Chapter 13 - Disorders of the motor
neurons, nerve roots and peripheral
nerves
Classification of nerve disorders
Nerve disorders can be subclassified by location, by their electrophysiological or
microscopic properties, by the nature of the symptoms and by cause. When epicritic
sensory information is missing, uncontrolled dyskinesia can develop, restless legs. If
the loss of epicritic sensation is severe, athetotic movements occur, as the patient
cannot feel precisely where in space his/her limbs are and is thus unable to correct
their position voluntarily. Neuropathic symptomas are caused by deafferentiation.
Autonomic symptoms:
Sweat secretion: intensified when the skin is cold (alcoholic neuropathy) or
reduced when the skin is above normal temperature (diabetic neuropathy)
Orthostatic hypotension: alcoholic or diabetic polyneuropathy, amyloidosis,
Guillain-Barré syndrome
Skin discouloration
Visceral autonomic disorders: impaired gastrointestinal motility, impaired
sexual function, bladder dysfunction and cardiac arrhythmias
Trophic disorders: ulcers, change in hair growth, skin abnormalities, bone
abnormalities, joint abnormalities (Charcot)
Cell body in anterior horn Motor neuronopathy
Cell body in dorsal Sensory neuronopathy
ganglion
Nerve root Radiculopathy
Nerve plexus Plexopathy/plexus neuritis
One peripheral nerve Mononeuropathy
Multiple peripheral nerves Multiple mononeuropathy/mononeuritis
multiplex
Many or all nerves (mainly Polyneuropathy
distal
Sensory axon Sensory neuropathy
Motor axon Motor neuropathy
Terminal branches of the Small-fiber neuropathy
axon
Diseases of the nerve cell body: neuronopathy
Anterior horn cell disorders (motor neuron disease) often have a very severe
progression. Hereditary types of spinal muscular atrophy are caused by aberrations
, of chromosome 5. Werdnig-Hoffmann disease (spinal muscular atrophy type 1,
infantile form) probably starts in the uterus (reduced baby movements) and
manifests itself before the 4th month of life in general muscle weakness and little
spontaneous motor function in the limbs. General paralysis finally develops, with
swallowing and respiratory disorders that cause the child to die before the end of
the second year of life.
Spinal muscular atrophy type 2 is an intermediate type that starts somewhat later
in life and has milder progression than the infantile form. The child does not reach
the walking age and develops tetraplegia with contractures and severe
kyphoscoliosis.
Kugelberg-Welander disease (type 3) is another recessive hereditary disorder. A
striking symptom is a finger tremor. Most patients with a later onset will be
wheelchair-bound a proportion of late-onset patients remain mobile for a long time.
The adult form is spinal muscular atrophy (type 4) starts after the 30th year of life.
The prognoses vary greatly, from rapidly progressive to very slowly progressive,
sometimes with weakness confined to one limb. This is referred to as 'progressive
spinal muscular atrophy'.
Other hereditary forms of spinal muscular atrophy are distal SMA, which starts at
the feet (claw foot) or hands, scapuloperoneal spinal muscular atrophy and
spinobulbar muscular atrophy (Kennedy's disease)
Amyotrophic lateral sclerosis involves degeneration of the peripheral motor neurons
in the anterior horns of the spinal cord and the medulla oblongata. The
degeneration also affects the central motor neurons (pyramidal tract) that have
their origin in the motor cortex. Muscular atrophy (amyotrophy) develops, along
with degeneration of the mainly lateral pyramidal tract (lateral sclerosis). The first
symptom is usually loss of strength. Pyramidal tract signs often only develop
secondarily, resulting in very brisk stretch reflexes of the atrophic muscles, usually
without any hypertonia. The initial symptoms generally affect one of the following
regions:
Small muscles of one hand (fine finger movement/precision grasp)
Extensors of one foot (foot drop)
Tongue and throat muscles (articulation, nasal speech and dysphagia.
Extensive neurogenic disorders are found in ALS, whereas nerve conduction velocity
remains intact. 5-10% have a familial form of ALS, caused by a mutation on
chromosome 21, superoxidase dismutase 1 gene. There is no causal therapy, but
riluzone, a glutamate inhibitor, has been shown to increase life expectancy.
Percutaneous endoscopic gastrostomy (PEG) prevents malnutrition due to
dysphagia.
If the clinical symptoms are confined to the peripheral motor neurons, the condition
is referred to as 'progressive spinal muscular atrophy'. There is also monomelic
amyotrophy (mainly found in young men), where the muscle weakness and atrophy
remain limited to one arm or leg.
Disorders of the nerve root: radiculopathy
Radiculopathy is characterized by segmental motor loss. Pain can sometimes by
aggravated by increased pressure. The main causes of monoradiculopathy are
compression by a lumbar or cervical hernia with a prolapsed disc, a trauma that has
torn of a cervical root, or an infection. Monoradiculitis can be caused by
neuroborreliosis or herpes zoster.
