SMA type and clinical feature
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Reading File
I now have comprehensive information. Here is a complete overview of SMA types and their clinical features.
Spinal Muscular Atrophy (SMA) — Types & Clinical Features
Background
SMA is caused by degeneration of anterior horn cells (lower motor neurons), predominantly due to homozygous deletion/mutation of the SMN1 gene on chromosome 5q11.2–13.3. The SMN2 gene copy number modifies severity: more copies → milder disease.
- Incidence: 1 in 6,000–10,000 live births
- Carrier frequency: ~1 in 35 (general population)
- Leading genetic cause of infant mortality
SMA Classification by Type
| Type | Eponym / Name | Age at Onset | Max Function Achieved | Survival/Prognosis | Inheritance |
|---|---|---|---|---|---|
| 0 | Prenatal SMA | Prenatal | Needs respiratory support at birth | Fatal at birth without respirator | AR (SMN gene) |
| 1 | Werdnig-Hoffmann disease (Infantile SMA) | Birth – 6 months | Sits only with support | Death by age 2 (without treatment) | AR (SMN gene) |
| 2 | Intermediate SMA | Before 18 months | Sits independently but never walks | Survives to adulthood, wheelchair-bound | AR (SMN gene) |
| 3 | Kugelberg-Welander disease (Juvenile SMA) | After 18 months | Walks independently (~25 steps) | Normal lifespan, slow progression | AR (SMN gene) |
| 4 | Pseudomyopathic / Adult-onset SMA | >5 years (most >30 years) | Walks normally | Slow progression | AR, AD, or sporadic (SMN, VAPB, dynactin) |
Detailed Clinical Features by Type
Type 0 (Prenatal)
- Reduced fetal movements in utero
- Respiratory distress at birth — requires immediate ventilator support
- Severe hypotonia; fatal without respiratory support
- Fewest SMN2 copies (often 1)
Type 1 — Werdnig-Hoffmann Disease
- Onset: birth to 6 months
- Severe hypotonia ("floppy baby")
- Characteristic "frog-leg" posture at rest — thighs externally rotated/abducted, knees flexed
- Weak cry, poor suck, feeding difficulties
- Respiratory distress (paradoxical breathing, bell-shaped chest)
- Cannot lift head when prone; marked head lag when pulled to sit
- Limb weakness: severe, generalized, proximal > distal
- Tongue fasciculations (a key sign of LMN involvement)
- Areflexia universally present
- Sensation and sphincter control: normal
- Eye movements: spared (bulbar nuclei and oculomotor nuclei relatively spared)
- Death typically by age 2 without treatment (respiratory failure)
- Only 2 copies of SMN2 → ~9% functional SMN protein
Type 2 — Intermediate SMA
- Onset: before 18 months (typically 6–18 months)
- Child achieves independent sitting but never walks
- Progressive proximal limb weakness — legs > arms
- Postural tremor of outstretched hands (minipolymyoclonus — characteristic)
- Fasciculations of tongue and limbs
- Scoliosis develops (major complication) due to paraspinal muscle weakness
- Respiratory involvement progresses — intercostal weakness, nocturnal hypoventilation
- Contractures and joint deformities over time
- Areflexia or hyporeflexia
- Bulbar involvement: mild; swallowing generally preserved initially
- Survives to adulthood but progressive deterioration
Type 3 — Kugelberg-Welander Disease
- Onset: after 18 months (up to early adulthood)
- Child achieves independent walking
- Proximal weakness: difficulty climbing stairs, rising from floor, Gowers' maneuver positive
- Limb-girdle pattern of weakness (pelvic girdle > shoulder girdle)
- Fasciculations visible in tongue and muscles
- Serum CK elevated up to 10× normal (unlike types 1 and 2 where CK is normal)
- Areflexia/hyporeflexia
- Mild scoliosis may develop
- Slow progression — most remain ambulatory for decades; normal lifespan
Type 4 — Adult-Onset SMA (Pseudomyopathic SMA)
- Onset: typically third decade (range 20–late 40s); accounts for <10% of all SMA
- Slowly progressive proximal limb-girdle weakness: difficulty walking, climbing stairs, rising from chair
- Fasciculations present in ~75% of patients
- Quadriceps weakness often prominent
- Muscle cramps occur (not prominent)
- Bulbar signs, scoliosis, and respiratory weakness are rare
- Pattern mimics limb-girdle muscular dystrophy → hence "pseudomyopathic"
- Dominant form (Finkel-type SMA): caused by VAPB mutations; some patients develop ALS phenotype instead
- Other genes implicated: DYNCH1H1, BICD2, TRPV4, CHCHD10
Key Shared Features Across All Types
| Feature | Detail |
|---|---|
| Motor neuron | LMN (anterior horn) — no UMN signs |
| Reflexes | Reduced or absent |
| Sensation | Normal (pure motor disease) |
| Cognition | Normal |
| Ocular muscles | Spared |
| Sphincters | Spared |
| Tongue fasciculations | Common (especially types 1 and 2) |
| Genetics | SMN1 deletion/mutation (chromosome 5q) in 95–98% of cases |
SMN2 Copy Number & Phenotype Correlation
| SMN2 Copies | Typical SMA Type |
|---|---|
| 1 | Type 0 (most severe) |
| 2 | Type 1 |
| 3 | Type 2 |
| 4–5 | Type 3–4 (mildest) |
Treatments (Modern)
- Onasemnogene abeparvovec (Zolgensma): AAV9 gene therapy — single IV infusion; FDA-approved for type 1, age <2
- Nusinersen (Spinraza): antisense oligonucleotide targeting SMN2 splicing; intrathecal, every ~4 months
- Risdiplam (Evrysdi): oral small-molecule SMN2 splicing modifier; approved for types 1, 2, and 3
Sources: Bradley and Daroff's Neurology in Clinical Practice; Braunwald's Heart Disease — Textbook of Cardiovascular Medicine
This is a shared conversation. Sign in to Orris to start your own chat.