Weakness is a common presentation, which is seen in disease processes affecting muscle (eg: muscular dystrophy, myositis), nerve and muscle junction (eg: myasthenia gravis), peripheral nerve (eg: neuropathy), plexus (eg: brachial/ lumbosacral plexopathy), nerve root (eg: radiculopathy or pinched nerve), motor neurons (eg: amyotrophic lateral sclerosis, spinal muscular atrophy), spinal cord (eg: Spinal cord injury, spinal stroke, spinal compression, transverse myelitis); and brain (eg: stroke, multiple sclerosis). Generalized weakness can sometimes be due to metabolic causes (eg: magnesium or thyroid hormone levels). All the above presentations can also be seen in psychological conditions (eg: conversion disorder).
What symptoms are seen with weakness?
Patients with weakness can have positive or negative symptoms. Positive symptoms include muscle cramps, contractures, pain/ myalgia, stiffness (myotonia), hypertrophy, and twitching/rippling. Negative symptoms include fatigue and atrophy (loss of muscle bulk). It’s important to know the evolution of these symptoms such as; acute (days), sub-acute (weeks), or chronic (months or years); constant or episodic; monophasic or relapsing; age at onset or lifelong (suggesting congenital disorders); progressive or static.
Weakness can develop over days to weeks, such as in Guillain-Barré syndrome, or slowly progressing over months to years as in Charcot-Marie-tooth disease. It can be localized, involving only certain muscle groups (eg: facioscapulohumeral muscular dystrophy) or generalized, affecting the entire body (eg: generalized myasthenia gravis). Weakness from genetic conditions usually presents at a younger age (eg: Duchenne muscular dystrophy). Family history is important in determining the pattern of inheritance.
What to do if a patient has weakness?
When physicians encounter a patient with weakness, it is important to know the onset of the symptoms, progression, and pattern of weakness. The key to diagnosis is a comprehensive history along with a detailed examination. This will narrow down which part of the nervous system might be affected and minimize the number of laboratory studies needed to confirm the diagnosis. Accordingly, work up with blood work, electrodiagnostic studies (needle EMG and nerve conduction studies), imaging, muscle-nerve biopsy and/or genetic testing may be considered. It is imperative to keep in mind that no two patients are the same, even with the same diagnosis.
Pasnoor M, Dimachkie MM. Approach to Muscle and Neuromuscular Junction Disorders. Continuum (Minneap Minn). 2019 Dec;25(6):1536-1563. doi: 10.1212/CON.0000000000000799. PMID: 31794459.