Adult spinal muscular atrophy is a genetic neurodegenerative disorder characterized by muscle wasting and weakness. In adults, normal life expectancy is achieved, as only mild weakness may be encountered. The diagnosis rests on confirming genetic mutations.
Adult spinal muscular atrophy or spinal muscular atrophy (SMA) type 4 usually begins to appear at 30 years of age and has a relatively benign course . The clinical features of this disease are very similar to SMA type 3, although the adult version is less severe. Life expectancy is normal in such patients . Type 1 and type 2 SMA can occur at any age between birth and 2 years.
Patients classically present with an asymmetric weakness of the proximal muscles of the arms and legs, resulting in hampering of the quality of life . Over time, the muscle wasting spreads to the other limb muscles before involving the central portions of the body. The weakness is associated with pain, in both the normal and affected muscles. Tremors and twitchings are also known to occur .
Involvement of the muscles of the thorax may result in respiratory compromise. The affliction of the brainstem may lead to bulbar symptoms such as dysphagia and dysarthria. There are usually no other central nervous system manifestations present. Reports have shown that these patients exhibit a higher degree of intelligence.
The examination may show all the characteristic findings of a lower motor neuron disease, namely absent/decreased reflexes, hypotonia, flaccid paralysis and muscle fasciculations.
Diagnosis of this disease is dependent on the anamnestic data. Amongst laboratory tests that may be performed, creatine kinase is the most useful, with elevated levels of the enzyme found. In some patients, the enzyme levels may be normal.
Genetic tests may be performed both prenatally and postnatally to check for deletions involving the SMN1 gene .
Electrocardiograms (ECGs) are usually normal in these patients .
A variety of other neuromuscular diseases may present in a similar manner. Differentiation is done by electrophysiological tests that demonstrate a reduction of transmitted nerve signals  . Sensory signals are usually normal in adult spinal muscular atrophy. Tests involving compound motor action potential may also be performed for diagnosis.
Muscle biopsy is another test that may help in differentiating adult spinal muscular atrophy from other similar muscular diseases  . These characteristically show atrophy of the muscle fibers with compensatory hypertrophy.
Differential diagnosis usually involves the other spinal muscle atrophies which include the X-linked spinal muscular atrophy, distal spinal muscular atrophy, autosomal dominant spinal muscular atrophy and juvenile asymmetric segmental spinal muscular atrophy. Postpolio syndrome and multifocal motor neuropathy are the other conditions that may present in a similar fashion.
There is a potential for patients with adult spinal muscular atrophy to be misdiagnosed due to the overlapping of clinical features. Over the years, patients labeled earlier as SMA have had their diagnoses revised to more treatable conditions including syringomyelia, amyotrophic lateral sclerosis, myopathy, axonal neuropathies and herniated lumbar discs. It is hence, recommended that patients be followed up for a long duration to establish the correct diagnosis.