Muscle Weakness Disorders (Neuromuscular Weakness)

How muscle contraction can fail

A voluntary movement passes along a chain:

1. Upper motor neurons in the brain and spinal cord send the command.
2. Lower motor neurons in the spinal cord send it out along peripheral nerves.
3. The neuromuscular junction passes the signal to the muscle with the chemical acetylcholine.
4. The muscle fibers contract.

A break at any point causes weakness, and each has a typical signature. For a practical guide to weakness in your legs, arms or ankles and when to get help, see muscle weakness symptoms.

Main groups of muscle weakness disorders

Brain and spinal cord

Stroke, multiple sclerosis, brain tumors, spinal cord injury and compression, and cervical myelopathy. Weakness comes with stiffness (spasticity), brisk reflexes and often sensory or bladder changes.

Motor neuron diseases

Amyotrophic lateral sclerosis (ALS) causes progressive, painless weakness with muscle wasting and twitching (fasciculations), often starting in one hand or foot, or with slurred speech. Spinal muscular atrophy is a genetic form, mainly in children, now treatable with gene-targeted therapies.

Peripheral nerve disorders

• Guillain-Barré syndrome: acute, ascending weakness with loss of reflexes after an infection.
• Chronic inflammatory demyelinating polyneuropathy: a slower, relapsing version.
• Peripheral neuropathy from diabetes, alcohol, B12 deficiency or chemotherapy, usually starting in the feet with numbness.
• Nerve entrapments: carpal tunnel, ulnar and peroneal nerve compression.

Neuromuscular junction disorders

• Myasthenia gravis: autoimmune antibodies block acetylcholine receptors. Weakness worsens with use and improves with rest, often affecting the eyelids, eye muscles, face and swallowing.
• Lambert-Eaton syndrome: weakness of the hips and thighs with dry mouth, often linked to small cell lung cancer.
• Botulism: a toxin that causes descending paralysis.

Muscle disorders (myopathies)

• Inflammatory myositis: dermatomyositis (with a rash on the eyelids and knuckles), polymyositis, immune-mediated necrotizing myopathy (sometimes linked to statins) and inclusion body myositis.
• Muscular dystrophies: inherited conditions such as Duchenne, Becker, myotonic and facioscapulohumeral dystrophy.
• Endocrine myopathies: hypothyroidism, hyperthyroidism, Cushing syndrome and long-term steroid use.
• Toxic and drug-induced myopathies: statins, alcohol, colchicine, some antiretrovirals.
• Metabolic myopathies: inherited problems with energy use, causing exercise intolerance, cramps and episodes of rhabdomyolysis.
• Periodic paralysis: episodes of weakness linked to high or low potassium.
• Rhabdomyolysis: acute muscle breakdown from extreme exercise, heat, crush injury, drugs or infection.

Systemic causes

Electrolyte imbalances (potassium, calcium, magnesium, phosphate), vitamin D deficiency, critical illness myopathy after intensive care, and cancer-related weakness. Ongoing inactivity leads to muscle atrophy, which itself worsens weakness.

Diagnosis

History and examination

The doctor maps which muscles are weak, how fast it developed, whether it fluctuates, and associated features: pain, rash, cramps, numbness, swallowing or breathing problems, family history, medicines and alcohol. Strength is graded on a 0 to 5 scale (the MRC scale).

Blood tests

• Creatine kinase (CK): high in most myopathies, myositis, muscular dystrophy and rhabdomyolysis. Usually normal in nerve and junction disorders.
• Electrolytes, thyroid function, vitamin D, B12, glucose, kidney and liver tests.
• Antibodies: acetylcholine receptor and MuSK antibodies for myasthenia, myositis-specific antibodies, and HMGCR antibodies for statin-associated necrotizing myopathy.

Nerve and muscle studies

Nerve conduction studies and electromyography (EMG) distinguish nerve, junction and muscle problems. Repetitive nerve stimulation and single-fiber EMG test the neuromuscular junction.

Imaging and biopsy

• MRI of the brain or spine for central causes. Muscle MRI shows inflammation or patterns of dystrophy and guides biopsy.
• Muscle biopsy: examines muscle tissue for inflammation, dystrophy or metabolic disease.
• Genetic testing for muscular dystrophies, spinal muscular atrophy and metabolic myopathies.
• CT of the chest in myasthenia gravis to look for a thymoma.

Treatment

Rehabilitation

Physical and occupational therapy maintain strength, range of motion and independence. Supervised, moderate exercise is safe and beneficial in most neuromuscular conditions. Braces such as ankle-foot orthoses, mobility aids, and breathing support (noninvasive ventilation) are used when needed.

Outlook

The outlook ranges widely. Weakness from electrolyte, thyroid, medicine or vitamin causes usually resolves completely within weeks. Guillain-Barré syndrome recovers well in most people over months. Myasthenia gravis and myositis are long-term but often well controlled. ALS and many dystrophies are progressive, though new therapies are changing the outlook for some.

Frequently asked questions

What are the main causes of muscle weakness?

Muscle weakness can come from the brain or spinal cord (stroke, MS), motor nerves (ALS, neuropathy, Guillain-Barré), the nerve-muscle junction (myasthenia gravis) or the muscle itself (myositis, dystrophy, thyroid or steroid myopathy). Electrolyte imbalance, medicines and inactivity are also common causes.

What blood test checks for muscle weakness?

Creatine kinase (CK) is the key test. It rises when muscle fibers are damaged, as in myositis, muscular dystrophy and rhabdomyolysis. Doctors also check electrolytes, thyroid function, vitamin D, and specific antibodies for myasthenia gravis or myositis.

What is the difference between myopathy and neuropathy?

Myopathy is a disease of the muscle itself, usually causing weakness of the hips and shoulders with normal sensation. Neuropathy is a disease of the nerves, usually causing numbness and tingling with weakness that starts in the hands and feet. EMG and nerve studies tell them apart.

Is myasthenia the same as myasthenia gravis?

Myasthenia is a general term meaning muscle weakness. Myasthenia gravis is a specific autoimmune disease of the nerve-muscle junction in which muscles tire with use and recover with rest, often affecting the eyelids, eye movements, face and swallowing.

Can muscle weakness be reversed?

Often, yes. Weakness from low potassium, thyroid disease, medicines, vitamin D deficiency, inactivity and inflammatory myositis usually improves with treatment and exercise. Nerve recovery after Guillain-Barré takes months. Progressive genetic and motor neuron diseases are harder to reverse but can be managed.