Muscle Weakness

See Also

Epidemiology

Weakness affects 5% of adults oiver age 60 years old

History

Onset
1. Acute onset within hours to days should prompt urgent assessment
  1. See Acute Motor Weakness Causes (Emergent Bilateral Motor Weakness)
  2. Acute Unilateral Weakness (aside from focal weakness) should prompt emergent CVA Evaluation
Distinguish weakness from Fatigue and Asthenia
1. Asthenia is exhaustion without weakness
2. Fatigue is inability to perform task repeatedly
3. Weakness is inability to perform task initially
4. Consider Frailty and Sarcopenia of advanced age
Distinguish weakness from pain-related disuse (e.g. specific joint Osteoarthritis)
Determine distribution
1. Unilateral Weakness (typically neurologic, especially if sensory involvement)
  1. Cerebrovascular Accident (CVA, critical to exclude in acute cases)
  2. Mononeuropathy (Focal Peripheral Neuropathy)
  3. Multiple Sclerosis
  4. Myasthenia Gravis
2. Symmetric proximal weakness (e.g. rising from chair, climbing steps, raising arms overhead)
  1. Myopathy
    1. See Myopathy Causes (e.g. Polymyositis, Polymyalgia Rheumatica, Thyroid disease)
3. Symmetric distal weakness (e.g. drops items, cannot open jars, foot drop)
  1. Polyneuropathy
  2. Guillain-Barre Syndrome (affects proximal Muscles as well)
  3. Amyotrophic Lateral Sclerosis
  4. Myopathy less commonly effects distal Muscles
    1. Statins, Alcohol may effect distal and proximal
    2. Inclusion body Myositis
    3. Myotonic dystrophy
Contributing factors
1. Medication history
  1. See Medication Causes of Myositis
2. Substance use history
  1. Alcohol Abuse
  2. Cocaine Abuse
  3. Huffing (esp. Glue)
3. Family History
  1. Genetic Muscular Dystrophy
  2. Disorders of metabolism

Exam
Neurologic Exam

Motor Strength
1. Evaluate specific upper and lower extremity testing (e.g. ulnar, median and Radial Nerve function)
2. Evaluate for symmetric versus asymmetric weakness
3. Upper Motor Neuron Lesion (everything is UP - tone, reflexes, toes)
4. Lower Motor Neuron Lesion (everything is DOWN - tone, reflexes, toes)
Sensory Exam
1. Evaluate for dermatomal distribution of sensory loss
Deep Tendon Reflexes
1. Hyperreflexia (Upper Motor Neuron Lesion, Hyperthyroidism, Hypercalcemia)
2. Hyporeflexia (Lower Motor Neuron Lesion, Hypothyroidism)
Cranial Nerve Exam
1. Ptosis and Diplopia may suggest Myasthenia Gravis
Evaluate for Sarcopenia and Frailty
1. Grip strength
2. Get Up and Go Test

Exam
General

Dermatomyositis changes (e.g. heliotrope rash, Gottron Papules)
Cushing's Disease changes (Obesity, Hypertension, Hirsutism, abdominal striae)
Thyroid changes (e.g. Goiter)

Evaluation

Is motor weakness NOT objectively identified on exam?
1. Conside Asthenia (exhaustion without weakness)
2. Consider Fatigue (inability to perform task repeatedly)
3. Consider Frailty and Sarcopenia of advanced age
4. Consider Dyspnea
Upper Motor Neuron Lesion (Muscle group weakness without atrophy, hyperreflexia)?
1. Acute Onset
  1. Emergency evaluation for Cerebrovascular Accident head imaging
2. Subacute Onset
  1. Evaluate with head imaging for CNS Lesion, Spinal Cord Syndrome, Multiple Sclerosis
Upper Motor Neuron Lesion AND Lower Motor Neuron Lesion
1. Evaluate for Amyotrophic Lateral Sclerosis (Neurology, Electromyogram, Nerve Conduction Studies)
Lower Motor Neuron Lesion (Flaccid Paralysis with Muscle atrophy, fasciculations and hyporeflexia) OR non-specific
1. Weakness is affected by repetition
  1. Weakness worsens with repetition: Myasthenia Gravis (associated with Ptosis, Diplopia, Dysarthria, Dysphagia)
    1. Anti-acetylcholine receptor Antibody positive
    2. Neurology Consultation and (if labs negative, Electromyogram, Nerve Conduction Studies)
  2. Weakness improves with repetition: Lambert-Eaton Syndrome
    1. Voltage-Gated Calcium Channel Antibody positive
    2. Neurology Consultation and (if labs negative, Electromyogram, Nerve Conduction Studies)
2. Sensory deficit
  1. Suggests Neuropathy (e.g. Diabetic Neuropathy)
  2. Evaluate with Serum Glucose, Vitamin B12
  3. Evaluate for Substance Abuse and toxin exposures
  4. Consider Neurology Consultation and Electromyogram, Nerve Conduction Studies
  5. Muscle biopsy may be considered in unclear cases
3. Asymmetric weakness
  1. Consider Compressive Neuropathy, Parsonage-Turner Syndrome
  2. Consider Neurology Consultation and Electromyogram, Nerve Conduction Studies
  3. Consider MRI Brain
4. Symmetric weakness
  1. Painful
    1. Consider Medication Causes of Myositis (esp. Statins and Quinolones)
    2. Consider Rhabdomyolysis and inflammatory Myopathy
    3. Evaluate with CK, myoglobin, ESR, CRP and consider ANA
    4. Consider Rheumatology Consultation
    5. Consider Neurology Consultation
    6. Muscle biopsy may be considered in unclear cases
  2. Not significantly painful
    1. See Muscle Weakness Causes
    2. Broad-based laboratory evaluation (e.g. chem8, TSH, CK, cortisol)
    3. Careful history including Substance Abuse, toxin exposures and Family History
    4. Consider Neurology Consultation and Electromyogram, Nerve Conduction Studies
    5. Muscle biopsy may be considered in unclear cases

Differential Diagnosis

See Muscle Weakness Causes
See Acute Motor Weakness Causes (Emergent Bilateral Motor Weakness)
Consider Frailty and Sarcopenia of advanced age
Consider primary neurologic disorders
1. See Neuropathy
2. Upper Motor Neuron Lesion (e.g. CVA, Multiple Sclerosis)
3. Lower Motor Neuron Lesion (e.g. Guillain-Barre Syndrome, Compressive Neuropathy)
Consider primary Muscle disorders
1. Myotonia (Muscle stiffness)
2. Myositis (e.g. Polymyositis, Dermatomyositis, Inclusion body Myositis)
3. Muscular Dystrophy (e.g. Becker Muscular Dystrophy, Limb-Girdle Muscular Dystrophy, Myotonic Dystrophy)
Consider toxic and metabolic causes
1. See Toxic Myopathy (Medication Causes of Myositis)
2. Electrolyte disturbance (e.g. Hypokalemia)
3. Endocrine disorder (e.g. Hyperthyroidism or Hypothyroidism)

Labs
As Indicated

See Neuropathy
Myopathy (see also Polymyositis)
1. Creatine Kinase (CPK)
2. Erythrocyte Sedimentation Rate
3. C-Reactive Protein
4. Serum chemistry (e.g. Chem 8)
  1. Electrolytes Glucose, Calcium, Magnesium, phosphate
5. Thyroid Stimulating Hormone (TSH)
6. Urinalysis
  1. Myoglobinuria
7. Vitamin B12
Other tests as indicated
1. 24-hour urine cortisol
2. Growth Hormone
3. Antinuclear Antibody
4. HIV Test
5. Lyme Disease Test
6. West Nile Virus Test
7. Dengue Fever Test
8. Lumbar Puncture
9. Anti-acetylcholine receptor Antibody (Myasthenia Gravis)
10. Weakness improves with repetition:

Imaging
As Indicated

MRI Brain

Diagnostics
As Indicated

Electromyogram
Nerve Conduction Studies
Muscle Biopsy
1. Reserved for specific Myopathy cases (objective weakness and positive testing), typically by neurology recommendation
2. Biopsy performed by surgical consultant of Muscle with mild to moderate weakness
3. Avoid biopsy of Muscles with severe weakness (nondiagnostic results with fat and fibrosis change)

References

Nayak (2017) Worl J Clin Cases 5(7): 270-9
1. https://www.wjgnet.com/2307-8960/full/v5/i7/270.htm
Chawla (2011) Front Neurol 2:49 +PMID:21886637 [PubMed]
1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3153853/
Larson (2020) Am Fam Physician 101(2):95-108 [PubMed]