Motor Neuron Diseases

Question 1

Spinal Muscular Atrophies

Answer 1

group of hereditary disorders characterized by proximal weakness
SMA I (Werdnig-Hoffman disease, acute infantile SMA) - Onset birth to 6 mos, cannot sit without support, mild join contractures, hypotonia, trouble with suck or swallow, lifespan  70% alive at 25 years old
SMA III (Kugelberg-Welander disease, chronic juvenile) - Onset after age 1, can walk, frequently falls, trouble walking up and down stairs at age 2-3 years, proximal limb weakness; legs more severely affected > arms, scoliosis common, normal lifespan
SMA IV (adult) onset usually in 2nd or 3rd decade of life, normal lifespan

Question 2

Genetics of SMA

Answer 2

AR (survival motor neural gene accoutns for SMA I, II and III, not IV)

Question 3

Electrodiagnosis of SMA

Answer 3

Sensory nerve conduction studies are normal, motor nerve conduction studies are usually normal or may show decreased amplituteds, needle EMG demonstrates fibrillation, potentials, positive sharp waves, and large motor units with decreased recruitment (neurogenic pattern)

Question 4

Treatment of SMA

Answer 4

Supportive care, trial of riluzole is underway

Question 5

Bulbar and Spinal Muscular Atrophy (Kennedy’s disease)

Answer 5

Clinical Features - Onset usually in 3rd or 4th decade, gynecomastia, testicular atrophy, impotence, progressive weakness affecting proximal muscles, bulbar weakness -> dysarthria, dysphagia, tongue fascciulations, reflexes decreased or absent, no UMN findings
Genetics -> X-linked - only males are slightly symptomatic; females are carriers (occasionally mild weakness), CAG trinucleotide repeat expansion on the androgen receptor gene

Question 6

Infections

Answer 6

Poliovirus - enterovirus rarely seen in US that affects the anterior horn cells and CN nuclei. Clinical features - aseptic meningitis followed by paralysis, usually asymmetric, decreased or absent reflexes, 10-15% of patients with motor system involvement develop CN dysfunction, Respiratory failure may occur (iron lung). Postpolio syndrom - combination of neuromuscular and orthopedic symptoms in patients with a h/o polio in the remote past -> fatigue, muscle pain, joitn pain, weakness, or previously affected or unaffected muscle atrophy, repiratory insufficiency, dysphagia. Pathophysiology -> loss of motor neurons th the time of acute disease -> motor units in which there are many more muscle fibers supplied by each motor neuron -> excessive diemand on motor units -> drop out of muscle fibers, motor units or both
West Nile Virus - Clinical Features - Acute, flaccid