The patient with weakness: brain, cord, nerve, junction or muscle
- Topic
How to tell where weakness comes from: the brain or spinal cord, a nerve, the nerve–muscle junction or the muscle itself.
See it in 3D: Motor cortex · Spinal cord · Grey matter of the cord · Neck nerve roots · Brachial plexus roots
What it is
True weakness is loss of muscle power, which is different from tiredness, pain that limits movement or general malaise. The pattern of weakness, the tone, the reflexes, the sensory findings and whether the weakness gets worse with effort point to one of five levels: upper motor neuron, lower motor neuron, peripheral nerve, neuromuscular junction or muscle. The myopathic syndrome, with symmetric proximal weakness and preserved sensation, is the pattern most often missed.
Key facts
- First confirm true weakness: ask what the patient cannot do (climb stairs, comb hair, hold a cup) rather than accepting 'weak'.
- Proximal weakness (rising from a chair, lifting the arms overhead) suggests muscle or the neuromuscular junction.
- Distal weakness with stocking-and-glove sensory loss suggests a polyneuropathy.
- Weakness that worsens with repetition and improves with rest points to the neuromuscular junction.
- Onset over minutes suggests stroke; over hours to days, Guillain–Barré syndrome or cord compression; over months, myopathy or motor neuron disease.
Where is the lesion? The five levels of weakness
| Level | Distribution | Tone and reflexes | Sensation | Clues |
|---|---|---|---|---|
| Upper motor neuron (brain, cord) | Hemibody or below a level; arm extensors and leg flexors | Increased tone, brisk reflexes, Babinski | Often affected along the same pattern | Pronator drift; sudden onset in stroke |
| Lower motor neuron (anterior horn, root) | Segment, root or myotome | Reduced tone and reflexes | Normal in anterior horn disease; dermatomal in root lesions | Wasting and fasciculations |
| Peripheral nerve | One nerve, or distal and symmetric in polyneuropathy | Reduced or absent reflexes | Numbness in the nerve territory or in a stocking-and-glove pattern | Ascending weakness with areflexia in Guillain–Barré |
| Neuromuscular junction | Eyelids, eye muscles, bulbar and proximal muscles | Normal tone and reflexes (reduced in Lambert–Eaton) | Normal | Fatigability; ptosis and double vision worse at the end of the day |
| Muscle (myopathy) | Symmetric and proximal; neck flexors | Normal tone; reflexes kept until late | Normal | Trouble rising from a chair, Gowers sign, waddling gait; muscle pain in some |
Clinical notes
- Myopathic syndrome: symmetric proximal weakness, normal sensation, reflexes preserved until late; seen in inflammatory myopathies, muscular dystrophies, thyroid and steroid myopathies.
- In a boy who climbs up his own legs to stand (Gowers sign), Duchenne muscular dystrophy is the classic cause.
- Myasthenia gravis: fluctuating ptosis, double vision and bulbar weakness that worsen with use.
- Lambert–Eaton syndrome: proximal leg weakness, low reflexes that may briefly improve after effort, dry mouth; it can accompany lung cancer.
- Generalized acute weakness with normal sensation can also come from low potassium or other electrolyte disorders.
In the lab
- Creatine kinase is high in most inflammatory myopathies and dystrophies.
- Electromyography and nerve conduction studies separate neuropathy, junction and muscle disorders.
- MRI of the brain or spine when the pattern points to an upper motor neuron lesion.
- Electrolytes (potassium, calcium, magnesium), glucose and thyroid function in acute or generalized weakness.
Test yourself
Think of the answer, then tap the question to see it.
Which pattern suggests a myopathy?
Symmetric proximal weakness, such as trouble rising from a chair or lifting the arms, with normal sensation and reflexes preserved until late.
Which feature points to a disorder of the neuromuscular junction?
Fatigability: weakness that worsens with repeated or sustained effort and improves with rest, often with ptosis and double vision.
What does weakness that ascends over days with loss of reflexes suggest?
Guillain–Barré syndrome, an acute polyneuropathy; breathing must be watched closely.
What is the Gowers sign?
A child rising from the floor by pushing the hands up along the thighs because of weak hip and thigh muscles; it is classic in Duchenne muscular dystrophy.
Connections
See it in 3D5
The structures of Durgical's 3D anatomy where it is found.
Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.
Sources: Weakness (MSD Manual Professional Edition, 2024); Overview of Peripheral Nervous System Disorders (MSD Manual Professional Edition, 2024).
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Representative model, for educational purposes only.