Introduction 

Muscle atrophy refers to the wasting or loss of muscle tissue. Often accompanied by muscle weakness, it may pose significant challenges to individuals’ mobility, strength, and overall quality of life. Understanding symptoms and clinical presentation of muscle atrophy is vital for prompt diagnosis and interventions. This discussion will delve specifically into the visible and tangible manifestations of muscle atrophy: ranging from general weakness, and reduced muscle mass to changes in posture and tone. By exploring in detail, we aim to clarify the physical effects of muscle atrophy, emphasising the importance of these indicators for effective treatment interventions to prevent complications and maximise functionality.¹ 

Understanding muscle atrophy 

Muscle atrophy (wasting) causes a gradual (focal or generalised) decline in muscle size and strength, impacting an individual’s functional abilities and general health, can be classified into the following subtypes: 

Disuse (physiologic) 

Disuse (physiologic) atrophy occurs by prolonged physical inactivity or (limb) immobility, resulting from a sedentary lifestyle, lack of exercise, bed rest, or medical conditions like muscular dystrophy, stroke or dermatomyositis that make movement difficult. Once muscles are not regularly used through physical activity, contractions are reduced, initiating less protein synthesis for muscle growth and repair. As the balance between protein synthesis and breakdown shifts, the net loss of muscle protein is induced. Under the muscle fibre composition alterations due to reduced protein synthesis and increased breakdown, muscles eventually decrease in size and strength.^1,2

Neurogenic 

Neurogenic atrophy is the most severe subtype, resulting from injury or disease affecting neuromuscular connections, such as amyotrophic lateral sclerosis (ALS), spinal muscular dystrophy (SMA), polio, and spinal cord injury (SCI). When motor neurons are damaged, inadequate or unable to trigger muscle contractions and activities, they become denervated and disused. Disruption in muscular protein (myosin) synthesis results in increased protein breakdown and decreased muscle breakdown..^1,2 

Metabolic and endocrine 

Metabolic and endocrine atrophy is induced by hormonal and metabolic imbalances that affect muscle health. Hormones such as growth hormones (GH), insulin-like growth factor I (IGF-I), testosterone, thyroid hormones (TH), and glucocorticoids (GCs) are essential for skeletal muscle growth, repair, and maintenance. Conditions like diabetes mellitus, thyroid disorders, malnutrition, cachexia, and prolonged steroid use that disrupt various hormonal production or functioning, could further proteolysis and autophagy activation, resulting in a loss of muscle mass.^1,2 

Age-related 

Sarcopenia is an age-related condition of gradual loss of muscle mass, strength, and function. Combined factors such as decreased GH and IGF-I levels, insulin resistance, prolonged inactivity, and sex hormone decline that impair protein metabolism could directly contribute to the development of sarcopenia. This results in the elderly experiencing muscle weakness, reduced mobility, overall functional decline and frailty. This makes them more prone to falls and fractures as time passes by.²

Symptoms of muscle atrophy 

Muscle weakness

Muscle atrophy often leads to notable general weakness in affected areas, making performing daily tasks difficult for individuals. Simple activities like independent standing, walking, lifting weights, speech and swallowing become increasingly challenging due to muscle strength reduction. It can worsen gradually, leading to an increase in dependency on routine movements and daily activities.¹

Decreased muscle mass

Decreased muscle mass has been a key symptom of muscle atrophy, with visible shrinkage and loss of bulk. While this can either be noticeably focal or generalised depending on etiologies, the affected area appears thinner or more frail in appearance. Hence, contributes to further strength reduction and physical limitations over time.¹ 

Fatigue

As weakened muscles tire quickly, fatigue becomes common among muscle atrophy. Even minor activities could induce unusual and unpredictable exhaustion, resulting in a persistent, overwhelming sense of tiredness. This then, impacts overall energy levels, interfering with daily activities and prolonged physical task engagement.^3,4

Reduced endurance

muscle atrophy, endurance usually diminishes significantly, while the ability to sustain long periods of physical activity decreases, leading to quicker fatigue.⁴ Tasks such as walking long distances and carrying weighted objects get progressively harder as a result of reduced stamina.⁴ 

Impaired muscle function

Impaired muscle functioning is a hallmark of atrophy, with the large and small muscles struggling to perform their intended roles. Tasks that require motor skills or strength could become increasingly strenuous, as movement becomes less precise and coordinated. 

Changes in posture

Muscle atrophy can lead to postural changes, instability and balancing issues as weakened muscles struggle to support the body position properly. Individuals may develop a slouched or hunched appearance, with difficulty in upright postural maintenance. These changes can induce pain and discomfort, contributing to musculoskeletal issues like stiffness, contractures, and scoliosis.⁵

Pain or discomfort

Stemming from various factors such as muscle weakness, imbalances, reduced blood flow, inflammation, nerve irritation, and extra loads to joints, pain and discomfort often accompany muscle atrophy. The affected areas may feel sore, stiff, and achy, particularly after physical activity. They can vary in intensity as muscle losses over time.⁵ 

Clinical presentations of muscle atrophy 

Muscle atrophy is typically diagnosed through physical and neurological exams. In addition, detailed symptomatic and medical history queries, combined with various diagnostic tests should also be sought. Below are the general clinical presentation of muscle atrophy: 

Loss of muscle bulk

Loss of muscle bulk is often an observable warning sign of muscle atrophy. This visible muscle size and volume reduction is beneficial for clinicians to identify the specific areas affected and its severity:

• Generalised muscle wasting often points to prolonged immobility, neuromuscular disorders, or systemic illness like cancer, diabetes or thyroid problems
• Asymmetrical (focal) muscle wasting inclines more to localised conditions like nerve damage or stroke

The extent of muscle loss is vital upon disease progression monitoring and treatment effectiveness evaluation.^1,6

Decreased muscle strength

Decreased muscle strength is an elemental clinical finding of muscle atrophy, revealing muscles’ normal force generation inability, resulting in difficulties handling daily tasks. This weakness occurs when atrophied muscles have fewer and smaller fibres reducing their capacity for effective contractions. Measuring muscle strength through a dynamometer or manual testing can provide objective data for quantifying the extent of weakness and identifying specific muscle involvement. Muscle strength reduction indicates the presence of conditions such as peripheral neuropathy, muscular dystrophies, and even neurodegenerative conditions like amyotrophic lateral sclerosis. Accurate assessment is foremost for tailoring rehabilitation programs and the need for assistive devices.⁶

Changes in muscle tone

Muscle tone is the partial, passive and continuous muscular contractions or resistance to movement that facilitates posture maintenance and muscles’ readiness for action. Muscle atrophy is typically presented with either hypotonia (reduced tone) or less commonly, hypertonia (increased tone). Hypotonia is often accompanied by muscle weakness, leading to muscular flaccidity, becoming softened, with little resistance to passive movements. Clinically, muscle tone changes are particularly vital to diagnosing neuromuscular disorders. In addition, Hypotonia is commonly associated with lower motor neuron lesions, peripheral neuropathies, or myopathies, where neuromuscular connections are disrupted. Hypertonia indicates upper motor neuron (UMN) lesions, although it is generally less direct to muscle wasting. These could help guide the investigation and diagnostic test to the specific neuroanatomical level.⁶

Muscle fasciculations

Muscle fasciculations are involuntary, brief, fine muscle contractions visible under the skin, induced by spontaneous, uncontrolled motor neuron discharges. While occasionally occurring among healthy individuals like BFS, their presence associated with muscle atrophy often underlies more severe neuromuscular diseases, like its common manifestation among amyotrophic lateral sclerosis (ALS). They can also indicate other forms of neurogenic atrophy, associated with nerve damage. observing and documenting fasciculations alongside other clinical signs helps narrow down the differential diagnosis. A nerve conduction test and EMG are often used to confirm the presence of fasciculations and assess the extent of neural involvement for accurate diagnosis, prognosis and treatment planning.⁶ 

Diminished reflexes 

Reflexes are the involuntary responsive movement to stimuli, mediated by the nervous system. While muscle atrophy is particularly paramount among lower motor neuron lesions, diminished or absent deep tendon reflexes (DTR) (such as knee jerk reflex) are commonly observed, due to motor neuronal loss or peripheral nerve damage-induced neural circuit disruptions. Regarding the diagnosis of neuromuscular damage location and extent, diminished reflexes have been significantly helpful in separating upper and lower motor neuron lesions. Reduced reflexes are particular among LMN lesions, whilst hyperreflexia (exaggerated reflexes) is presented among UMN lessons. Diminishing DTR is indicative of conditions like spinal muscular atrophy (SMA), Guillain-Barre syndrome (GBS), or chronic peripheral neuropathies. An accurate DTR assessment can assist in localising the lesion and guide further tests like MRI or nerve conduction studies.⁶ 

Diagnosis 

Despite the self-reported symptoms and observable clinical presentations and neurological examinations done along, healthcare professionals may order tests including blood sampling, X-rays, nerve conduction study, electromyography (EMG), computed tomography (CT), magnetic resonance imaging (MRI) scans, as well as muscle or nerve biopsy to determine the underlying cause of muscle wasting.¹

Differential diagnosis 

Regardless of the above diagnostic tests, muscle atrophy is discriminated against by several muscle-related conditions regarding their causes, symptoms, and implications.

Muscle hypertrophy 

Muscle hypertrophy is the gross muscle appearance or the increased diameter of myofibers. It is caused either physiologically by increased resistance training, physical workload, or pathologically due to neuropathies or myopathies processes that stimulate muscle growth.⁷ 

Muscle dystrophy 

Muscle dystrophy is a cluster of genetic disorders causing progressive muscle weakness and degeneration, such as Duchenne muscular dystrophy (DMD), or Becker muscular dystrophy (BMD), originating from abnormal protein production.¹ 

Muscle spasms

Muscle spasms (cramps) are the sudden, involuntary, forcible contractions that induce pain and discomfort in one or several muscle(s) groups, due to fatigue, extreme heat, dehydration, electrolyte imbalances, and stress, which typically do not associate with muscle atrophy. 

Muscle strain

Muscle strains are painful injuries when muscle fibres are stretched or torn due to overexertion or sudden movements, particularly among athletes, resulting in pain, swelling, and movement difficulties. 

Summary 

Muscle atrophy can be self-reported as weakness, constant fatigue, reduced mass and endurance, functional impairments, postural changes, with pain and discomfort. Alongside the clinical manifestations of hypotonia, muscle fasciculations, and diminished reflexes, muscular deterioration results from various causes like disuse or neurological conditions. Illustrating the symptoms and clinical presentation of muscle atrophy in-depth could facilitate the identification and push forward relevant studies and research, which is crucial for early assessment, detection, and timely rehabilitation management to prevent further muscle loss, mobility challenges and complications. Hence, maintaining or even enhancing the functional abilities and quality of life for the affected population.