Spinal muscular atrophy in detail

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Here is a comprehensive overview of Spinal Muscular Atrophy (SMA).

Spinal Muscular Atrophy (SMA)

Definition

SMA is a family of hereditary lower motor neuron diseases characterized by progressive degeneration of anterior horn cells in the spinal cord (and sometimes brainstem motor nuclei), leading to muscle weakness and atrophy. It is the leading genetic cause of infant death and one of the most common autosomal recessive disorders.

Genetics & Molecular Pathophysiology

Gene	Chromosome	Role
SMN1 (Survival Motor Neuron 1)	5q13.2	Produces full-length, functional SMN protein
SMN2	5q13.2 (inverted duplication)	Produces ~10–15% functional SMN protein; remainder is truncated (exon 7 skipped)

~95% of SMA cases: Homozygous deletion or mutation of SMN1 (loss of function)
SMN2 copy number is the primary disease modifier — more copies = milder phenotype
SMN protein is essential for the formation and trafficking of RNA complexes (snRNPs) across the nuclear membrane, critical for pre-mRNA splicing in all cells, but motor neurons are uniquely vulnerable (Harrison's, p. 12410)
Deficiency leads to selective degeneration of large motor neurons → denervation atrophy of skeletal muscle

Importantly, SMN2 cannot fully compensate for SMN1 loss because of a single nucleotide difference (C→T at position +6 in exon 7) that causes exon 7 skipping during splicing, yielding a truncated, unstable protein (SMNΔ7).

Classification by Type

Type	Onset	Motor Milestones	SMN2 Copies	Natural Course
Type 0 (prenatal)	In utero	No movement at birth	1	Fatal within weeks
Type 1 (Werdnig-Hoffmann)	< 6 months	Never sit independently	2	Death by 2 yrs (if untreated)
Type 2 (intermediate)	6–18 months	Sit but never stand	3	Survives to adulthood; wheelchair-bound
Type 3 (Kugelberg-Welander)	> 18 months	Walk independently	3–4	Near-normal lifespan; may lose ambulation
Type 4 (adult-onset)	> 21 years	Walk throughout adult life	4+	Near-normal lifespan

Clinical Features

Type 1 (Most Common, Most Severe)

Profound hypotonia ("floppy infant")
Symmetric proximal > distal weakness
Tongue fasciculations
Areflexia
Paradoxical breathing (diaphragm relatively spared → bell-shaped chest)
Bulbar weakness → poor suck, swallowing difficulties
Preserved sensation and cognition
Alert, bright facial expression (classic finding)

Type 2

Achieve sitting, never standing/walking
Scoliosis (universal), joint contractures
Respiratory compromise (restrictive lung disease)
Tremor of outstretched hands (minipolymyoclonus)

Type 3

Walk independently but fall frequently
Proximal limb weakness (Gowers' sign)
Waddling gait
May lose ambulation during 2nd–3rd decade
CK mildly elevated

Type 4 (Adult-Onset)

Slowly progressive proximal weakness
Fasciculations
Minimal respiratory involvement

Diagnosis

1. Genetic Testing (First-Line)

MLPA or real-time PCR for homozygous deletion of exon 7 of SMN1 — confirms diagnosis in ~95% of cases
SMN2 copy number should be determined simultaneously (prognostic)
Full SMN1 sequencing for compound heterozygotes

2. Supportive Investigations

Test	Finding
EMG/NCS	Fibrillations, positive sharp waves, large polyphasic motor unit potentials; NCS normal
Muscle biopsy	Grouped denervation atrophy (large groups of atrophic fibers), hypertrophic fibers; rarely needed now
CK	Normal to mildly elevated
Newborn screening	Now included in many national programs (dried blood spot PCR)

Differential Diagnosis

Duchenne/Becker muscular dystrophy
Congenital myopathies (nemaline, centronuclear)
Myotonic dystrophy (congenital)
Pompe disease
Congenital myasthenic syndromes
Botulism (infantile)
X-linked spinobulbar muscular atrophy (Kennedy disease) — adults

Treatment

Disease-Modifying Therapies (DMTs)

Drug	Mechanism	Route	Approval
Nusinersen (Spinraza)	Antisense oligonucleotide (ASO) — modifies SMN2 splicing to include exon 7, increasing functional SMN protein	Intrathecal (every 4 months after loading)	FDA 2016
Onasemnogene abeparvovec (Zolgensma)	AAV9 gene therapy — delivers functional SMN1 copy	Single IV infusion	FDA 2019 (< 2 yrs)
Risdiplam (Evrysdi)	Small molecule SMN2 splicing modifier	Oral daily	FDA 2020

Key notes (Harrison's, p. 13374):

Zolgensma is a true disease-modifying gene therapy; requires pre-treatment and 30-day post-treatment corticosteroids to prevent hepatotoxicity (liver transaminase monitoring essential)
Nusinersen efficacy data from trials (ENDEAR, CHERISH) showed significant motor milestone gains, especially when started early; requires repeat intrathecal dosing every 4 months
Risdiplam offers the advantage of oral administration, with efficacy comparable to nusinersen in several types
Pre-symptomatic treatment (via newborn screening) yields dramatically superior outcomes vs. symptomatic treatment

SMA ASO motor neuron rescue — immunofluorescence cross-sections of spinal cord showing ChAT+ motor neuron preservation with ASO treatment vs. untreated SMA

Spinal cord cross-sections (L1–L2, postnatal day 9) comparing ChAT-stained motor neurons in SMA mice: untreated mice show marked ventral horn motor neuron loss (top/bottom left panels), while ASO-treated mice show dose-dependent rescue of motor neurons.

Supportive & Multidisciplinary Care

Domain	Intervention
Respiratory	Non-invasive ventilation (BiPAP), cough assist, airway clearance; tracheostomy in severe cases
Nutritional	Gastrostomy feeding (Type 1/2), anti-reflux measures
Orthopedic	Scoliosis surveillance and surgery, joint contracture management, seating/positioning
Rehabilitation	Physical therapy, occupational therapy, speech-language pathology
Neurology	Regular monitoring of motor function (HFMSE, RULM, MFM scales)
Genetics	Carrier testing of family members; genetic counseling

Prognosis

Untreated Type 1: median survival < 2 years; death from respiratory failure
Treated (pre-symptomatic) Type 1: majority achieve sitting and many achieve walking milestones — a transformative change from the natural history
Types 2–3: lifespan near-normal with good respiratory management; quality of life depends on functional preservation
Type 4: slow progression, near-normal lifespan

Newborn Screening

SMA is now included in the Recommended Uniform Screening Panel (RUSP) in the US and many other countries. Early identification before symptom onset allows treatment initiation when the largest number of motor neurons remain viable — this is the single most important prognostic factor.

Key Takeaways

SMA is caused by SMN1 loss → deficient SMN protein → motor neuron death
SMN2 copy number is the main disease severity modifier
Four major types (0–4) defined by onset age and motor milestones
Three approved DMTs: nusinersen (intrathecal ASO), onasemnogene (gene therapy), risdiplam (oral)
Pre-symptomatic treatment via newborn screening produces far better outcomes than treating symptomatic patients
Multidisciplinary care (respiratory, nutritional, orthopedic) remains essential alongside DMTs

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