Spinal muscular atrophy (SMA) is a serious genetic disorder that targets motor neurons in the central nervous system (CNS) leading to progressive muscular atrophy, weakness, and paralysis.1,2 The condition affects 1 in 11,000 births but is often undiagnosed on clinical evaluation .1 The availability of new treatments for SMA has changed the natural course of this disease. Early diagnosis and treatment of SMA can limit disease progression in children and adults, extend life expectancy, and improve quality of life (QOL).
The SMA is divided into 5 types according to age at onset and maximum motor function achieved (Table 1) .1,2 Type 0, the most severe form of SMA, begins in utero; After birth, babies have difficulty moving, swallowing, and breathing and can survive for days to months if left untreated.2 Type 1, the most common form, is diagnosed in infancy and is associated with the inability to work without assistance The expected stage of development (age, 9 months) .1,2 Type 2 includes children who can sit but cannot walk without support.1,2 Type 3 includes patients who can walk in childhood but have weak motor skills, and type 4 begins in adulthood in patients with no previously identified neurologic motor deficits.1,2
Table 1. Symptoms of spinal muscular atrophy according to type 1,2
Each type of SMA is associated with symmetrical muscle weakness in the periphery, with weakness initially evident in the lower extremities.1,2 Cognitive function is not impaired in any type of this disease. The disease is a leading cause of infant mortality due to a single gene, but longevity is typically not affected with SMA types 3 and 4.2-4
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Epidemiology
This disorder is most commonly caused by genetic deletions or mutations in the survival motor neuron 1 gene (SMN1) that cause it to produce low levels of SMN protein. The inheritance pattern is autosomal recessive and the carrier frequency ranges from 1 in 45 to 1 in 100 people.2 Asian ethnicity is associated with the highest carrier frequency (2.4%). 1
Due to the high mortality rates of SMA types 0 and 1 and the quality of life effects of all types of SMA, carrier screening for all ethnic groups should be considered. 4
Clinical presentation of SMA types
With SMA type 1, infants have extremity hypotension but normal facial expressions.2 The screams are often weak and bulbar symptoms appear, leading to progressive feeding problems and aspiration of respiratory distress and failure.2 Breathlessness can be the symptom in infants that are not seen during routine health checks. 2
Type 2 SMA has similar clinical diagnostic evidence, but the bulbar symptoms and breathing problems are not as profound as type 1.2 SMA. Infants fail to reach the motor development milestones of pulling up and walking by 15 months of age
For types 3 and 4, the patient’s and parent’s medical history is key to the diagnosis.2 Because patients can walk and function as expected into adolescence or adulthood, clinical symptoms of muscle weakness are often ignored or overlooked, resulting in delayed diagnosis leads (see page 3, Delayed). Diagnosing SMA Type 3: A Case Study) 2
Clinicians should be aware of the genetic prevalence of SMA and consider the condition in the differential diagnosis of newly emerging muscle weakness (Table 2)
Table 2. Differential diagnosis of new onset muscle weakness in children and adults2
This article originally appeared on Clinical Advisor
From July / August 2021 of the Neurology Advisor
