Spinal Muscle Atrophy Clinical Presentation

Updated: Nov 05, 2018
  • Author: Ashish S Ranade, MBBS, MS, MRCS; Chief Editor: Jeffrey A Goldstein, MD  more...
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With type I spinal muscle atrophy (SMA; also known as spinal muscular atrophy), most mothers report abnormal inactivity of the fetus in the latter stages of pregnancy. Babies with type I SMA face many physical challenges, including muscle weakness, trouble breathing, coughing and swallowing. The patient with type I SMA is unable to roll over or sit. Progressive clinical deterioration occurs. Death usually occurs from respiratory failure and its complications in patients by age 2 years.

Patients with type II SMA have normal development for the first 4-6 months of life. They may be able to sit independently, but they are never able to walk. 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.

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 are initially able to walk, but as they grow, their mobility is increasingly limited, and they eventually may need to use a wheelchair. The life span 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 control or lift the head. Areflexia is universal.

Patients with type II SMA have head control, and 75% of these patients can sit independently. Muscular weakness is greater in the lower extremities than in the upper extremities. Patellar reflex is absent. 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 walk early in life and maintain their ambulatory capacity into adolescence. Weakness may cause foot drop, and patients have limited endurance. A third of the patients become wheelchair bound as adults (mean age, 40 years).

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 reported that this finding was associated with elevated serum creatine kinase (CK). [25]  This combination was only observed in males; no females in his series had hypertrophy of the calves. [25]

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.