Spinal Muscular Atrophy II
Spinal muscular atrophy II disease onset is usually more insidious than that of SMA I. The findings of generalized hypotonia, symmetrical weakness, and delayed motor milestones are hallmarks of SMA II.
Weakness also involves proximal muscles more than distal muscles and lower extremity more than upper extremity. A fine tremor of the fingers and hands occurs in a minority of patients. This “polyminimyoclonus” may be attributed to spontaneous, repetitive rhythmical discharges by the motor neurons that innervate a large territory of muscle. Wasting tends to be more conspicuous in SMA II versus SMA I. DTRs are depressed and usually absent in the lower extremities. Appendicular or thoracic muscle wall fasciculations may be observed. Tongue fasciculations have been observed in 30% to 70% of SMA II patients (97,113,114) Progressive kyphoscoliosis and neuromuscular restrictive lung disease is almost invariably seen in the late first decade. Contractures of the hip flexors, tensor fasciae latae, hamstrings, triceps surae, and elbow and finger flexors are quite common. Hypotonic hip dislocations have been noted commonly in SMA II patients. Sensory examination is completely normal, and extraocular muscles and the myocardium are spared. In a large series from Germany (98), of 104 cases classified as SMA II (sits alone, never walks), 98% survived to the age of 10 and 77% to the age of 20. Thus, a longer lifespan is possible with adequate supportive care.SMA II is a slowly progressive condition affecting proximal musculature more than distal. The calculated grade of progression for SMA may be less than one-half manual muscle testing units decline per decade (69). Longitudinal series of 12-39 months' duration have shown essentially stable strength measurements but slow loss of function (115,116).
Pathologic changes on muscle biopsy have been consistent with hypotrophic change in fetal muscle development. Other changes are consistent with a more active denervating process. Thus, SMA includes a component of myofiber atrophy comparable to that seen in other denervating diseases and is not a pure hypotrophic process occurring during early fetal development.