14A. Motor neuron diseases
Motor neuron diseases are diseases which selectively affect motor neurons in the spinal cord and motor cortex. They’re progressive diseases which lead to severe disability and death.
The most important types are:
- Amyotrophic lateral sclerosis (ALS)
- Hereditary spastic paraplegia
- Spinal muscular atrophy (SMA)
ALS occurs sporadically, while spinal muscular atrophy and hereditary spastic paraplegia are genetic disorders.
Clinical features
In ALS both upper and motor neuron lesions are present simultaneously. The patient initially has weakness in the hands, feet, or bulbar muscles (causing dysarthria or dysphagia), but later progresses to having symptoms in all muscles. They can develop “drop head” due to weak neck muscles. LMN lesion manifests as muscle atrophy and fasciculations, while UMN lesion manifests as hyperreflexia, positive Babinski, and spasticity. Atrophy of the tongue and hand muscles is typical.
In hereditary spastic paraplegia only upper motor neuron lesion is present. Only the lower limbs are affected, causing spastic gait and weakness.
In spinal muscular atrophy only peripheral motor neuron lesion is present. The symptoms depend on the age of onset, which can be anything from early infancy to young adults. All muscles can be affected.
Diagnosis and evaluation
Physical examination to detect lower and upper motor neuron lesions is important. Lower motor neuron lesions can be identified by EMG, while upper motor neuron lesions can be identified by MRI or motor evoked potentials (MEP). Genetic testing confirms hereditary spastic paraplegia and SMA.
It’s important to exclude other neurological disorders. In motor neuron disorders there are no sensory, autonomic or visual problems.
Treatment
The treatment is mostly supportive. The patient eventually becomes unable to swallow and breathe, which require enteral feeding and ventilation. Riluzole, a glutamate inhibitor, prolongs survival in ALS by a few months.
Prognosis
The prognosis of ALS is very poor. Most survive only a few years. The prognosis of hereditary spastic paraplaegia is good, and the prognosis of SMA depends on the age of onset (the earlier onset the worse prognosis).