Viral Myositis

Causes

• Influenza A and B - the classic and most common cause, especially in children ("benign acute childhood myositis")
• Enteroviruses: coxsackievirus, echovirus
• Adenovirus, parainfluenza virus, Epstein-Barr virus, cytomegalovirus, arbovirus, dengue virus
• Hepatitis B and C
• HIV and HTLV-1 (retroviruses) - cause a more chronic, polymyositis-like syndrome rather than an acute illness
• COVID-19 - reported to cause myositis, possibly via direct viral effect or triggering autoimmunity

Clinical presentation

• Occurs a few days after the initial respiratory/flu symptoms begin to improve, not during the acute febrile phase
• Sudden onset of severe, focal calf pain and tenderness ("severe calf myalgia") causing difficulty walking or refusal to walk
• This is distinct from ordinary myalgia (body aches) that accompanies flu itself - it comes on later, is more focal (calves), and is more intense
• Usually self-limited, resolving within a few days with supportive care

• Preceding or concurrent viral prodrome: fever, cough, sore throat, rhinorrhea, GI symptoms - though these may have resolved before myositis appears
• Muscle pain, tenderness, and weakness (proximal in some infections, focal/calf in influenza)
• Elevated creatine kinase (CK), sometimes markedly
• EMG may show myopathic changes
• Muscle biopsy in influenza myositis typically shows fiber degeneration/necrosis with relatively little inflammatory infiltrate (Rheumatology, 2-Volume Set, p. 1936-1943)

Severe end of the spectrum

Diagnosis

• Clinical picture in the setting of recent viral illness
• Elevated CK (can be markedly elevated)
• Urinalysis/renal function to exclude myoglobinuria-related kidney injury in severe cases
• EMG if diagnosis is unclear (myopathic pattern)
• Muscle biopsy is not usually needed in typical influenza-associated cases but helps exclude autoimmune inflammatory myopathy (dermatomyositis, polymyositis, inclusion body myositis, necrotizing autoimmune myopathy) when the picture is atypical, prolonged, or the CK/weakness is disproportionate

Treatment

• Rest, hydration (important to prevent renal complications from myoglobinuria), and analgesia (NSAIDs can help myalgia)
• Monitor CK and renal function in more severe cases
• Antiviral therapy for the underlying infection if indicated (e.g., oseltamivir for influenza, though this doesn't necessarily shorten the myositis itself)
• IV fluids and closer monitoring if rhabdomyolysis develops
• For HIV myopathy, treatment addresses the underlying HIV and may require adjusting antiretroviral regimen if NRTI toxicity is suspected

When to worry / red flags

• Inability to walk or bear weight
• Extreme muscle tenderness or swelling
• Dark urine (suggesting myoglobinuria)
• Signs that suggest an alternative diagnosis (e.g., ongoing high fever, focal warmth/erythema suggesting bacterial pyomyositis, or a rash/systemic features suggesting an autoimmune myopathy)

Recent literature

Curosurf (poractant alfa) dosing

• 2.5 mL/kg (200 mg/kg) birth weight, given intratracheally as soon as possible after diagnosis of RDS

• Up to 2 repeat doses of 1.25 mL/kg (100 mg/kg) each
• Given at intervals of roughly 12 hours (Harriet Lane notes repeat dosing can occur as early as 6 hours if still intubated and requiring ≥30% FiO2 with PaO2 ≥80 torr)
• Given if RDS persists or respiratory status deteriorates (ongoing high oxygen/ventilation requirement)

• 5 mL/kg (400 mg/kg) birth weight (sum of initial + 2 repeat doses)

• Intratracheal only, via a 5-French feeding catheter
• Suction the infant before administration
• Each dose is split into two aliquots, administered with the infant positioned right-side-down then left-side-down so each lung gets an aliquot
• Manually ventilate with 100% O2 (40-60 breaths/min) for about 1 minute between aliquots
• Monitor SpO2 and lung compliance closely after each dose - transient bradycardia, desaturation, reflux of surfactant up the ET tube, and airway obstruction can occur
• Pulmonary hemorrhage is a recognized, uncommon complication

RAM Cannula Sizing for a 23-Week Infant

Neotech RAM Cannula - full size range (smallest to largest)

Bottom line

• Micro Preemie (white) is the smallest size in the RAM Cannula line and is the correct starting choice for a 23-week gestation infant.
• The septal space (distance between prongs) is only 2.5 mm with a 3 mm outer diameter, specifically designed to fit the smallest nares without causing nasal septal trauma or excessive occlusion.
• Correct sizing matters clinically: a cannula that's too large can obstruct nares and impair pressure delivery/venting (risking excessive/unpredictable CPAP pressure transmission), while one that's too small won't seal enough to deliver effective pressure. Nasal septal breakdown is also a recognized risk in ELBW infants with prolonged RAM cannula/CPAP use, so frequent skin/septum checks are important regardless of size chosen.
• Sizing should still be confirmed at the bedside by checking prong fit against the nares (there should be a small visible air leak, not a tight seal) rather than by weight alone, since actual nares size can vary between infants of similar gestational age.