Spinal Muscle Atrophy Clinical Presentation

Updated: Aug 11, 2020
  • Author: Ashish S Ranade, MBBS, MS, FRCS(Glasg); Chief Editor: Jeffrey A Goldstein, MD  more...
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Spinal muscle atrophy (SMA; also known as spinal muscular atrophy) is a single-gene disorder with a spectrum of clinical presentation. The clinical presentation includes a wide range of phenotypes that are classified into groups on the basis of age of onset and maximum level of motor function achieved, but hypotonia and/or muscle weakness and atrophy are common signs and symptoms.

With type I SMA, most mothers report abnormal inactivity of the fetus in the latter stages of pregnancy. Babies with type I SMA face many physical challenges, including trouble breathing, coughing and swallowing.Patients with type I SMA are unable to roll over or sit. Progressive clinical deterioration occurs. Death usually occurs from respiratory failure and its complications by age 2 years.

Patients with type II SMA have normal development for the first 4-6 months of life. They are able to sit independently but are never able to walk independently. They require a wheelchair for locomotion. They have a longer life span than patients with type I SMA. Some patients with type II SMA live into the fifth decade of life, with more than 70% still living at 25 years of age.

In patients with type III SMA, the presenting complaint is difficulty climbing stairs or getting up from the floor (due to hip extensor weakness). Individuals affected by SMA type III initially are able to walk independently, but as they grow, their mobility is increasingly limited, and they eventually may need to use a wheelchair. The lifespan is nearly normal. [24]


Physical Examination

SMA is often diagnosed clinically on the basis of the child's physical appearance. The diagnosis may be suspected when children are noted to be weak or to have a delay in their developmental milestones, such as holding their head up, rolling over, sitting independently, standing, or walking later than would be expected.

After a thorough medical history is reviewed and a physical examination is performed, the primary care provider may order genetic testing through a blood sample, or the child may be referred to a neurologist who will also perform an examination and then order genetic testing (again through a blood sample) to confirm the diagnosis. 

Type-specific findings

Newborns with type I SMA are floppy and inactive. They move the extremities little, if at all. The hips are flexed, abducted, and externally rotated. The knees are flexed. Because the distal musculature is usually spared, the fingers and toes move. Infants cannot sit independently or control or lift the head. They have a weak cry and cough. They have difficulty with swallowing and feeding. They have a bell-shaped trunk with chest-wall collapse and abdominal protrusion with paradoxical breathing. Areflexia is universal.

Patients with type II SMA have head control and can sit independently, though about 25% may lose this ability in their mid-teenage years. They have bulbar weakness resulting in difficulty with coughing, swallowing and clearing tracheal secretions. They have weak intercostal muscles, diaphragmatic breathing, and fine tremors with extended fingers or with attempted hand grips. Muscular weakness is greater in the lower extremities than in the upper extremities. Patients develop lower-extremity contractures, and about 50% lose the ability to walk by age 12 years.

Patellar reflex is absent in type II SMA. The young may demonstrate bicipital and triceps tendon reflexes. Tongue fasciculations are present, as are upper-extremity tremors. Scoliosis is universal, and most patients develop hip dislocation, either unilateral or bilateral, when younger than 10 years.

Patients with type III SMA are able to walk independently early in life and maintain their ambulatory capacity into adolescence. They may have difficulty with coughing and swallowing with nocturnal hypoventilation.They develop muscle weakness, aching, and joint overuse symptoms. Weakness may cause foot drop, and patients have limited endurance. A third of the patients become wheelchair-bound as adults (mean age, 40 years). 

Functional decline in later-onset SMA types is associated with age of onset of symptoms and maximal function acheived. Loss of function may have a significant impact on their quality of life. Stabilization of functional abilities may be important for individuals with later-onset SMA. [25]

Other findings

A long C-shaped thoracolumbar scoliotic curve is present in patients with type II SMA and in half of patients with type III SMA. The curve progresses to a severe and incapacitating deformity if not treated. About 30% of patients have kyphotic deformities as well.

Pseudohypertrophy of the calf is present, which may confound the diagnosis (ie, with Duchenne muscular dystrophy and Becker muscular dystrophy). Bouwsma et al reported that this finding was associated with elevated serum creatine kinase (CK). [26]  This combination was only observed in males; no females in the series had hypertrophy of the calves. [26]

Tongue fasciculations are pathognomonic of SMA (all types), as opposed to all other neuromuscular diseases of infancy. The presence of tongue fasciculations can aid in the diagnosis, in that 56% of patients exhibit this symptom.