Practice Essentials
Spinal muscular atrophies (SMAs) represent a rare group of inherited disorders that cause progressive degeneration of the anterior horn cells of the spinal cord. The exact cause of the degeneration is unknown. Loss of these cells results in a progressive lower motor neuron disease that has no sensory involvement and that is manifested as hypotonia, weakness, and progressive paralysis. Kugelberg Welander spinal muscular atrophy (also known as Wohlfart-Kugelberg-Welander syndrome or mild SMA) is a milder form of SMA, with symptoms typically presenting after age 18 months.
SMAs were first described in the 1890s, by Guido Werdnig, a physician from the University of Vienna, in his lecture “On a Case of Muscular Dystrophy with Positive Spinal Cord Findings.” Soon after, Professor Johann Hoffmann from Heidelberg University presented a paper describing a syndrome of progressive atrophy, weakness, and death during the early childhood period of siblings with genetically normal parents. Both physicians conducted autopsies on their patients and found severe atrophy of the ventral roots of the spinal cord. They also found histologic evidence of loss of motor neurons in the anterior horn cells of this region. Hoffmann called the syndrome spinale muskelatrophie (spinal muscular atrophy).
In the early 1960s, Byers and Banker classified SMA into categories based on the severity and age of onset of the symptoms, in an effort to predict prognosis. Their system, summarized below, became the basis for the most widely recognized system now used for the classification of SMA.
Type I
Onset of symptoms before age 6 months
Also known as infantile onset SMA, or Werdnig-Hoffmann disease
Type II
Onset of symptoms at age 6-18 months
Also known as chronic SMA, juvenile SMA, or intermediate SMA
Type III
Onset of symptoms after age 18 months, usually in late childhood or adolescence
Also known as Kugelberg Welander SMA, or mild SMA
Although Byers and Banker’s classification system focuses on only the above 3 categories, many sources refer to a fourth type of SMA.
Type IV
This category is reserved for onset of symptoms during early adulthood.
This disorder usually carries a much more favorable prognosis than do the other types of SMA.
This article focuses only on SMA types III and IV.
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Signs and symptoms of spinal muscular atrophy
Signs and symptoms of SMA include the following:
Proximal muscle weakness is seen in SMA, with the pelvic girdle being more affected than the shoulder girdle
Patients have decreased muscle tone
Patients have diminished deep tendon reflexes; ankle reflexes, however, may be preserved until very late in the disease’s progress
Fasciculations may be present in the tongue or shoulder girdle muscles (especially after manual muscle testing)
Minipolymyoclonus, a fine, irregular tremor of the outstretched fingers, may be seen
Calf pseudohypertrophy has occasionally been noted, but muscle wasting of the affected musculature is more prominent
Patients may have a positive Gower sign and a waddling gait
Approximately one third of patients have facial and masseter muscle weakness
Sensory examination findings are normal
Workup in spinal muscular atrophy
Routine diagnostic testing for SMA involves targeted mutation analysis to detect deletion of exons 7 and 8 of SMN1.
Serum creatine kinase levels may be elevated but usually not to the extent that they are elevated in persons with muscular dystrophy. Serum aldolase levels also are commonly elevated in persons with types III and IV SMA.
Other tests include the following:
Muscle biopsy – Reveals evidence of neurogenic atrophy and chronic reinnervation in SMA
Electromyography (EMG) and nerve conduction studies (NCS) – Diffuse abnormalities on EMG are seen in the extremities and bulbar musculature
Management of spinal muscular atrophy
Rehabilitation
Spinal muscular atrophy (SMA) has no known cure; thus, most care for the patient with SMA is focused on symptomatic control and preventative rehabilitation.
Maintaining the patient’s joint mobility is very important because the goal is to decrease the incidence of contractures. Plantar flexion contractures are the most common.
Ankle-foot orthotics worn at night may help to provide prolonged, passive stretching to prevent worsening of ankle plantar flexion contractures.
Stretching and strength training in patients under the care of an experienced physical therapist are very important components of the preventative rehabilitation approach.
Occupational therapy is useful for teaching the patient ways to increase his/her independence in activities of daily living (ADL).
Patients with SMA may require consultation with a speech therapist if dysphagia is present or diet modification is needed.
Surgery
If scoliosis develops in a patient with SMA, spinal instrumentation and fusion may be necessary.
Some upper extremity function can be lost after fusion.
Tendon lengthenings may be needed to improve joint position.