Causes of arm weakness
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Common Neuromuscular Causes of Arm Weakness
Arm weakness can result from a variety of neuromuscular conditions. Diseases affecting the muscles themselves, such as myopathies (including inclusion body myositis and hereditary myopathies), often cause weakness in specific muscle groups. For example, amyotrophic lateral sclerosis (ALS) typically affects the intrinsic hand muscles, while inclusion body myositis more often involves the extrinsic hand muscles. Motor neuron diseases like ALS and spinal muscular atrophy (SMA) can cause both proximal and distal arm weakness, often progressing in a predictable sequence regardless of where symptoms first appear. In these conditions, muscle atrophy and changes in muscle fiber composition, as well as mitochondrial dysfunction, contribute to the observed weakness 13510.
Nerve-Related Causes: Peripheral Nerve and Nerve Root Lesions
Peripheral nerve injuries are a frequent cause of arm weakness. Mononeuropathies, such as lesions of the ulnar, radial, median, or anterior interosseous nerves, can lead to weakness in specific muscles depending on the nerve affected. Suprascapular nerve entrapment is an under-recognized cause of upper arm pain and weakness, often resulting from repetitive movements, heavy lifting, or stretch injuries. This condition can present with muscle atrophy and weakness, particularly in the infraspinatus and supraspinatus muscles, and may be confirmed with electrodiagnostic studies 1278.
Radiculopathies, or nerve root problems, can also cause arm weakness and are often differentiated from peripheral nerve lesions by careful muscle testing and diagnostic rules such as the "5.3.5 rule," which helps pinpoint the affected nerve or root without requiring detailed neuroanatomical knowledge .
Muscle and Tendon Injuries
Acute injuries, such as biceps brachii tendon rupture, can cause sudden arm weakness, often accompanied by visible deformity and reduced strength in specific movements like elbow flexion. These injuries are usually related to trauma or overuse and may be managed conservatively or surgically, depending on the severity and patient activity level .
Systemic and Autoimmune Disorders
Autoimmune conditions like myasthenia gravis can cause fluctuating weakness in the arm, particularly affecting the wrist and finger extensors. Chronic inflammatory demyelinating polyneuropathy (CIDP) is another example, often presenting with symmetrical proximal weakness in both arms and legs 13.
Stroke and Central Nervous System Causes
After a stroke, arm weakness can result from both direct muscle weakness and abnormal muscle synergies (involuntary movement patterns). Both factors contribute to impaired arm movement and coordination, and careful assessment is needed to distinguish between them for effective rehabilitation .
Distinguishing True Weakness from Other Causes
It is important to differentiate true muscle weakness from other issues such as joint pain, swelling, or incoordination due to cerebellar disease, which may mimic weakness but are not caused by loss of muscle strength .
Conclusion
Arm weakness can arise from a wide range of causes, including neuromuscular diseases, nerve injuries, muscle and tendon injuries, autoimmune disorders, and central nervous system conditions. Accurate diagnosis relies on careful clinical assessment, targeted diagnostic tests, and an understanding of the typical patterns of weakness associated with each condition 12345678+2 MORE.
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