Comprehensive Study Notes on Spinal Muscular Atrophy (SMA)

Overview and Pathophysiology of Spinal Muscular Atrophy (SMA)

• Definition: Spinal Muscular Atrophy (SMA) is a recessive degenerative motor neuron disease.

• Primary Genetic Cause: The condition is caused by a deletion or mutation in the survival motor neuron 1 ($SMN1$) gene.

• Mechanism of Disease:

  • Children with SMA lack the necessary $SMN1$ protein.

  • This deficiency leads to the eventual degeneration of the anterior horn cells.

  • The result is subsequent progressive muscle weakness and atrophy.

• The Role of the $SMN2$ Copy Gene:

  • Recent research indicates that human bodies also create an $SMN2$ copy gene.

  • The number of $SMN2$ copies a child possesses may decrease the overall disease severity.

• Genetic Prevalence: SMA is an autosomal recessive disease and is classified as the second most common group of fatal recessive diseases following cystic fibrosis.

Clinical Presentation and Physical Symptoms

• Symmetry: Children typically present with symmetrical weakness of the skeletal muscles.

• Distribution of Weakness:

  • Weakness is most profound in the proximal musculature, specifically the neck, trunk, pelvis, and shoulder girdles.

  • Significant weakness is commonly observed in the triceps, deltoids, iliopsoas, and quadriceps.

  • There is typical preservation of muscles in the hands, biceps, and hamstrings.

• Neurological Findings:

  • Hyporeflexia or Areflexia: Common across all types of SMA. The absence of deep tendon reflexes is attributed specifically to the loss of alpha motor neurons.

  • Sensation: These children have normal sensation because the disease affects only the motor neurons.

  • Intellect: Intellectual development is typically normal.

  • Hypotonia: Presents as a lack of resistance to passive motion across the limbs.

• Involuntary Movements:

  • Fasciculations: Brief muscle contractions that may not be visible to the naked eye; these are particularly common in the tongue.

  • Tremors: Common in the hands.

  • Management Indicator: Both fasciculations and tremors often increase when a child is fatigued, serving as a clinical signal to decrease treatment intensity or load.

• Associated Complications: Presentations often include feeding difficulties, impaired breathing, and secondary conditions such as contractures and scoliosis.

Diagnosis and Classification Systems

• Suspected Diagnosis: Identification is based on clinical examinations and laboratory procedures.

• Laboratory Procedures:

  • Electromyography ($EMG$).

  • Muscle ultrasound.

  • Muscle biopsy.

• Confirmatory Testing: Final diagnosis is confirmed via genetic testing.

• Functional Classification: Traditionally, SMA was classified based on the highest level of functional ability achieved:

  • Non-sitters: Traditionally SMA Type I.

  • Sitters: Traditionally SMA Type II.

  • Walkers: Traditionally SMA Type III.

Detailed Classification by Onset and Progression

• SMA Type I (Infantile Onset):

  • Diagnosis Window: Typically between $0$ to $4 \text{ months}$.

  • Prevalence: Accounts for approximately $60\%$ of children with SMA.

  • Progression: Rapidly progressive with severe weakness.

  • Historical Outlook: Death was common within a few months or years due to pneumonia, respiratory complications, or apnea.

  • Modern Outlook: Advances in therapy and early detection mean children are living longer without severe weakness and may achieve sitting or walking without support.

  • Functional Status: Traditionally inability to perform anti-gravity movements; the pelvis is usually more affected than the shoulder girdle. Head control is often failed or significantly impaired.

• SMA Type II (Childhood Onset):

  • Diagnosis Window: Diagnosed before $18 \text{ months}$.

  • Progression: Initial progression of slowly progressing weakness.

  • Functional Status: Traditionally classified as "sitters." They may have periods of stability in gross motor skills followed by decline.

  • Adulthood: Survival into adulthood is typical, depending on aggressive respiratory management and the management of secondary deformities.

  • Mobility: Can stand with support and practice ambulation with support and bracing.

• SMA Type III (Juvenile Onset):

  • Diagnosis Window: Between $1$ to $10 \text{ years old}$.

  • Progression: Slower progression with mild impairments; life expectancy is generally normal.

  • Functional Characteristics: Similar progression to Duchenne Muscular Dystrophy ($DMD$). Signs include difficulty arising from the floor (Gowers' maneuver), difficulty climbing stairs, and a waddling gait pattern.

• SMA Type IV (Adult Onset):

  • Diagnosis Window: After age $35$.

  • Progression: Mild weakness and a normal lifespan.

Sub-Types of SMA Type III

• SMA Type III A:

  • Diagnosis: Before the child is $2 \text{ years old}$.

  • Prognosis: $50\%$ of these children will retain the ability to walk past the age of $12$.

• SMA Type III B:

  • Diagnosis: After the age of $2 \text{ years old}$.

  • Prognosis: $50\%$ of these children will retain the ability to walk past their $44\text{th}$ birthday.

  • Clinical Significance: Identifying the specific subtype assists in long-term management and planning for adulthood, as prolonged walking decreases secondary conditions.

Physical Therapy (PT) Interventions

• Primary Roles of PT: Prevention of complications, preservation of function, and maximizing quality of life. Guidelines follow the International Classification of Functioning, Disability and Health ($ICF$) model.

• Evidence-Based Interventions:

  • Stretching and night splinting.

  • Concentric exercising.

  • Respiratory training.

  • Standing programs.

  • Power mobility and assistive technology.

• Type-Specific Management:

  • Type I: Focus on positioning to maintain flexibility using wedges or towel rolls; focus on head control and respiratory care.

  • Type II: Challenge sitting posture, encourage standing between $12$ and $18 \text{ months}$. Use of gait trainers and early fitting for a $KFL$ (Knee-Foot-Limb/Leg) brace or $KAFO$. Trunk support or corsets are used to manage or prevent scoliosis until the age of $10$ to $12 \text{ years old}$. Aquatics are beneficial to move without the effects of gravity.

  • Type III: Focus on strength and endurance training, keeping fatigue in mind. Use of adaptive equipment to prolong independent walking and transfer training.

• Dosage Recommendation: Recent research in the Pediatric Physical Therapy journal suggests that using a stander $3$ to $7 \text{ times per week}$ is feasible and tolerated for both Type I and Type II patients.

Medical Management and Emerging Research

• Evolution of Care: While there is no current cure, the disease is treatable, and the natural history is changing due to proactive treatments.

• Pharmacological and Genetic Advancements:

  • Gene Replacement Therapies: Aim to correct the faulty $SMN1$ gene.

  • Spinraza: A drug designed to modulate the low-functioning $SMN2$ gene.

  • Other Clinical Trials: Focus on neuroprotection of the motor neuron and protection of muscle function.

• Standards of Care: The revised standard of care now explicitly includes physical therapy and rehabilitation (stretching, strengthening, aerobics, and bracing).

Early Detection and Screening

• The Crucial Window: Early identification is critical to changing the disease course, particularly for Type I infants, to allow for the administration of new therapeutic approaches before significant degeneration occurs.

• Clinical Red Flags:

  • Fatiguing easily.

  • Low muscle tone (hypotonia).

  • Decreased or absent reflexes.

  • Delays in developmental milestones.

  • Poor anti-gravity movements.

  • "Frog-leg" posturing.

  • Tongue fasciculations and hand tremors.

• Newborn Screening in the US:

  • Currently, $31$ states screen for SMA in newborns.

  • This covers approximately $65\%$ of all newborn babies in the United States.

  • Early referral to neurology is essential upon suspicion or positive screening.