Hereditary and Acquired Motor Neuron Diseases
Definitions
Diseases characterized by progressive degeneration and loss of motor neurons in spinal cord, motor nuclei of brainstem, motor cortex, or all three sites.
Spinal Muscular Atrophies (SMA) of Childhood
SMA: lower motor neuron signs only.
Infantile, childhood, or juvenile forms differ in age at onset, severity. All autosomal recessive; all map to 5q11-q13 with
mutations in survival motor neuron (SMN) gene (allelic heterogeneity).
mutations in survival motor neuron (SMN) gene (allelic heterogeneity).
Infantile SMA (type 1; Werdnig-Hoffmann disease): evident at birth or before age 6 months. Common cause of floppy infant syndrome. Sucking problems, feeble or no limb movements, tongue fasciculations, absent tendon reflexes. Proximal muscles affected before distal. Respiratory failure later. About 85% die before age 2 years. Survivors do not sit.
SMA type 2: onset age 6 to 12 months; can sit, not walk.
SMA type 3 (Kugelberg-Welander syndrome): onset in late childhood or adolescence; can walk, slowly progressive gait disorder, proximal arm weakness, absent tendon reflexes, fasciculation of limb muscles and tongue. Other cranial muscles spared.
Laboratory Data
EMG: denervation with normal nerve conduction. Serum CK may be increased. Diagnosis: DNA analysis.
Focal Muscular Atrophies of Childhood and Adolescence
Lower motor neuron signs only. Most autosomal recessive.
Fazio-Londe disease: onset late childhood or adolescence; dysarthria, dysphagia, lingual fasciculation without limb weakness.
Scapuloperoneal, fascioscapulohumeral forms: distinction from corresponding muscular dystrophies depends on DNA analysis.
Childhood SMA with hexosaminidase deficiency: onset in childhood or adolescence. Autosomal recessive. Some have upper motor neuron signs as well.
Motor Neuron Diseases of Adult Onset
X-Linked Recessive SMA (Kennedy Syndrome)
Lower motor neuron signs only.
Onset after age 40; dysarthria, dysphagia. Limb weakness later; usually proximal; fasciculation of tongue, limbs. Tendon reflexes absent. Mutation: expansion of CAG repeat; longer repeat length associated with increased severity.
Amyotrophic Lateral Sclerosis (ALS)
Also known as Lou Gehrig’s disease. Upper and lower motor neuron signs. In some patients, only lower motor neuron signs present; called “progressive muscular atrophy” (PMA) but more than half of these cases show degeneration of corticospinal tracts at autopsy.
Definition
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