44.2 Myalgias, Rhabdomyolysis & Post-Viral Pain Syndromes

Diagnostic Framework for Generalized Muscle Pain & Weakness

The clinical evaluation of a patient presenting with generalized muscle pain (myalgias) or weakness requires a systematic distinction between true objective muscle weakness (inability of a muscle to exert normal contractile power against resistance) and asthenia or antalgic limitation (fatigue, lethargy, or reluctance to move due to joint or tendon pain).

Neurologic vs. Myopathic Localization

• Myopathic Weakness: Characteristically manifests as symmetrical proximal muscle weakness (shoulder abductors, hip flexors, neck flexors) with preservation of distal dexterity until late stages. Patients report difficulty rising from a low chair without pushing with arms, climbing stairs, or lifting objects overhead (e.g., placing dishes on high shelves). Deep tendon reflexes and sensation remain intact.
• Neuropathic Weakness: Typically asymmetric, involves distal muscle groups (foot drop, weak wrist extension), and is accompanied by objective sensory loss (numbness, paresthesias) and depressed or absent deep tendon reflexes.
• Neuromuscular Junction Disorders (e.g., Myasthenia Gravis): Characterized by fluctuating, fatigable weakness exacerbated by repetitive exertion and relieved by rest, with prominent cranial nerve involvement (ptosis, diplopia, dysarthria, dysphagia) and normal muscle enzymes (CK).

Idiopathic Inflammatory Myopathies: Polymyositis vs. Dermatomyositis

Polymyositis (PM) and Dermatomyositis (DM) are systemic autoimmune connective tissue diseases characterized by chronic immune-mediated striated muscle inflammation resulting in progressive proximal muscle weakness and elevated serum muscle enzymes.

               COMPARISON OF POLYMYOSITIS & DERMATOMYOSITIS

   Feature                 Polymyositis (PM)               Dermatomyositis (DM)
   ═════════════════════════════════════════════════════════════════════════════════════════
   Immunopathogenesis      Cell-mediated: CD8+ cytotoxic   Humoral/vascular: CD4+ T cells,
                           T cells invade non-necrotic     B cells, complement (C5b-9) MAC
                           endomysial muscle fibers        damages perimysial capillaries

   Muscle Biopsy           Endomysial mononuclear          Perimysial and perivascular
                           infiltrates; muscle necrosis    infiltrates; perifascicular atrophy

   Cutaneous Signs         ABSENT                          PATHOGNOMONIC:
                                                           Heliotrope rash, Gottron papules,
                                                           shawl sign, V-sign, holster sign

   Malignancy Risk         Modest increase (~1.5-2x)       VERY HIGH (20% to 30% of adults);
                                                           Mandatory comprehensive screening

   Autoantibodies          Anti-Jo-1 (antisynthetase);     Anti-Mi-2, anti-TIF1-gamma,
                           Anti-SRP, ANA                   anti-NXP-2, anti-MDA5

   First-Line Therapy      High-dose oral corticosteroids  High-dose oral corticosteroids
                           (prednisone 1 mg/kg/day) +      (prednisone 1 mg/kg/day) +
                           steroid-sparing immunosuppressant steroid-sparing immunosuppressant
   ═════════════════════════════════════════════════════════════════════════════════════════

1. Polymyositis (PM)

• Pathophysiology: MHC Class I antigens (HLA-ABC) are aberrantly expressed on striated sarcolemma. Autoreactive CD8+ cytotoxic T lymphocytes and macrophages invade non-necrotic endomysial muscle fibers, releasing perforin and granzyme to cause myofiber necrosis.
• Clinical Presentation: Insidious, progressive, painless or mildly aching symmetrical proximal weakness developing over weeks to months. Pharyngeal and upper esophageal striated muscle involvement occurs in up to 30% of patients, presenting with dysphagia, nasal regurgitation, and high aspiration pneumonia risk.
• Laboratory Findings: Serum Creatine Kinase (CK) is markedly elevated (5 to 50 times the upper limit of normal; typically 1,000 to >10,000 U/L). Elevated aldolase, lactate dehydrogenase (LDH), and serum transaminases (AST/ALT; frequently misinterpreted as liver disease until CK is checked).
• Antisynthetase Syndrome: A recognized clinical subgroup associated with anti-Jo-1 (anti-histidyl-tRNA synthetase) antibodies, characterized by the clinical triad of inflammatory myopathy, interstitial lung disease (ILD), non-erosive polyarthritis, Raynaud phenomenon, and "mechanics' hands" (hyperkeratotic, cracked, scaling fissures along the palmar and lateral surfaces of the fingers).

2. Dermatomyositis (DM) & Pathognomonic Cutaneous Signs

• Pathophysiology: Complement-mediated microangiopathy. Deposition of the C5b-9 membrane attack complex (MAC) within endomysial capillary endothelial cells leads to capillary thrombosis, microvascular ischemia, and characteristic perifascicular myofiber atrophy.
• Pathognomonic Cutaneous Signs:
  • Heliotrope Rash: Violaceous (purplish-pink) erythema involving the upper eyelids, often accompanied by periorbital edema.
  • Gottron Papules: Pathognomonic violaceous or erythematous, flat-topped, lichenoid papules and plaques overlying the dorsal surfaces of the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints.
  • Gottron Sign: Symmetrical macular erythema with or without scaling over the extensor surfaces of the elbows, patellae, and medial malleoli.
  • Shawl Sign: Widespread violaceous macular erythema distributed across the posterior neck, upper back, and shoulders.
  • V-Sign: Confluent erythematous eruption over the anterior lower neck and upper chest (photo-distributed).
  • Holster Sign: Violaceous macules on the lateral hips and thighs (trochanteric regions).
  • Periungual Telangiectasias: Capillary dilation, tortuosity, and cuticular overgrowth along the proximal nailfold.

The Mandatory Malignancy Workup in Dermatomyositis

[!IMPORTANT] CANCER SCREENING MANDATE IN DERMATOMYOSITIS Up to 20% to 30% of adult patients with dermatomyositis have an underlying occult malignancy (paraneoplastic phenomenon). The highest relative risk is for ovarian cancer in females, followed by breast, lung, gastrointestinal (colorectal, gastric, pancreatic) cancers, and non-Hodgkin lymphoma (as well as nasopharyngeal carcinoma in patients of Southeast Asian ancestry).

Every adult newly diagnosed with dermatomyositis must undergo comprehensive, age- and sex-appropriate malignancy screening: complete pelvic examination with transvaginal pelvic ultrasound and serum CA-125 in women, mammography, PSA in men, colonoscopy, computed tomography (CT) of the chest, abdomen, and pelvis, and fecal occult blood testing. Screening should be repeated annually for at least 3 years following diagnosis.

Polymyalgia Rheumatica (PMR) & Giant Cell Arteritis (GCA)

Polymyalgia Rheumatica is a common chronic inflammatory condition of older adults characterized by aching and morning stiffness in the shoulder girdles, hip girdles, and neck.

Clinical Presentation & Diagnostic Criteria

• Demographics: Almost exclusively affects individuals aged >=50 years (peak incidence between 70 and 80 years); females outnumber males 2:1.
• Clinical Presentation: Acute or subacute onset (days to weeks) of bilateral, symmetrical aching and stiffness in the proximal girdles: shoulders, pelvic girdle (hips/buttocks/thighs), and cervical spine.
• Profound Morning Stiffness: Cardinal symptom lasting >45 to 60 minutes (often lasting multiple hours); patients report severe difficulty getting out of bed, rising from a toilet, or lifting their arms to put on a jacket or brush their hair.
• Physical Examination Hallmark:
  • Active range of motion of the shoulders and hips is severely restricted due to pain.
  • TRUE MUSCLE STRENGTH IS NORMAL (5/5) once pain is accounted for! There is NO intrinsic muscle weakness, and passive range of motion is relatively preserved.
  • Musculoskeletal ultrasound characteristically reveals bilateral subacromial/subdeltoid bursitis, biceps tenosynovitis, and trochanteric bursitis.
• Laboratory Findings:
  • Markedly elevated ESR (>40-50 mm/hr, often >100 mm/hr) and elevated C-reactive protein (CRP).
  • Serum Creatine Kinase (CK) and aldolase are completely NORMAL (essential differentiating factor from polymyositis).
  • Mild normocytic normochromic anemia of chronic disease; negative rheumatoid factor and anti-CCP.

Dramatic Response to Low-Dose Corticosteroids

• First-Line Regimen: Oral Prednisone 15 to 20 mg daily.
• Diagnostic and Therapeutic Hallmark: Patients exhibit a dramatic, rapid clinical response within 48 to 72 hours (often describing near-total resolution of pain and stiffness). If a patient fails to show substantial improvement within 7 days, the diagnosis of PMR must be fundamentally questioned.
• Tapering Schedule: Continue initial dose for 2 to 4 weeks until symptoms remit and ESR/CRP normalize, then taper gradually over 12 to 24 months to avoid inflammatory relapse.

Mandatory Screening for Giant Cell Arteritis (GCA / Temporal Arteritis)

[!CAUTION] THE PMR-GCA LINK: PREVENTING IRREVERSIBLE BLINDNESS Approximately 15% to 20% of patients with PMR have concomitant Giant Cell Arteritis, and 40% to 50% of patients with GCA have PMR.

Clinicians must screen for GCA symptoms at EVERY clinical encounter: • New-onset unilateral temporal or frontal headache • Scalp tenderness (pain while brushing hair or resting head on a pillow) • Jaw claudication (fatigue and pain in the masseter muscles while chewing food; highest positive likelihood ratio for GCA) • Visual disturbances: amaurosis fugax (transient painless monocular vision loss), diplopia, or visual field defects • Thickened, tender, nodular, or pulseless temporal artery

IF GCA IS SUSPECTED: Immediately initiate HIGH-DOSE oral Prednisone (40 to 60 mg daily) or IV methylprednisolone (1,000 mg daily if visual symptoms are present) and arrange an urgent temporal artery biopsy. NEVER DELAY CORTICOSTEROID THERAPY PENDING BIOPSY; immediate high-dose steroids prevent catastrophic, permanent blindness from anterior ischemic optic neuropathy (AION).

Rhabdomyolysis: Pathophysiology & Etiologies

Rhabdomyolysis is an acute, life-threatening syndrome caused by skeletal muscle necrosis and the catastrophic release of intracellular myocellular contents (myoglobin, creatine kinase, electrolytes, purines) into the systemic circulation.

Pathophysiologic Cascade

Direct mechanical trauma, sustained muscle ischemia, or severe metabolic ATP depletion impairs the sarcolemmal Na+/K+-ATPase and Ca2+-ATPase pumps. Intracellular sodium accumulates, reversing the Na+/Ca2+ exchanger and driving a massive, uncontrolled influx of ionized calcium into the myoplasm. Intracellular hypercalcemia activates calcium-dependent neutral proteases (calpains) and phospholipase A2, initiating myofibrillar autolysis, mitochondrial destruction, and sarcolemmal rupture. Massive quantities of myoglobin, creatine kinase, potassium, phosphorus, and nucleic acids flood the extracellular space.

                      COMMON ETIOLOGIES OF RHABDOMYOLYSIS

   Etiologic Category      Specific Causes & Clinical Contexts
   ═════════════════════════════════════════════════════════════════════════════════════════
   Trauma & Compression    • Prolonged immobilization ("down time" after fall, stroke, coma,
                             or overdose of opioids, sedatives, or ethanol)
                           • Crush injuries, vehicular entrapment, compartment syndrome
                           • High-voltage electrical injuries, severe third-degree burns

   Exertional Overload     • Extreme strenuous exertion in unconditioned individuals (CrossFit,
                             marathon running, military boot camp)
                           • Exertional heat stroke, strenuous exertion with sickle cell trait
                           • Severe prolonged seizures, status epilepticus, delirium tremens

   Drugs & Toxins          • Statins (HMG-CoA reductase inhibitors), especially combined with
                             gemfibrozil or CYP3A4 inhibitors (clarithromycin, azoles)
                           • Illicit psychostimulants: Cocaine, methamphetamine, MDMA (ecstasy)
                           • Alcohol abuse (toxic myopathy, immobilization, hypokalemia)
                           • Neuroleptic Malignant Syndrome (NMS), Malignant Hyperthermia

   Metabolic & Endocrine   • Severe hypokalemia (<2.5 mEq/L) causing impaired muscle blood flow
                           • Severe hypophosphatemia (<1.0 mg/dL) depleting muscle ATP
                           • Diabetic ketoacidosis (DKA), Hyperosmolar Hyperglycemic State (HHS)
                           • Severe profound hypothyroidism (myxedema coma)

   Systemic Infections     • Viral: Influenza A and B, SARS-CoV-2 (COVID-19), enteroviruses, EBV
                           • Bacterial: Legionella pneumophila, Streptococcus pneumoniae, sepsis
   ═════════════════════════════════════════════════════════════════════════════════════════

Clinical Presentation, Complications & Urinalysis Hallmarks

The Classic Triad

The classic clinical triad consists of:

1. Severe Myalgias: Muscle pain and profound tenderness, most prominent in proximal muscle groups (thighs, calves, lower back).
2. Generalized Proximal Muscle Weakness: Difficulty standing, walking, or lifting arms.
3. Dark Reddish-Brown Urine: Described as "tea-colored", "cola-colored", or smoky urine. Note: The full triad is present in fewer than 10% of patients at initial presentation! Swelling, edema, and firmness of affected muscle compartments may be the only physical signs.

Diagnostic Hallmarks: Serum CK & The Urinalysis Discrepancy

• Serum Creatine Kinase (CK): The single most sensitive and definitive diagnostic test. A diagnosis of rhabdomyolysis is established when serum CK exceeds 5 times the upper limit of normal (typically >1,000 U/L). In clinically significant rhabdomyolysis, levels routinely exceed 10,000 to >100,000 U/L (and can reach 500,000+ U/L). Serum CK rises within 2 to 12 hours of muscle injury, peaks at 24 to 72 hours, and declines by 40% to 50% daily once muscle necrosis ceases.
• The Pathognomonic Urinalysis Discrepancy:
  • Urine Dipstick: STRONGLY POSITIVE (3+ to 4+) FOR "BLOOD" (HEME).
  • Microscopic Examination: ABSENCE OF RED BLOOD CELLS (0 to 2 RBCs/HPF).
  • Mechanism: The urine dipstick uses an orthotolidine reagent that reacts with the heme moiety present in both hemoglobin and myoglobin. In rhabdomyolysis, massive circulating free myoglobin is filtered by the glomeruli into the tubular lumen (myoglobinuria). Because the dipstick cannot differentiate myoglobin from intact erythrocytes, it triggers a strong positive reaction, but microscopic examination reveals no intact red blood cells.

Life-Threatening Acute Complications

1. Acute Kidney Injury (AKI): Occurs in 15% to 50% of patients. Three synergistic mechanisms drive renal injury:
   • Renal Vasoconstriction: Intravascular volume depletion and cytokine release trigger intense renal arteriolar vasoconstriction.
   • Intratubular Cast Formation: In the acidic environment of the distal convoluted tubule (urine pH <5.6), filtered myoglobin precipitates with Tamm-Horsfall mucoprotein, forming obstructive pigmented casts that block tubular lumen flow.
   • Direct Cytotoxicity & Lipid Peroxidation: Ferrihemate released from degraded myoglobin catalyzes the generation of reactive oxygen species (Fenton reaction), causing peroxidation of proximal tubular cell membranes and direct acute tubular necrosis (ATN).
2. Hyperkalemia: Catastrophic, rapid release of intracellular potassium into the extracellular compartment can produce fatal cardiac arrhythmias (peaked T waves, PR prolongation, wide QRS, ventricular fibrillation, asystole).
3. Hyperphosphatemia & Early Hypocalcemia: Intracellular phosphate binds circulating ionized calcium, causing metastatic calcium phosphate deposition into damaged necrotic muscle beds. Warning: Later in the recovery phase, this deposited calcium is remobilized back into circulation, leading to late hypercalcemia.
4. Hyperuricemia: Massive hepatic conversion of intracellular purines released from degenerated muscle cell nuclei.
5. Compartment Syndrome: Intracellular swelling within unyielding fascial envelopes compromises microvascular perfusion; requires immediate monitoring of compartment pressures and emergent fasciotomy if pressure exceeds 30 mmHg or delta pressure (diastolic BP minus compartment pressure) is <30 mmHg.
6. Disseminated Intravascular Coagulation (DIC): Triggered by the release of intracellular thromboplastin and procoagulants into systemic circulation.

Evidence-Based Treatment Protocol for Rhabdomyolysis

1. Immediate, Aggressive Intravenous Crystalloid Resuscitation

Aggressive volume expansion is the cornerstone of therapy to prevent myoglobin cast precipitation and acute tubular necrosis:

• Fluid Selection: Isotonic crystalloids—Normal Saline (0.9% NaCl) or Lactated Ringer's.
• Initial Rate: Administer 400 to 1,000 mL/hr intravenously immediately upon clinical suspicion.
• Urine Output Target: Titrate intravenous fluids to achieve and maintain an aggressive target urine output of 200 to 300 mL/hr (3 to 5 mL/kg/hr) until myoglobinuria has cleared and serum CK levels trend below 5,000 U/L.

2. Cardiac Telemetry & Aggressive Electrolyte Management

• Obtain an immediate 12-lead ECG and initiate continuous telemetry monitoring for signs of hyperkalemic cardiotoxicity.
• Management of Hyperkalemia:
  • Membrane stabilization: Intravenous Calcium Gluconate (10 mL of 10% solution over 2-3 minutes) if ECG changes (peaked T waves, QRS widening) are observed.
  • Intracellular potassium shifting: Regular insulin 10 units IV with 50 mL 50% Dextrose (D50W), nebulized albuterol (10-20 mg).
  • Potassium elimination: Oral sodium zirconium cyclosilicate (Lokelma) or emergent hemodialysis if refractory.

[!CAUTION] DO NOT ROUTINELY ADMINISTER CALCIUM FOR ASYMPTOMATIC HYPOCALCEMIA Intravenous calcium should NOT be administered to treat early hypocalcemia in rhabdomyolysis unless the patient exhibits life-threatening hyperkalemic cardiotoxicity or severe symptomatic tetany/seizures. Administering supplemental calcium drives further calcium precipitation into damaged muscle tissue, worsening rhabdomyolysis and exacerbating severe, dangerous rebound hypercalcemia during the recovery phase.

3. Role of Urine Alkalinization (Sodium Bicarbonate) & Diuretics

• Routine intravenous sodium bicarbonate infusion or mannitol administration is NOT supported by clinical trials over aggressive saline hydration alone.
• Bicarbonate infusion (100-150 mEq sodium bicarbonate in D5W) can be selectively considered ONLY if severe systemic metabolic acidosis exists (arterial pH <7.1) or if urine pH remains <6.5 despite adequate hydration, but it carries a significant risk of worsening hypocalcemia and alkalemia. Aggressive isotonic hydration remains the primary therapeutic mandate.

Post-Viral Myalgias & Chronic Post-Viral Pain Syndromes

Viral infections are among the most frequent precipitants of acute generalized myalgias in ambulatory family medicine. While most episodes resolve spontaneously within 7 to 14 days, certain pathogens can trigger protracted, debilitating musculoskeletal pain syndromes.

Causative Pathogens & Clinical Characteristics

• Influenza A and B: Causes acute, intense, diffuse myalgias affecting the back, thighs, and calves, accompanied by fever, chills, and respiratory symptoms. Serum CK may show mild, transient elevations (<2-3 times normal), but frank rhabdomyolysis is rare.
• SARS-CoV-2 (COVID-19 & Post-COVID Conditions / PASC): Acute COVID-19 frequently presents with prominent myalgias. Up to 15% to 25% of patients develop Post-Acute Sequelae of SARS-CoV-2 ("Long COVID"), characterized by persistent diffuse musculoskeletal pain, exercise intolerance, post-exertional malaise (PEM), profound cognitive fatigue ("brain fog"), and autonomic dysfunction (postural orthostatic tachycardia syndrome - POTS).
• Chikungunya Virus: An Aedes mosquito-borne alphavirus endemic to the Caribbean, South America, and tropical regions. Characterized by acute high fever and excruciating, debilitating symmetric polyarthralgia and myalgias predominantly affecting small distal joints (wrists, ankles, hands). Up to 40% to 50% of infected individuals develop chronic chikungunya arthralgia/myalgia lasting months to years post-infection.
• Epstein-Barr Virus (EBV): Acute infectious mononucleosis frequently triggers protracted post-viral fatigue and generalized muscle aching persisting for months.

Diagnostic Approach & Supportive Management

• Workup: Rule out inflammatory myopathies (serum CK and aldolase are normal or only minimally elevated), systemic vasculitis/PMR (ESR and CRP are typically normal or only mildly elevated), and primary thyroid disease (normal TSH).
• Management Principles:
  • Reassurance regarding the absence of progressive destructive joint or muscle disease.
  • Multimodal non-opioid analgesia: scheduled acetaminophen and short courses of oral NSAIDs for acute inflammatory flares.
  • Non-pharmacologic rehabilitation: structured, low-intensity graded exercise therapy (avoiding severe exertion that precipitates post-exertional malaise), pacing strategies, and cognitive behavioral therapy.