Spinal Muscular Atrophy
- Progressive muscle weakness
- SMN gene mutation
- Varying severity
- Primary orthopaedic concerns: hip dislocations, scoliosis, and lower extremity contractures
Description:Spinal muscle atrophy (SMA) involves a loss of alpha motor neurons in the anterior horn of the spine. This leads to muscle weakness and atrophy.
Epidemiology:SMA is the most common genetic disorder resulting in death during childhood. It occurs in one out of 10,000 live births.
Clinical Findings:The primary finding is profound muscular weakness. The deep tendon reflexes are absent. The patient may have fasciculations, with tongue fasciculations being pathognomonic. Scoliosis will usually be present at 2-3 years of age.
Imaging Studies:Screening exams should include pelvis and spine radiographs.
Treatment:The primary affected musculoskeletal aspects of SMA, the hips, spine and lower extremities should be evaluated at each visit.
Hips: Hips are commonly dislocated in SMA and reduction is controversial. Many centers do not perform surgery to relocate because of the absence of motor function and that most patients are wheelchair ambulators, while others do, citing risk of late pain.
Spine: Scoliosis is common in SMA and may present early. Patients may be braced early with the goal of delaying surgery. Growing instrumentation is a consideration for early onset patients. Posterior spinal instrumentation and fusion should be performed to the pelvis in mature patients.
In December 2016, the FDA approved nusinersen (Spinraza), the first drug approved to treat children (including newborns) and adults with SMA. Nusinersen is anantisense oligonucleotide (ASO) designed to treat SMA caused by mutations in chromosome 5q that lead to SMN protein deficiency. As the delivery is intrathecal, any planned spinal fusion should allow for access to the intrathecal space via a “skip segment” of fusion.
The treatment of lower extremity contractures is controversial because of a high recurrence rate. Also, surgical intervention rarely improves ambulation ability.
Complications:Prognosis is varied by subtype. Nutritional deficiency and respiratory failure are problematic, with early mortality for type 1 SMA. Later onset is associated with milder symptoms and improved prognosis.
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References:NIH Genetics Home Reference: Spinal Muscular Atrophy http://ghr.nlm.nih.gov/condition/spinal-muscular-atrophy
Mesfin A, Sponseller PD, Leet AI. Spinal muscular atrophy: manifestations and management. J Am Acad Orthop Surg. 2012 Jun;20(6):393-401.
Sucato DJ. Spine deformity in spinal muscular atrophy. J Bone Joint Surg Am. 2007 Feb;89 Suppl 1:148-54. Review. Erratum in: J Bone Joint Surg Am. 2007 May;89(5):1090-1