summary Spinal Muscular Atrophy is a common genetic disease caused by an autosomal recessive mutation in the survival motor neuron gene. The condition presents with progressive motor weakness, scoliosis, hip dislocations, and lower extremity contractures. Diagnosis is made by DNA analysis and muscle biopsy. Treatment involves a multidisciplinary approach to address motor weakness, scoliosis, and lower extremity contractures. Treatment of associated hip dislocation is observation. Epidemiology Incidence most common genetic disease resulting in death during childhood 1 in 10,000 live births Anatomic location progressive weakness starts proximally and moves distally Etiology Pathophysiology caused by progressive loss of alpha-motor neurons in anterior horn of spinal cord Genetics inheritance autosomal recessive mutation survival motor neuron (SMN) gene mutation present in 90% of cases of SMA a telomeric gene deletion SMN critical to RNA metabolism and is a mediator of apoptosis there are two SMN genes all patients with SMA lack SMN-I protein severity of disease based on number of functional copies of SMN-II Associated conditions orthopaedic manifestations of SMA hip dislocation and subluxation scoliosis lower extremity contractures Classification Types of Spinal Muscle Atrophy Type Name Presentation Prognosis Type I Acute Werdnig-Hoffman disease Present at < 6 months Absent DTR Tongue fasciculations Poor, usually die by 2 yrs. Type II Chronic Werdnig-Hoffman disease Present at 6-12 months Muscle weakness worse in LE Can sit but cant walk May live to 5th decade Type III Kugelberg-Welander disease Present at 2-15 years Proximal weakness Walk as children, wheelchair as adult Normal life expectancy - may need respiratory support Presentation Symptoms symmetric progressive weakness that is more profound in lower-extremity than upper extremity more profound proximally than distally Physical exam absent deep tendon reflexes distinguishes from Duchenne's muscular dystrophy where DTR are present fasciculations present Imaging Radiographs scoliosis series pelvis Evaluation Diagnosis based on DNA analysis muscle biopsy prenatal diagnosis is possible Treatment Nonoperative Nusinersen has been FDA approved for treatment of SMA. It is administered intra-thecally. Operative treat associated orthopaedic disorders (details below) hip dislocation scoliosis lower extremity contractures Hip Dislocation Overview hip subluxation and dislocation occur in 62% with type II SMA, and less frequently in Type III. Treatment nonoperative observation alone (leave dislocated) indications standard of care as dislocations typically remain painless and high recurrence rate if open reduction attempted Scoliosis Overview the development of scoliosis is almost universal usually occurs by age 2 to 3 years often progressive Treatment nonoperative bracing indications devices may delay but not prevent surgery in children younger than ten years operative PSF with fusion to pelvis indications progressive curve technique address hip contractures and any other lower extremity contractures before PSF to ensure seating balance to allow for intrathecal Nusinersen to be given after spine surgery, perfrom a laminectomy in the lower lumbar spine that is kept free of fusion. perform fusion of spine around this laminectomy outcomes for improved wheelchair sitting may lead to temporary loss of upper extremity function Combined PSF with anterior releases/fusion indications curves >100 degrees very young child with high risk of crankshaft phenomenon contraindications pulmonary compromise typically not necessary due to the high flexibility of SMA curves Hip, knee, and ankle contractures Overview Common in the hip and knee Nonambulators also develop ankle equinus Treatment Physical therapy Surgical release is controversial as function in nonwalkers is rarely improved and recurrence is common Prognosis Severity of disease based on number of functional copies of SMN-II Type 1 SMA has the worst prognosis, with majority of patients dying by age 2