Diseases affecting the lower motor and sensory neurons can be classified by the area of pathologic involvement and are divided into 2 groups: anterior horn cell diseases and peripheral nerve disorders. Anterior horn cell (also known as motor neuron) diseases affect the most proximal portion of the motor unit and are primarily inherited or infectious. The spinal muscular atrophies are the major inherited disorders of the anterior horn cell. Infectious anterior horn cells disorders include paralytic poliomyelitis, which is largely eradicated from the Western world, in addition to other enterovirus, arbovirus, and West Nile virus. Peripheral nerve disorders affect the nerve segments distal to the anterior horn cell and may be divided into hereditary neuropathies and acquired neuropathies. Hereditary neuropathies (Charcot-Marie-Tooth) are more common in children than are acquired neuropathies. Acquired neuropathies include autoimmune inflammatory neuropathies such as Guillain-Barre syndrome and chronic inflammatory demyelinating polyneuropathy, facial neuropathies, infectious neuropathies, traumatic neuropathies, perinatal brachial plexus injuries, toxic neuropathies, and vitamin deficiency neuropathies.
ANTERIOR HORN CELL DISORDERS
Etiology and Pathogenesis
Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by the degeneration of spinal cord motor neurons, resulting in progressive muscular atrophy and weakness (Table 561-1). The classic form of the disorder is autosomal recessive and caused by a homozygous deletion or mutation involving the survival of motor neuron 1 (SMN1) gene on chromosome 5q11.2-q13.3. Approximately 95% of patients with SMA have this genetic defect, which results in a decreased level of survival of motor neuron protein. A small proportion of patients (4%) are affected with other causative mutations with genetic and clinical heterogeneity, termed non-5q SMA. The incidence of SMA is estimated at 1 in 6000 to 11,000 live births with a carrier frequency of 1 in 38 to 70. It is the most common genetic cause of death in infants.
TABLE 561-1SPINAL MUSCULAR ATROPHY (SMA) SUBTYPES ||Download (.pdf) TABLE 561-1SPINAL MUSCULAR ATROPHY (SMA) SUBTYPES
|Type ||Prevalence ||Onset ||Clinical Features ||Life Expectancy |
|SMA type 1 ||60% ||0–6 mo ||Unable to sit up ||2 y |
|SMA type 2 ||27% ||7–18 mo ||Able to sit, but unable to stand or walk ||> 2 y (often 2nd-3rd decades) |
|SMA type 3 ||12% ||> 18 mo ||Able to sit, stand, and walk at least until puberty ||Adult |
|SMA type 4 ||1% ||> 18 y ||Ambulatory during adult years ||Adult |
SMA is commonly classified into 4 clinical subtypes based on severity as defined by the maximal functional level achieved, with type 1 patients (the most frequent) defined as nonsitters, type 2 as sitters, type 3 as walkers, and type 4 as adult onset with mild phenotype (see Table 561-1). In addition, there is a relatively uncommon category used by some called ...