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OVERVIEW OF MOTOR NEURON DISEASE

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This chapter discusses a group of diseases that affect motor neurons in isolation, collectively referred to as motor neuron disease. Motor neuron disease can affect upper motor neurons (motor neurons of the central nervous system), lower motor neurons (motor neurons of the peripheral nervous system), or both (Table 28–1). The primary symptom of motor neuron disease is weakness. The distribution of weakness, associated signs (upper motor neuron vs lower motor neuron vs both; see “Upper Motor Neuron Lesions Versus Lower Motor Neuron Lesions” in Ch. 4), and pace of progression differ depending on the disease. Just as there are upper and lower motor neurons for the muscles of the extremities, the motor cranial nerves contain lower motor neurons under the control of upper motor neuron cortical input (corticobulbar tract). The muscles of the face, larynx, and pharynx (bulbar muscles) are affected in many motor neuron diseases.

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TABLE 28–1Clinical Features of Motor Neuron Disease.
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AMYOTROPHIC LATERAL SCLEROSIS (ALS) AND ITS VARIANTS

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Clinical Features of Amyotrophic Lateral Sclerosis (ALS)

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The most common motor neuron disease is amyotrophic lateral sclerosis(ALS). ALS affects the motor system both centrally and peripherally leading to weakness in the limbs and bulbar and respiratory muscles, with both upper motor neuron signs and lower motor neuron signs on examination.

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ALS has a pure central nervous system/upper motor neuron variant known as primary lateral sclerosis, a pure peripheral nervous system/lower motor neuron variant called progressive muscular atrophy, and a pure brainstem variant known as progressive bulbar atrophy. These variants are less common and may accumulate additional features over time, developing into ALS (i.e., lower motor neuron signs emerge in primary lateral sclerosis, upper motor neuron signs develop in progressive muscular atrophy, extremity weakness arises in progressive bulbar atrophy).

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ALS typically begins after the age of 50, but can affect younger patients. Early-onset cases are often familial. Slowly progressive weakness most commonly begins in one limb and progresses over time to involve the other limbs, the tongue, the larynx, the pharynx, and the respiratory muscles. Less commonly, the disease can also begin in the bulbar muscles and spread to the limbs. Pseudobulbar affect (laughing and/or crying out of proportion to the emotion felt) can occur in later stages of the disease. Eye ...

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