Spinal Muscular Atrophy (SMA)

Spinal Muscular Atrophy (SMA): is a rare genetic neuromuscular disorder characterized by progressive degeneration of motor neurons in the spinal cord and brainstem. These motor neurons are responsible for transmitting signals from the brain to skeletal muscles. Their loss leads to muscle weakness, atrophy, and functional decline.

SMA primarily affects infants and children, though milder forms may present in adolescence or adulthood. Early diagnosis and advances in disease-modifying therapies have significantly improved survival and functional outcomes.

Pathophysiology and Genetic Basis:

SMA is caused by mutations or deletions in the Survival Motor Neuron 1 (SMN1) gene, which is essential for motor neuron survival.

Most individuals with SMA lack a functional SMN1 gene.

A second gene, SMN2, produces limited amounts of SMN protein. The number of SMN2 gene copies strongly influences disease severity—higher copy numbers are generally associated with milder phenotypes.

SMA is inherited in an autosomal recessive pattern. Parents are typically asymptomatic carriers.

Clinical Features and Symptoms

Common clinical manifestations include:

• Progressive muscle weakness, predominantly affecting proximal muscles (shoulders, hips, trunk)

• Hypotonia (“floppiness”), especially in infants

• Delayed motor milestones (head control, sitting, standing, walking)

• Postural instability and balance difficulties

• Respiratory muscle weakness leading to breathing difficulties

• Feeding and swallowing impairments in more severe forms

Symptom onset and severity vary widely depending on SMA type and genetic factors.

Classification of SMA:

SMA is clinically classified based on age of onset and highest motor function achieved:

• Type 0: Prenatal onset, severe weakness and respiratory failure

• Type 1 (Werdnig–Hoffmann disease): Onset <6 months, inability to sit independently

• Type 2: Onset 6–18 months, ability to sit but not walk

• Type 3 (Kugelberg–Welander disease): Onset after 18 months, independent walking initially

• Type 4: Adult onset, mild and slowly progressive weakness

Diagnosis and Early Detection

Diagnosis is confirmed through genetic testing, identifying SMN1 gene deletions or mutations.

Many countries now include SMA in newborn screening programs, enabling presymptomatic diagnosis and earlier intervention.

Clinical assessment focuses on motor strength, reflexes, respiratory status, and motor development. Prenatal genetic testing may be offered to families with known carrier status.

Early diagnosis is strongly associated with improved motor outcomes and survival.

Medical Treatment and Advances

Disease-Modifying Therapies (DMTs)

Modern SMA management includes therapies targeting the underlying genetic defect:

• Nusinersen (Spinraza): Intrathecal therapy that enhances SMN protein production

• Onasemnogene abeparvovec (Zolgensma): One-time gene replacement therapy delivering a functional SMN1 gene

• Emerging therapies targeting SMN2 splicing and muscle preservation are under ongoing investigation

Early initiation of treatment yields the greatest functional benefit.

Supportive and Multidisciplinary Care

Despite pharmacological advances, comprehensive supportive care remains essential:

• Respiratory support (airway clearance, non-invasive ventilation)

• Nutritional and swallowing management

• Orthopedic monitoring (scoliosis, joint contractures)

• Regular functional and respiratory follow-up

• Coordinated multidisciplinary care involving neurology, rehabilitation, pulmonology, and nutrition

Role of Physical Therapy in SMA

Physical therapy is a cornerstone of long-term SMA management and focuses on:

• Preserving joint range of motion and muscle flexibility

• Preventing contractures and deformities

• Enhancing postural control and functional mobility

• Supporting respiratory function through breathing and airway clearance techniques

• Promoting safe participation in daily activities

Family education, home programs, and long-term follow-up are critical to maximizing independence and quality of life.

References :

Battista, V., et al. (2022). Standards of care in spinal muscular atrophy. Journal of Neuromuscular Diseases, 9(2), 151–167.

Finkel, R. S., Mercuri, E., Meyer, O. H., et al. (2018). Diagnosis and management of spinal muscular atrophy: Part 2. Neuromuscular Disorders, 28(3), 197–207.

Mercuri, E., Darras, B. T., Chiriboga, C. A., et al. (2018). Nusinersen versus sham control in later-onset SMA. The New England Journal of Medicine, 378(7), 625–635.

Prior, T. W. (2020). Spinal muscular atrophy: A time for screening. Current Opinion in Pediatrics, 32(6), 723–728.

StatPearls. (2024). Spinal muscular atrophy. National Center for Biotechnology Information (NCBI).