In one sentence Neuromuscular disorders begin when the neurons that command voluntary muscle become unhealthy or die, so the message never reaches the muscle and it weakens and wastes away.
A nerve signal is a chain, and any broken link weakens the muscle
A nerve signal is a relay, not a single event. A motor neuron sits in the spinal cord or the brainstem, and its long fibre, the axon, runs out to the muscle. At the end of the axon the signal crosses a microscopic relay, the neuromuscular junction, and makes the muscle fibre contract. Sensory neurons run the other way, carrying touch, pain and temperature back to the brain. Because the chain has several links, a disorder can begin at any of them — the neuron, its axon, the relay or the muscle itself — but the visible result is the same: the muscle receives no reliable command. It first becomes weak, and then, having lost its supply or its use, it shrinks, or atrophies. This is a breakdown of communication between the nervous system and the muscles, and it is the single mechanism behind every disorder in this unit. Note that the muscle is often the innocent last station: the disease may lie far away from it, in the cord or in the nerve.
A patient reports tripping on kerbs and dropping cups, and the nurse notices that the muscles between the thumb and index finger look hollow. Those muscles have not been injured; they have lost the nerve signals that kept them alive and strong.
Why does a muscle waste away when the nerve that supplies it dies?
Show the answer
Because the neuron carries the electrical command that keeps the muscle working; without that signal the muscle cannot contract normally and it shrinks.
Read detailed explanations
- Localising the lesion: four levels and the signs that separate them
Localising the lesion is the step that turns a symptom into a diagnosis. Start above the muscle. A motor unit is one motor neuron plus every muscle fibre its axon branches to supply, and the number of fibres per unit varies with the precision the movement needs: a few for an eye muscle, several hundred for a thigh muscle. Damage to the neuron itself, an anterior horn cell lesion as in amyotrophic lateral sclerosis or spinal muscular atrophy, produces lower motor neuron signs: wasting, involuntary muscle twitching called fasciculation, reduced tone, and diminished or absent reflexes, with sensation preserved.
Damage to the peripheral nerve produces much the same picture in the territory of that nerve but often adds sensory loss and pain; if many nerves are affected in a glove-and-stocking pattern the cause is a polyneuropathy, while one nerve alone suggests a mononeuropathy from compression or injury. A root lesion at the spine, a radiculopathy, follows a dermatomal strip and is usually painful. Damage above the spinal cord, the upper motor neuron, gives the opposite signs: increased tone, exaggerated reflexes, stiff and slower movement, and an extensor plantar response, without early wasting. A junction disorder gives fatigable weakness with normal tone, normal reflexes and normal sensation. A muscle disorder gives weakness that is usually proximal, with normal sensation and normal or only mildly reduced reflexes.
Electromyography separates them further: in a nerve lesion the muscle at rest shows spontaneous fibrillation potentials and the number of motor units firing is reduced, whereas a myopathic muscle shows many small, short, polyphasic units. Nerve conduction studies add the conduction velocity, normally about 50 to 60 metres per second in the arm, which falls sharply when myelin is destroyed. Because amyotrophic lateral sclerosis damages both the anterior horn cell and the upper motor neuron, the same patient can show wasting and fasciculation in the hands together with brisk reflexes and spasticity in the legs, a combination no single-site disease produces.
- What each test proves, and what a normal result does not exclude
Creatine kinase is an enzyme released when muscle fibres are damaged, so it is high in muscular dystrophy and in myositis and usually normal in a disorder of the nerve or of the junction; however it also rises after exercise, after an intramuscular injection and in some healthy people, so it indicates a direction rather than a diagnosis and must be read with the clinical picture. Electromyography uses a fine needle electrode to listen to the muscle at rest and during contraction. Spontaneous activity at rest, the fibrillation potentials and positive sharp waves, signals denervated or inflamed fibres, and the shape and size of the motor unit action potentials separate myopathic from neuropathic disease.
Nerve conduction studies stimulate the nerve at one point and record the response at another; the latency and the conduction velocity distinguish a demyelinating process, in which speed collapses, from an axonal one, in which amplitude falls while speed is preserved. In suspected myasthenia gravis, repetitive nerve stimulation can show a decrement in the response, and single-fibre studies can demonstrate the variable transmission directly. Magnetic resonance imaging shows structure, and gadolinium contrast shows active inflammation, but it cannot see individual nerve fibres. A lumbar puncture examines cerebrospinal fluid for cells, protein and markers such as oligoclonal bands, and it carries a risk of post-procedure headache, so it is used where central disease is suspected.
Muscle biopsy remains the tissue proof: frozen sections with immunohistochemistry reveal missing structural proteins, inflammatory infiltrates or ragged fibres. Genetic testing has moved from single-gene tests to panels and exome sequencing, and because many of these conditions are inherited in an autosomal dominant, autosomal recessive or X-linked pattern, a positive result carries implications for parents, siblings and children, so genetic counselling is part of the test, not an afterthought. Finally, sensitivity matters: a normal test early in a disease does not exclude it, and the clinical examination remains the reference standard.
- The complications that decide the outcome: swallowing, breathing and the immobile patient
Bulbar muscles are the muscles of the mouth, throat and tongue, and when they weaken the patient develops dysarthria, which is slurred speech, and dysphagia, which is difficulty swallowing. Dysphagia is dangerous because food, drink or saliva can enter the airway; the result is aspiration, and aspiration leads to pneumonia, a leading cause of death in these disorders. The nurse watches for coughing or choking during meals, a wet or gurgling voice afterwards, a rising temperature and new breathlessness, keeps the patient upright during and after eating, offers thickened fluids and small mouthfuls, and involves a speech and language therapist and a dietitian. A gastrostomy tube may be needed when swallowing becomes unsafe or weight falls. Breathing fails more quietly.
Weak diaphragm and intercostal muscles reduce the volume the patient can move, so the vital capacity falls, the cough becomes too weak to clear secretions, and carbon dioxide accumulates, producing morning headache, disturbed sleep, daytime sleepiness and orthopnoea, which is breathlessness when lying flat. These are late and easily missed signs, so ventilatory capacity is measured rather than guessed, and non-invasive ventilation, secretion clearance and avoidance of sedative drugs that depress breathing are the mainstays. Immobility brings its own predictable damage: joint contractures, pressure ulcers, deep vein thrombosis, constipation and loss of bone density.
Multidisciplinary care therefore runs alongside diagnosis from the beginning, with physiotherapy to preserve range and strength, occupational therapy to adapt the home, attention to pain, cramps, fatigue and mood, and early discussion of what the patient wants if the disease progresses. Some commonly used medicines, including certain antibiotics and anaesthetic agents, can worsen transmission at the neuromuscular junction, so every new prescription is checked against the diagnosis.
