General weakness: causes and diagnosis

Generalized weakness is a subjective feeling of decreased strength throughout the body, often described by the patient as "no strength," "feeling like cotton wool," or "difficulty supporting the weight of the limbs." It is important to distinguish weakness from fatigue and drowsiness: with true weakness, muscle strength is reduced, whereas with fatigue and drowsiness, objective strength may remain normal. Misinterpretation leads to diagnostic errors and delayed treatment. [1]

Generalized weakness is not a diagnosis, but a syndrome that occurs in dozens of conditions, from anemia and electrolyte imbalances to myopathies, polyneuropathies, neuromuscular junction diseases, and spinal cord disorders. The first clinical step is to confirm the presence of "true" weakness with strength testing and localize the level of damage: the central nervous system, peripheral nerves, neuromuscular junction, or the muscles themselves. [2]

The prevalence of this complaint is high in general practice and increases with age due to sarcopenia, comorbidity, and polypharmacy. In the elderly cohort, weakness is often associated with falls, decreased gait speed, and risk of hospitalization, so routine assessment of strength and functional testing are essential elements of the examination. [3]

Some patients present with "functional" complaints of weakness in the absence of structural damage; in such cases, criteria for functional neurological disorders and early referral to physiotherapy with training in symptom self-management are helpful. [4]

Code according to ICD 10 and ICD 11

In the International Classification of Diseases, Tenth Revision, two approaches are used in clinical practice: the symptom code "Weakness" R53.1 and the code "Muscle weakness, generalized" M62.81. The former is used when weakness is considered a symptom of an unspecified condition, while the latter is used when the clinical focus is on decreased muscle strength as a manifestation of a muscle disorder. The choice of code determines routing and statistics. [5]

The International Classification of Diseases, Eleventh Revision, instead of a single "weakness code," uses categories in the chapter "Symptoms, signs, or clinical findings, not elsewhere classified" and codes for diseases where weakness is the leading symptom, such as the "Primary muscle diseases" section, as well as the diagnostic category "Dissociative neurological disorder with paresis or weakness" for functional symptoms. This reflects the principle that the established cause, not just the symptom, should be coded. [6]

Table 1. Code according to ICD 10 and ICD 11

Situation	International Classification of Diseases, 10th revision	Note	International Classification of Diseases, 11th revision	Note
Weakness as a symptom	R53.1 "Weakness"	A symptom when the cause is not specified	MG section "General symptoms"	Symptoms are coded if there is no established cause.
Generalized muscle weakness	M62.81	Emphasis on decreased strength as a muscle disorder	FB32-FB3Z "Muscle diseases"	Specific muscle nosology code
Functional weakness	-	-	6B60.6 "Dissociative neurological disorder with paresis or weakness"	If the symptom is functional in nature
[7]

Epidemiology

Fatigue is one of the most common complaints in adult clinical practice. In the elderly, the proportion of individuals with reduced muscle strength, as measured by tests, reaches significant levels, which is associated with sarcopenia, polymorbidity, and physical inactivity. Because weakness is a syndrome, precise estimates depend on the population and measurement method. However, experts recommend routine strength screening as a marker of adverse outcome risk. [8]

According to reviews of frailty diagnostics in adults, true frailty occurs in a significant proportion of patients over 60 years of age and requires objective grading using the UK Medical Research Council scale. This improves the reproducibility of observations and allows for monitoring of progress during therapy. [9]

Sarcopenia, an age-associated decline in muscle strength and mass, has been recognized as a distinct condition since 2019: the European Working Group consensus identifies low muscle strength as a key characteristic. This has significant implications for the epidemiology of frailty in older age groups. [10]

In hospital and intensive care settings, weakness is often due to “ICU-acquired weakness,” critical illness polyneuromyopathy, and immobilization, which increases the length of hospital stay and rehabilitation needs.[11]

Reasons

The etiologic spectrum is broad. Systemic causes include anemia, infections, inflammatory diseases, endocrine disorders, electrolyte and metabolic imbalances, drug effects, and nutritional deficiencies. Neuromuscular causes include myopathies, polyneuropathies, neuromuscular junction disorders, and central tract lesions.[12]

Common and potentially treatable causes include iron deficiency, vitamin B12 deficiency, hypothyroidism, hypo- and hyperkalemia, hypercalcemia, hyponatremia, hyperglycemia, chronic renal failure, heart failure, and polypharmacy, particularly the myopathic effects of glucocorticoids and statins. Early laboratory testing of these factors increases diagnostic yield. [13]

Neuromuscular causes include inflammatory myopathies, hereditary and metabolic myopathies, demyelinating and axonal polyneuropathies, and diseases of the neuromuscular junction, such as myasthenia gravis. In each case, the rate of development, symmetry, and distribution of weakness, as well as the involvement of the respiratory and bulbar muscles, are important. [14]

A separate group consists of sarcopenia and detraining, where the dominant symptoms are decreased grip strength, walking speed, and endurance. These conditions are modifiable through resistance training and optimized protein intake, making them crucial to identify. [15]

Table 2. Main groups of reasons and examples

Group	Examples	Clinical clues
Systemic	Anemia, hypothyroidism, vitamin B12 deficiency, electrolyte disturbances	General symptoms: paleness, dry skin, orthostasis
Neuromuscular	Inflammatory myopathies, polyneuropathies, myasthenia	Symmetry, reflexes, fatigue, bulbar signs
Central	Myelopathies, stroke, demyelination	Pathological reflexes, spasticity, conduction symptoms
Functional	Functional neurological disorders	Inconsistency of symptoms, positive clinical signs
[16]

Risk factors

Age and physical inactivity increase the risk of sarcopenia, falls, and weakness. Low protein and energy intake, vitamin D deficiency, chronic diseases, and prolonged immobilization increase strength loss. These factors are modifiable and should be identified at the initial consultation. [17]

Polypharmacy and long-term use of glucocorticoids, sedatives, and some lipid-lowering agents increase the risk of drug-induced myopathy and weakness. Regular medication reviews reduce the likelihood of adverse effects. [18]

Micronutrient deficiencies, particularly vitamin B12, iron, and copper, are common in individuals with dietary restrictions, post-bariatric surgery, and malabsorption, making screening for these deficiencies an important component of frailty assessment.[19]

Critical illness and intensive care unit stay are associated with critical illness myopathy and long-term weakness requiring early rehabilitation.[20]

Pathogenesis

Decreased strength occurs with disorders at any level, from the cerebral cortex to the contractile apparatus of the muscle fiber. Central lesions disrupt descending motor neuron stimulation, peripheral lesions disrupt axonal conduction and synaptic transmission, and myopathies disrupt calcium signaling and contractile proteins. Distinguishing between the levels of damage determines the diagnostic strategy. [21]

Systemic causes of weakness include tissue hypoxia, substrate deficiency, and electrolyte imbalance, which impair membrane excitability and contractility. Correction of systemic disturbances often rapidly improves strength, emphasizing the importance of an early, basic laboratory assessment. [22]

Sarcopenia is characterized by neuromuscular changes in motor units, decreased muscle mass and quality, and low muscle strength is considered a key clinical marker. This determines the priority of resistance training and protein supplementation in treatment. [23]

Functional weakness is associated with disturbances in movement patterns without structural damage; the diagnosis is supported by positive clinical signs and responds to specialized physical therapy aimed at restoring normal movement patterns. [24]

Symptoms

The primary complaint is a feeling of lack of strength during everyday and professional tasks: climbing stairs, holding objects, rising from a chair, and bearing body weight. Decreased endurance, slow walking speed, unsteadiness, and an increased need for rest are often associated. [25]

In cases of true weakness, the physician objectively identifies a decrease in strength using manual testing and the MRC scale. It is important to distinguish weakness from pain, deconditioning, and fear of movement. Standardized assessment allows for monitoring progress during treatment. [26]

Associated symptoms depend on the cause: in case of anemia - shortness of breath during exertion and dizziness, in case of endocrine disorders - cold intolerance and weight gain, in case of neuromuscular diseases - fatigue phenomena, bulbar and respiratory symptoms. [27]

Red flags include rapid onset of weakness, ascending spread, respiratory and swallowing difficulties, and recent iatrogenic exposures. These signs require immediate, in-depth investigation and often hospitalization.[28]

Table 3. Red flags for generalized weakness

Sign	Possible reasons	Actions
Rapid progression within hours to days	Acute polyradiculoneuropathy, myelitis, hyperkalemia	Urgent hospitalization, respiratory monitoring
Bulbar symptoms, respiratory failure	Diseases of the neuromuscular junction, polyneuropathy	Measurement of vital capacity, consultation with a resuscitator
Pronounced electrolyte shifts	Hypo- hyperkalemia, hypercalcemia	Immediate correction of electrolytes
Post-intensity weakness	Critical illness neuromyopathy	Early rehabilitation, prevention of complications
[29]

Classification, forms and stages

Clinically, a distinction is made between true muscle weakness, fatigue, and mixed conditions. True weakness is confirmed by an objective decrease in strength, while fatigue is confirmed by the inability to maintain effort at normal instantaneous force. This distinction is crucial for the selection of examinations. [30]

Based on the level of damage, central, peripheral, synaptic, and muscular forms are distinguished, each with its own clinical clues and tests. This "from symptom to level" approach systematizes the examination and reduces the time to diagnosis. [31]

Depending on the course of the disease, a distinction is made between acute, subacute, and chronic weakness. Acute weakness is most often associated with electrolyte imbalances and acute neurological syndromes, subacute weakness is associated with endocrine and inflammatory pathology, and chronic weakness is associated with sarcopenia and hereditary diseases. [32]

Functional motor disorders constitute a separate category and are diagnosed based on positive clinical signs, which allows avoiding overdiagnosis of “psychosomatics” and referring the patient to evidence-based physiotherapy. [33]

Complications and consequences

Untreated weakness leads to falls, loss of independence, hospitalizations, and decreased quality of life. For older adults, this carries the risk of long-term loss of mobility and the need for support or caregiver assistance. [34]

In patients after intensive care, weakness is associated with prolonged rehabilitation, muscle atrophy, and decreased exercise tolerance. Early initiation of kinesitherapy and prevention of complications improve outcomes. [35]

In neuromuscular diseases, complications include respiratory failure, aspiration, and nutritional deficiencies, requiring respiratory monitoring and multidisciplinary management.[36]

Even with reversible systemic causes, repeated episodes of weakness impair functional reserve, so secondary prevention and post-discharge management are critical.[37]

When to see a doctor

You should consult a doctor if weakness persists for more than 7-14 days, progresses, limits daily activities, or is accompanied by weight loss, fever, pain, sensory disturbances, dizziness, or shortness of breath on exertion. These symptoms require evaluation and basic laboratory testing. [38]

Seek immediate emergency care if you experience rapidly worsening weakness, difficulty holding your head up, difficulty speaking or swallowing, shortness of breath at rest, or an inability to rise from a chair without using your hands. These are potential signs of a life-threatening condition. [39]

Elderly patients and those following long-term hospitalization should be routinely assessed for strength, gait speed, and risk of falls, even in the absence of overt complaints. Early detection of weakness can prevent complications. [40]

In case of repeated episodes of weakness against the background of polypharmacy, it is advisable to review medications for myopathic and sedative effects, with the participation of a clinical pharmacologist. [41]

Diagnostics

Step 1: Confirm "true" weakness. Assess strength manually using the Medical Research Council (MRC) strength scale in 6-8 key muscle groups and record the scores. Use grip dynamometry and the chair rise test if necessary. [42]

Step 2. Neuroanatomical localization. Based on reflexes, tone, distribution of weakness, and sensory disturbances, decide whether the lesion is likely central, peripheral, synaptic, or muscular. This determines the examination trajectory and specialists to be referred. [43]

Step 3. Basic laboratory workup for everyone. Complete blood count, ferritin and iron levels, vitamin B12 and folate, creatine kinase, electrolytes with calcium and magnesium, urea and creatinine, glucose and glycated hemoglobin, thyroid function tests, and inflammatory markers. This workup covers common and treatable causes. [44]

Step 4. Targeted testing. Serology for infections, immunological panels, electromyography and nerve conduction studies, magnetic resonance imaging of the brain and spinal cord, and cerebrospinal fluid analysis are prescribed as indicated. Muscle biopsy is considered if myopathy is suspected after electrophysiology. [45]

Step 5. Sarcopenia screening and functional assessment. Use the European Working Group criteria: low strength as a key feature, supported by muscle mass and quality, and physical performance assessment to grade severity. [46]

Table 4. Starting laboratory panel and diagnostic goals

Test	What does it reveal?	Clinical value
Complete blood count, ferritin	Anemia, iron deficiency	A common and reversible cause of weakness
Electrolytes, calcium, magnesium	Ionic shifts	Urgent correction in case of severe disorders
Creatine kinase	Myolysis, myopathy	Suggests a muscular etiology
Vitamin B12, folate	Deficits	Neurological and hematological effects
Thyroid function	Hypo- hyperthyroidism	Often amenable to correction
[47]

Table 5. The UK Medical Research Council scale for assessing muscle strength

Score	Definition
5	Normal force
4	Movement against moderate resistance
3	Movement against gravity throughout its entire amplitude
2	Motion with the exclusion of gravity
1	A barely noticeable reduction
0	There is no abbreviation
[48]

Differential diagnosis

True weakness versus fatigue. In fatigue, the patient is "out of breath," but the maximum one-time force may be normal; in true weakness, peak force is reduced. The distinction is made through repeated strength tests and functional testing. [49]

Neuromuscular diseases. Myopathies produce proximal, symmetrical weakness with possible elevation of creatine kinase; polyneuropathies produce distal weakness with sensory disturbances; diseases of the neuromuscular junction produce fluctuating weakness and bulbar symptoms. Electrophysiology and serology provide clarification. [50]

Central lesions. Myelopathies and focal lesions of the central nervous system are accompanied by pathological reflexes, spasticity, and conduction symptoms; any suspicion requires imaging as an emergency indication. [51]

Functional motor disorders. Diagnosis is made on the basis of positive signs and requires early, specially trained physiotherapy rather than exclusion of "everything under the sun." [52]

Table 6. Differential diagnosis based on key features

Sign	Myopathy	Polyneuropathy	Neuromuscular junction	Central lesion
Distribution	Proximal	Distal	Fluctuating	Conductor
Reflexes	Maintained or reduced	Reduced	Usually saved	Strengthened
Sensitivity	Usually normal	Violated	Normal	Conduction disorders
Creatine kinase	Often elevated	Norm	Norm	Norm
[53]

Treatment

Treatment tactics are always etiotropic and include symptomatic rehabilitation. For systemic causes, the priority is to eliminate deficiencies, correct electrolytes, inflammation, and endocrine disorders; for neuromuscular diseases, specialized therapy and multidisciplinary management are essential. [54]

The first support block is physical therapy with an emphasis on resistance training 2-3 times per week, progressive load, power exercises, balance, and functional tasks. For older patients with sarcopenia, this is a first-line intervention that improves gait strength and speed. [55]

Nutritional support includes adequate protein intake. Consensus guidelines for individuals over 65 recommend 1.0-1.2 g of protein per kilogram of body weight per day, with higher amounts recommended for individuals with active training or medical conditions, based on individual tolerance and under physician supervision. The diet is divided into 3-4 meals to optimize muscle protein synthesis. [56]

Vitamin D supplementation is advisable in cases of confirmed deficiency. Reviews show a modest positive effect of supplementation on strength and balance in the elderly, although universal prescription without deficiency is not justified; dosages are individualized, often in the range of 800-1000 international units per day, with monitoring of levels and safety. [57]

In cases of anemia, etiotropic correction is used: iron supplements for deficiency, parenteral forms for intolerance to oral forms, and treatment of the source of blood loss; in cases of vitamin B12 deficiency, parenteral initiation with transition to maintenance regimens. Improvement in strength usually parallels the correction of laboratory parameters. [58]

For hypothyroidism, levothyroxine replacement therapy is prescribed, titrated based on the clinical picture and laboratory values. For electrolyte disturbances, standardized correction regimens are implemented, beginning with priority treatment of life-threatening potassium and calcium shifts. These measures often quickly reduce weakness. [59]

For diseases of the neuromuscular junction and inflammatory myopathies, immunomodulatory approaches are used as indicated. Early specialized care reduces disability, and rehabilitation is integrated from the first weeks of treatment to prevent deconditioning and contractures. [60]

After resuscitation, the benefits of early mobilization and individualized physical therapy programs have been proven. Focus is on verticalization, transfer training, strengthening of proximal groups, fall prevention, and patient training in fatigue self-management. [61]

In functional weakness, the best results are achieved with physical therapy programs that focus on restoring normal movement patterns, using an explanatory model, demonstrations, and a gradual increase in activity; medications are an adjunct here. [62]

An important cross-sectional component is medication audit: minimizing sedative, myopathic, and anticholinergic effects, deprescribing when not indicated, and coordinating treatment plans between specialists. This reduces drug-induced weakness and the risk of falls. [63]

Table 7. Directions of therapy and expected effects

Direction	Target	Expected effect
Resistance training	Increase strength and power	Increased strength, faster walking
Protein optimization	Support anabolism	Improved mass and function when combined with training
Replenishing vitamin D in case of deficiency	Reduce the risk of falls, improve balance	Modest increase in strength and stability
Correction of anemia and deficiencies	Eliminate systemic limiting factors	Reduced shortness of breath, increased endurance
Immune and specialized methods	Treat the underlying disease	Reducing disability
Rehabilitation after resuscitation	Restore autonomy	Reducing recovery times
[64]

Prevention

Daily physical activity with a strength training component, adequate protein intake, and correction of vitamin D deficiency, if present, reduce the risk of weakness, falls, and functional dependence in the elderly. These are basic measures for "healthy aging." [65]

Prevention of drug-induced complications includes regular review of therapy, caution with long-term use of glucocorticoids and sedatives, and patient education about signs of muscle toxicity.[66]

Reducing the risk of weakness after severe illness is achieved through early mobilization, nutritional support, and a discharge rehabilitation plan agreed upon between the hospital and primary care. [67]

Educational programs for symptom self-management and fall prevention, including balance training and home modifications, complement medical interventions and improve independence.[68]

Forecast

The prognosis is determined by the cause, age, comorbidities, and time to treatment. With reversible systemic disorders, strength is often restored after correction; with neuromuscular disorders, the prognosis varies and depends on timely specialized therapy and rehabilitation. [69]

Sarcopenia responds to resistance training and nutritional optimization, but requires consistent intervention; cessation of activity leads to loss of achieved improvements. This explains the importance of a long-term plan with gradual progression of exercise. [70]

After critical illness, recovery can take months; early initiation of rehabilitation and a multidisciplinary approach accelerates return of function and reduces the risk of rehospitalization. [71]

In functional weakness, correct communication of the diagnosis and specialized physiotherapy are associated with good symptom reversibility and restoration of activity. [72]

Questions and Answers

How does true weakness differ from fatigue and drowsiness?
True weakness is characterized by decreased maximal muscle strength, as confirmed by tests. Fatigue is a decreased ability to sustain effort, while drowsiness is a disturbance of wakefulness; these conditions can coexist but require different approaches. [73]

What tests are almost always needed at the start?
Complete blood count, ferritin, vitamin B12, glucose and glycated hemoglobin, electrolytes with calcium and magnesium, creatine kinase, thyroid function and inflammation indicators. This panel covers common and potentially reversible causes. [74]

Are there "vitamins for weakness" without an examination?
There are no universal supplements. Vitamin D is replenished when a deficiency is confirmed; protein in the diet is increased to target levels in the absence of contraindications, always in combination with strength training. Decisions are made based on an examination. [75]

Which exercises really help?
The strongest evidence supports resistance training: 2-3 times a week, progressive overload, leg and core exercises, plus balance and functional tasks. The plan is tailored to the individual and combined with nutritional support. [76]