Introduction
- Definition: Spinal Muscular Atrophy
(SMA) is a genetic neuromuscular disorder caused by mutations in the survival
motor neuron gene 1 (SMN1 gene), leading to deficiency of survival
motor neuron protein.
- Significance: Spinal muscle atrophy
(SMA) is characterized by muscle weakness and atrophy resulting from the
degeneration and loss of motor neurons in the anterior horn of the spinal
cord. The clinical manifestations of this disease vary considerably; onset
can begin before birth (in utero), manifesting as decreased fetal
movement, or it can occur in adulthood.
Basic Knowledge & Epidemiology
- Pathogenesis:
- SMN1 mutation → reduced SMN
protein.
- SMN protein essential for snRNP assembly and mRNA processing.
- Epidemiology:
- Incidence: 1/10,000–1/6,000.
- Carrier frequency: 1/40–1/50.
Clinical Classification
- Type I (Infantile, Werdnig‑Hoffman)
- Accounts for approximately 45%.
- Onset time: < 6 months after birth.
- Severe hypotonia, feeding/swallowing difficulty, paradoxical
breathing.
- 80% die before age 1, most children with this condition die of
respiratory failure before the age of 2.
- Type II (Intermediate, Dubowitz)
- Accounts for approximately 30% to 40%.
- Onset time: 6-18 months after birth.
- Can sit but not stand/walk.
- Progressive weakness, scoliosis, respiratory issues.
- Survive to 10-20 years old. The intelligence is normal.
- Type III (Juvenile, Kugelberg‑Welander)
- Accounts for approximately 20%.
- Onset time: > 18 months after birth.
- Initially able to walk, later wheelchair dependent.
- The prognosis is relatively good, and patients can walk for
many years, although spinal deformities may occur later in life.
- Life expectancy is not shortened or only slightly reduced.
- Type IV (Adult‑onset)
- Onset at age 30-60.
- Slow progression, normal life expectancy.
Diagnosis
- Clinical Features:
- Symmetric proximal weakness, hypotonia, absent reflexes.
- Cognition preserved.
- Genetic Testing:
- MLPA, qPCR, RFLP, DHPLC and other methods were used to detect
homozygous deletion mutations in exons 7 or 7 and 8 of the SMN1 gene.
- SMN2 copy number informs prognosis.
- Auxiliary Tests:
- CK: mild elevation.
- EMG: neurogenic changes.
- Muscle biopsy: fiber atrophy.
Treatment & Management
- Multidisciplinary Care:
- Neurology, rehabilitation, nutrition, pulmonology.
- Regular assessments every 6 months.
- Rehabilitation:
- Stretching, orthoses, standing frames.
- Swimming, cycling, low‑intensity exercise.
- Swallowing Therapy:
- Oral motor training, tongue/lip exercises.
- Respiratory Management:
- Cough assist, BiPAP, tracheostomy in severe cases.
- Orthopedic Care:
- Monitor scoliosis; brace >20°, surgery >50°.
- Nutrition:
- Gastrostomy feeding if dysphagia.
- Calcium, vitamin D supplementation.
- Bone Health:
- Monitor bone density; bisphosphonates.
- Emergency Care:
- Family training for airway clearance.
- Immunization:
- Influenza, pneumococcal, RSV vaccines.
- Psychological Support & Transition:
- Counseling, pediatric → adult care.
- Genetic Counseling:
- Recurrence risk 25%.
- Prenatal diagnosis recommended.
Pharmacological Advances
- Nusinersen (Antisense Oligonucleotide Therapy)
- Alters SMN2 splicing to increase functional SMN protein.
- Intrathecal injection every 4 months.
- Improves survival and motor milestones.
·
Gene Replacement Therapy
(AVXS‑101, Onasemnogene Abeparvovec)
- AAV vector delivers functional SMN1 gene.
- One‑time intravenous infusion.
- Clinical trials show significant motor improvement.
·
Small Molecule Therapies
(e.g., Risdiplam, SMN‑C3)
- Oral administration.
- Promote exon 7 inclusion in SMN2 transcripts.
- Phase II/III trials demonstrate efficacy.
·
Supportive Medications
- Salbutamol (Ξ²‑agonist).
- Vitamin D, calcium, bisphosphonates.
- Symptomatic relief and bone protection.
Conclusion
- SMA treatment requires integrated supportive care and novel pharmacological therapies.
- Early diagnosis and intervention are critical.
- Advances in gene therapy and splicing modifiers are transforming prognosis.
