Causes of unilateral arm weakness
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Common Organic Causes of Unilateral Arm Weakness
Unilateral arm weakness can result from a variety of organic (physical or structural) causes. Stroke is a well-known cause, often presenting with sudden onset weakness, sometimes accompanied by facial droop or leg involvement, and is a medical emergency requiring prompt assessment and treatment 34. Lesions in the brain, such as infarcts or tumors affecting the premotor cortex, can also cause moderate unilateral weakness, particularly impacting shoulder and hip muscles, and may be associated with limb-kinetic apraxia . Cervical disc herniation is another important cause, typically presenting with neck and arm pain due to nerve root compression, but can sometimes cause isolated arm weakness without other neurological symptoms .
Peripheral nerve disorders, such as Parsonage-Turner Syndrome (PTS), can lead to acute or subacute onset of shoulder and arm weakness, often following an episode of severe pain. PTS is thought to be triggered by factors like infection, vaccination, or surgery, and is confirmed by electrodiagnostic studies . Rarely, progressive conditions like Mills’ syndrome, a form of motor neuron disease, can present with slowly worsening unilateral arm and leg weakness, spasticity, and abnormal reflexes . In children and young adults, Hirayama disease—a rare, self-limiting motor neuron disorder—should also be considered, especially if the weakness progresses slowly .
Functional and Non-Organic Causes of Unilateral Arm Weakness
Not all cases of unilateral arm weakness have an identifiable structural or organic cause. Functional neurological disorder (FND), also known as non-organic or psychogenic weakness, can mimic organic disease but is not due to structural damage. FND can present acutely or chronically, sometimes with associated sensory changes or loss of consciousness, and is more common in women . Clinical examination techniques, such as the Hoover sign, the elbow flex-ex sign, and the Tsimerinov sign, are useful in distinguishing non-organic from organic weakness. These signs rely on detecting unexpected strength or movement in the “weak” limb during specific maneuvers, which would not occur in true organic weakness 16.
Unusual and Reversible Causes
Although most cases of hypokalemic paralysis present with symmetric weakness, rare cases can cause unilateral limb weakness. This condition is due to low potassium levels and is reversible with prompt correction of the electrolyte imbalance. It is important to consider this diagnosis, as delayed treatment can have serious consequences .
Diagnostic Approach and Importance of Differentiation
A thorough history, physical examination, and appropriate investigations (such as imaging and electrodiagnostic studies) are essential to determine the underlying cause of unilateral arm weakness. Differentiating between organic and non-organic causes is crucial to avoid unnecessary tests and treatments, and to ensure appropriate management 136.
Conclusion
Unilateral arm weakness can result from a wide range of causes, including stroke, brain lesions, cervical disc herniation, peripheral nerve disorders, rare motor neuron diseases, metabolic disturbances, and functional neurological disorders. Careful clinical assessment and targeted diagnostic tests are key to identifying the underlying cause and guiding effective treatment 12345678+1 MORE.
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