OVERVIEW

What is spinal muscular atrophy?

Spinal muscular atrophy (SMA) is a hereditary neurodegenerative disorder primarily caused by degeneration of anterior horn cells in the spinal cord and lower brainstem motor nuclei. Its clinical manifestations mainly include varying degrees of hypotonia and muscle atrophy, which may severely affect respiratory muscles and become life-threatening in advanced cases.

The disease often runs in families and may onset at any age from fetal development to adulthood. Treatment options are limited, and the prognosis is generally poor.

Is spinal muscular atrophy common?

It is a clinically rare disease, with studies indicating an incidence of 4–10 cases per 100,000 live births.

What are the types of spinal muscular atrophy?

Based on age of onset and disease severity, SMA is classified into types I–IV:

• Type I: Infantile spinal muscular atrophy (Werdnig-Hoffmann disease);

• Type II: Intermediate SMA, typically occurring in children aged 6–18 months;

• Type III: Juvenile SMA (Kugelberg-Welander disease), usually manifesting from early childhood to adolescence, with most cases developing before age 5;

• Type IV: Adult-onset (late-onset) chronic spinal muscular atrophy.

SYMPTOMS

What are the manifestations of spinal muscular atrophy?

Patients with spinal muscular atrophy primarily exhibit progressive muscle atrophy and weakness, along with low muscle tone. The affected areas and clinical features vary depending on the age of onset:

• Infantile muscular atrophy may begin in some cases during the fetal stage, while others develop symptoms in infancy. Affected infants often have weak crying, difficulty feeding, respiratory and swallowing difficulties, joint dislocation, and generalized weakness. In severe cases, respiratory muscles may be involved, and most of these infants die within one to two years.

• Intermediate spinal muscular atrophy has milder clinical symptoms than the infantile type, mainly affecting infant development. Children often cannot stand or walk independently and may experience muscle fasciculations. Some may exhibit a waddling gait ("duck walk") due to pelvic girdle muscle weakness. About one-third may develop facial muscle weakness, leading to reduced facial expressions. Most children with this type survive into childhood or adolescence, with some reaching adulthood, but many die from respiratory complications.

• Juvenile spinal muscular atrophy mostly affects adolescent males. Early symptoms may include proximal limb weakness, particularly in the thighs and hips, leading to difficulty climbing stairs or squatting. Walking may involve a protruding abdomen, waddling, or a duck-like gait. Over time, shoulder and upper limb muscles weaken, making arm lifting difficult, and eventually, distal limb muscles are affected.

• Adult-onset spinal muscular atrophy typically begins between ages 17–30, with insidious onset and slow progression. Early symptoms may include painful muscle cramps, followed by progressive proximal limb weakness, atrophy, and fasciculations, gradually impairing running, climbing stairs, walking, and arm lifting. Some patients may develop endocrine dysfunction, such as gynecomastia in males. Other subtypes may present with distal muscle weakness (hands, feet) or involve peroneal or scapular muscles.

What severe consequences can spinal muscular atrophy cause?

Spinal muscular atrophy can impair childhood growth and development, and in severe cases, respiratory muscle involvement may lead to respiratory paralysis.

What conditions should spinal muscular atrophy be differentiated from?

Spinal muscular atrophy should be distinguished from other disorders causing muscle atrophy, weakness, or hypotonia, particularly congenital myopathies, spinal cord diseases, muscular dystrophy, metabolic myopathies, and polyneuropathies.

CAUSES

What are the causes of spinal muscular atrophy?

This disease is a genetic disorder, but its inheritance pattern is complex, including autosomal recessive/dominant inheritance and X-linked inheritance. Several pathogenic genes have been identified, including the SMN gene, but the specific pathogenesis of the disease is not yet fully understood.

DIAGNOSIS

What is the diagnostic basis for spinal muscular atrophy?

SMA should be suspected in any infant with unexplained muscle weakness or hypotonia. Clinicians make a definitive diagnosis based on the age of onset, symptoms and signs of lower motor neuron disease such as muscle weakness, reduced muscle tone, hyporeflexia or areflexia, and fasciculations, combined with auxiliary examinations and genetic testing results.

What tests are required for patients with spinal muscular atrophy? Why are these tests necessary?

• Hematological tests, including serum creatine kinase, are usually normal or mildly elevated in SMA patients;

• Muscle imaging (CT, ultrasound, or MRI) aids in diagnosis and differentiation of muscle disorders;

• Neurophysiological studies: Helps distinguish SMA from peripheral nerve or muscle diseases and reflects disease severity and progression;

• Muscle biopsy: Assists in diagnosing SMA and differentiating it from other muscle disorders;

• Genetic testing: Confirms the diagnosis.

What precautions should be taken for neurophysiological testing in spinal muscular atrophy patients?

Neurophysiological testing is an important method for diagnosing muscle and peripheral nerve diseases.

Before the test, patients should disclose any history of cardiac pacemakers. During the procedure, they must follow the doctor's instructions and avoid unnecessary movements. Some tests may involve mild electrical sensations, and slight needle-like discomfort may occur afterward, which usually subsides with rest.

TREATMENT

Which department should I visit for spinal muscular atrophy?

Pediatrics or Neurology.

Can spinal muscular atrophy heal on its own?

No.

Does spinal muscular atrophy require hospitalization?

Hospitalization is needed for diagnosis.

How is spinal muscular atrophy treated?

There is no effective cure, but supportive therapies like nutrition, respiratory support, physical therapy, and symptomatic treatment can help prolong survival:

• Respiratory support includes airway secretion clearance and non-invasive/invasive ventilation.

• Nutritional support includes nasogastric feeding or gastrostomy for swallowing difficulties, preventing reflux, and dietary adjustments to ensure adequate nutrition.

• Physical therapy includes spinal braces to delay scoliosis caused by muscle weakness and mobility aids to improve movement.

Emerging gene-modifying therapies like antisense oligonucleotide Nusinersen show promise, but further large-scale studies are needed to confirm efficacy and safety.

DIET & LIFESTYLE

What should patients with spinal muscular atrophy pay attention to in their diet?

Patients with spinal muscular atrophy should maintain a healthy and regular diet, ensuring adequate intake of protein and vitamins.

What should patients with spinal muscular atrophy pay attention to in daily life?

The disease primarily affects mobility. When impairment is mild, patients should stay active with assistive devices while avoiding falls. In severe cases, patients may become bedridden, requiring caregivers to provide close monitoring and prevent complications such as pressure sores and venous thrombosis. Additionally, the disease may involve respiratory muscles, so patients should avoid triggers like colds or infections that could worsen the condition.

Does spinal muscular atrophy require follow-up examinations? How often?

Yes, clinical evaluations should be conducted at least every 6 months, with shorter intervals for patients experiencing severe muscle weakness.

PREVENTION

Can spinal muscular atrophy be prevented?

This disease is hereditary. If there are patients with this disease in the family, prenatal diagnosis and genetic counseling during pregnancy can help promote eugenics.

How can patients with spinal muscular atrophy prevent complications?

The disease may affect the respiratory muscles. Patients should try to avoid triggers such as colds and infections that may worsen the condition, maintain a regular lifestyle, and ensure proper nutrition.