs involving both the lower and upper motor neurons. The disease usually begins in the spinal cord, with weakness in extremities, increased or diminished to absent tendon reflexes, Babinski sign, increased or diminished muscle tone, muscle wasting, and fasciculations. Bulbar symptoms usually develop late in the course of the disease but, in a small number of cases, introduce the disease. The bulbar palsy combines the features of brainstem motor neurons (bulbar palsy) with those of corticobulbar tracts (pseudobulbar palsy). It presents with facial weakness, nasal or dysarthric speech, brisk or absent gag and jaw reflexes, dysphagia, atrophy and fasciculation of the tongue, and emotional lability with spontaneous laughing or crying. The course averages from 2 to 10 years.

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