General
- Degenerative diseases of motor neurons.
- There are five subtypes of degenerative motor neuron diseases, of which ALS is the most common
Three patterns of involvement
- Mixed UMN & LMN degeneration
- Amyotrophic lateral sclerosis (ALS)
- UMN degeneration
- Primary lateral sclerosis
- Rare, onset after age 50.
- No LMN signs. Slower progression than ALS (yrs to decades).
- Pseudobulbar palsy is common.
- Usually does not shorten longevity.
- May present with falling due to balance problems or low back and neck pain due to axial muscle weakness
- LMN degeneration
- Progressive muscular atrophy (PMA)
- Spinal muscular atrophy (SMA)
- Congenital degeneration of anterior horns. Autosomal recessive SMN1 mutation (encodes survival motor neuron protein) → defective snRNP assembly → LMN apoptosis. Spinal muscular atrophy type 1 (most common) is also called Werdnig-Hoffmann disease.
- LMN signs only (symmetric weakness). “Floppy baby” with marked hypotonia (flaccid paralysis) and tongue fasciculations.
Spectrum of diseases
- Classical ALS: Affects both the upper and lower motor neuron globally
- Progressive muscular atrophy.
- Mainly lower motor neuron disorder
- Primary lateral sclerosis
- Mainly upper motor neuron disorder
- Progressive bulvar palsy
- Both upper and lower motor neuron deficit that is isolated in the cranial nerves
- Flail arm syndrome
- Progressive lower motor neuron Symptoms isolated at proximal arm
- More benign disease
- Flail legs syndrome
- Progressive lower motor neuron Symptoms isolated at distal leg
- More benign disease
- ALS plus syndrome
- Als is pure motor disease but if have concurrent following diseases it is called ALS plus
- Frontotemporal dementia (FTD),
- Autonomic insufficiency,
- Parkinsonism,
- Supranuclear gaze paresis,
- Sensory loss.
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