Pediatric Infectious Myositis

Disease/Disorder

Definition

Pediatric infectious myositis encompasses inflammatory myopathies of skeletal muscle caused by viral, bacterial, fungal and/or parasitic agents. Benign acute childhood myositis (BACM), the most common subtype, is a self-limited post viral myopathy primarily affecting the calves of school aged children. This article addresses presentation and management of these presentations, with emphasis on BACM as the most common presentation.¹

Etiology

Children have more immature muscle cells which are more prone to viral myositis.¹ The most common viral etiology in children is influenza B, followed by influenza A, parainfluenza, and adenovirus.² Bacterial myositis are mostly attributed to S. aureus and Group A Strep while C. albicans is the most common fungal agent.³

Epidemiology including risk factors and primary prevention

Pediatric infectious myositis predominantly affects boys aged 5-10 years during influenza season (late winter/early spring), with male to female ratio of approximately 2:1.^4,5 Patients are at risk for infectious myositis with muscle damage, injury, or vascular insufficiency, which acts as the first hit. A second hit can cause disease when the child develops bacteremia, viral illness, or other infectious process where the pathogen gains access to the damaged muscle. Primary prevention includes minimizing infection risk by increased sanitation and cooking and washing foods appropriately.³ Although no study has directly evaluated influenza vaccination for myositis prevention, given that approximately 80% of viral myositis is influenza attributable, annual influenza vaccination per AAP recommendations would be expected to reduce incidence by preventing precipitation infection. A Post Covid era study observed a 140% increase in BACM incidence paralleling influenza resurgence after pandemic related immune debt.  If infection occurs, further prevention includes abscess drainage, debridement, or indicated pharmacological treatment of primary infectious cause to minimize complications.⁶

Patho-anatomy/physiology

Myositis occurs when the normal architecture of the protective barriers and fibers are disrupted through trauma, surgery, or ischemia which allows the infectious agent to gain access to the muscle fibers.³ Bacterial myositis (pyomyositis) tends to present more focally but occurs via hematogenous spread, while viral, fungal, and parasitic agents come from diffuse, systemic infections. The quadriceps and gluteus muscle groups are commonly affected in pyomyositis, but any group can be involved.³

Disease progression including natural history, disease phases or stages, disease trajectory (clinical features and presentation over time)

In BACM, a viral prodrome (fever, cough, coryza) precedes myositis onset by 1–3 days. Bilateral calf pain, refusal to walk, and toe-walking gait peak at presentation and resolve within 3–5 days, with CK normalizing within a median of 7 days. Full recovery without residual damage is expected.⁷

Bacterial pyomyositis follows a more protracted, staged course

• An invasive phase with low-grade fever, malaise, and swelling over 1–3 weeks
• A suppurative stage at 10–21 days with localized pain, severe myalgia, and abscess formation on imaging; and
• A late stage with sepsis and systemic toxicity requiring urgent intervention.

Specific secondary or associated conditions and complications

Abscess formation (pyomyositis), bacteremia, sepsis, and septic shock, and rhabdomyolysis are important, concerning sequelae of infectious myositis.¹ Local infection can result in compartment syndrome and muscle necrosis which can progress to bacteremia, shock, and multiorgan failure if not treated.³ Immunocompromised states lead to increased vulnerability and worse outcomes.

Essentials of Assessment

History

It is fundamental to take a detailed history, focusing on symptoms such as a flu-like illness days before onset of disease, fever, symmetrical or asymmetrical muscle pain, as well as inability or refusal to walk. The classic history, a school aged child with bilateral leg pain and difficulty walking 1-3 days after a respiratory illness. The physician should also inquire about recent trauma, travel or intense physical activity.^4,8

Physical examination

Some of the physical examination findings may include

• Vital signs: May present with fever, but mostly stable vital signs.
• Inspection: Occasional mild swelling of involved extremities, as well as a wide-based, toe walking gait may be observed.
• Palpation: Patients may report intense tenderness to palpation either symmetrically (BACM) or asymmetrically (pyomyositis), which may resolve after 2-4 days.
• Range of Motion (ROM): May present with a plantar flexion hold as well as refusal of active and passive ROM. ROM of the knees and hips tend to be intact.
• Neurological: Sensory, motor, deep tendon and plantar reflex testing tend to be intact.¹⁰

Functional assessment

BACM is a self-limited illness and due to its short course, there is no functional assessment validated specifically for it. The best way to assess function is with clinical criteria and observation, focusing on gait abnormalities, tenderness, strength and recovery timeline.^2,4,8

Laboratory studies

At presentation, obtain CK, CBC with differential (leukopenia is common), CMP emphasizing on renal function and electrolyte disturbances, urinalysis for myoglobinuria and viral testing. At follow up in 2-3 weeks, repeat CK to confirm normalization. Failure to normalize prompts further investigation for alternative diagnoses.^4,8

Imaging

In the case of BACM, MRI is not currently recommended but may be used only to rule out other differential diagnoses such as trauma, osteomyelitis, deep vein thrombosis or malignancy. As for infectious myositis, ultrasound can be used as primary screening to identify superficial intramuscular abscess collection or inflammation, whereas MRI can be used to screen for early inflammatory changes as well as extent and location of the infection.^2,8

Supplemental assessment tools

No validated supplemental assessment tools exist, specifically for BACM; diagnosis relies on clinical recognition, laboratory findings, and imaging when indicated. However, when red flags are present — age <2 years, family history of neuromuscular disorders, recurrent episodes, abnormal neurological examination, CK ≥5,000 U/L, dark urine, or abnormal renal function — further evaluation should include urinary organic acids, myositis-specific autoantibodies, NGS myopathy panel, and neurophysiological studies.⁴

Key distinguishing features: BACM shows pain-limited function with intact reflexes, whereas inflammatory myopathies present with progressive proximal weakness and characteristic skin findings, and dystrophinopathies show chronic progressive weakness with muscle atrophy.^11,12 Metabolic myopathies were identified in approximately 3% of BACM-presenting patients in one series, supporting screening in atypical cases.⁴

Early predictions of outcomes

Some early predictions of a favorable prognosis include normalization of lab values as well as resolution of complaints in a short period of time. The condition tends to have a good prognosis, with full resolution of symptoms and no residual damages.^2,8

Environmental

BACM is mostly associated with Influenza A and B viruses and tend to occur in the late winter and early spring. Bacterial pyomyositis is mostly seen with Staphylococcus aureus, Clostridium botulinum species and Streptococcus pyogenes infections. Environmental and exposure modifications as well as vaccination are recommended.^2,3

Social role and social support system

Compared to conditions like chronic inflammatory myositis, infectious myositis tends to be a self-limited condition for which minimal social intervention is needed. Hence, consider a brief school absence during the acute phase. Family assistance might be helpful during the acute phase for ambulation.^2,3

Professional issues

Although rare, one of the most common complications of non-viral myositis is rhabdomyolysis. Clinical acumen and close monitoring of renal function, serum electrolytes and cardiac function are essential for quick management and hospitalization if necessary.¹⁰

Rehabilitation Management and Treatments

Available or current treatment guidelines

Viral myositis is self-limited; rehabilitation centers on supportive care with rest, hydration, and ibuprofen as first line analgesia.¹⁰ Activity modification is recommended in the acute phase, with gradual return to ambulation as pain permits, likely within 3-5 days. Once severe diagnoses have been excluded, children may be managed with clinical and laboratory follow up in 2-3 weeks.¹⁰ For bacterial myositis, rehabilitation begins after infection control (i.e. antibiotics and drainage) with early mobilization, range of motion, and physical therapy to restore function and prevent contractures.

At different disease stages

• New onset/acute (viral): rest, activity modification, and analgesia. Encourage gentle range of motion of affected muscles and joints as tolerated. Monitor gait and pain daily; most children resume walking within 3 days.^3,13
• New onset/acute (bacterial): following initiation of antibiotics +/- surgical drainage, begin passive range of motion to prevent contractures. Weight bearing as tolerated once pain and inflammatory markers improve.
• Subacute: gradual return to full activity with supervised mobilization. Physical therapy for strengthening if residual functional deficits persist.³
• Chronic/stable: address any sequelae (contractures, deconditioning) with targeted physical and occupational therapy. Monitor full return to age appropriate activities.

Coordination of care

Effective care involves collaboration among the pediatrician, neurologist, infectious disease specialist, and rehabilitation team. Rapid diagnostics, targeted antibiotics, hydration, and gradual mobilization are monitored clinically and with labs. Clinical follow-up can be arranged with full clinical and laboratory recovery expected at 2 weeks.³

Patient & family education

Patient and family education emphasizes rest, hydration, and symptom management. Families should encourage the use of antipyretics and pain relief measures as directed and closely monitor for worsening pain, fever, inability to walk, muscle swelling, or dark urine. Reassurance is important, as most children recover fully without lasting complications.

Measurement of treatment outcomes

Outcomes are assessed through impairment, activity and participation, and environmental support. Impairment-based measures include muscle strength testing using the Manual Muscle Test (MMT), functional endurance via the Timed Up and Go (TUG) or 2-Minute or 6-Minute Walk Test, range of motion with goniometry, neurological examination, and laboratory markers such as creatine kinase and inflammatory markers to monitor disease activity. Activity-based outcomes evaluate mobility, gait, and the ability to perform age-appropriate tasks using the Childhood Myositis Assessment Scale (CMAS) and the Childhood Health Assessment Questionnaire (CHAQ).⁴

Translation into practice: Practice “pearls”/performance improvement in practice (PIPs)/changes in clinical practice behaviors and skills

BACM is a clinical diagnosis based on pattern recognition: the characteristic trial of post viral bilateral calf (leg) pain, gait disturbance, and moderately elevated CK in a school aged child, confirmed by self-resolution within 3 to 5 days; this is not strictly a diagnosis of exclusion.¹⁰ True muscle weakness is absent in BACM; only pain limiting function occurs. Actual weakness should prompt evaluation for inflammatory or dystrophic myopathies. Red flags warranting further workup include age under 2 years, family history of neuromuscular disorders, recurrent episodes of abnormal neurological findings, dark urine, or abnormal renal function.⁴ Unnecessary MRI or muscle biopsies should avoided in typical presentations.

Cutting Edge/Emerging and Unique Concepts and Practice

New technology has paved way for identifying the pathogen and gene expression of the pathogens in tissue. Molecular sequencing, proteomic-based mass spectrometry, and bioinformatics enhance pathogen identification and clarify post-translational modifications, helping to understand infection mechanisms and guiding future therapies and prevention.¹⁰

Gaps in the Evidence-Based Knowledge

No study has directly evaluated whether influenza vaccination reduces BACM incidence. Standardized diagnostic criteria and CK thresholds distinguish BACM from metabolic or dystrophy myopathies are lacking. Most evidence derives from small case series; prospective studies are needed to define vaccination’s preventive role and optimal follow up protocols.^10,12

References

1. Oboli VN, Baum CR. Infectious Myositis. Pediatr Rev. 2025;46(11):663-665. doi:10.1542/pir.2024-006634
2. Rennie LM, Hallam NF, Beattie TF. Benign acute childhood myositis in an accident and emergency setting. Emerg Med J. 2005;22(10):686-688.
3. Magee H, Goldman RD. Viral myositis in children. Can Fam Physician. 2017;63(5):365-368..
4. Brisca G, Mariani M, Pirlo D, et al. Management and outcome of benign acute childhood myositis in pediatric emergency department. Ital J Pediatr. 2021;47:57. doi:10.1186/s13052-021-01002-x
5. Narayanappa G, Nandeesh BN. Infective myositis. Brain Pathol. 2021;31(3):e12950. doi:10.1111/bpa.12950
6. Attaianese F, Costantino A, Benucci C, Lasagni D, Trapani S. Benign acute children myositis: 5 years experience in a tertiary care pediatric hospital. Eur J Pediatr. 2023;182(10):4341-4349. doi:10.1007/s00431-023-05115-9
7. Weber S, Schlaeppi C, Barbey F, et al. Clinical Characteristics and Management of Children and Adolescents Hospitalized With Pyomyositis. Pediatr Infect Dis J. 2024;43(9):831-840. doi:10.1097/INF.0000000000004382
8. Crum-Cianflone NF. Bacterial, fungal, parasitic, and viral myositis. Clin Microbiol Rev. 2008;21(3):473-494. doi:10.1128/CMR.00001-08
9. Majava E, Renko M, Kuitunen I. Benign acute childhood myositis: a scoping review of clinical presentation and viral etiology. Eur J Pediatr. 2024;183(11):4641-4647. doi:10.1007/s00431-024-05786-y
10. Kerr J, Macartney K, Britton PN. Influenza-associated myositis: a single-centre, 5-year retrospective study. Eur J Pediatr. 2021;180(2):577-584. doi:10.1007/s00431-020-03835-w
11. D’Amico S, Gangi G, Barbagallo M, et al. Benign acute childhood myositis: our experience on clinical evaluation. Neuropediatrics. 2022;53(6):418-422. doi:10.1055/a-1792-7606
12. Rider LG, Miller FW. Deciphering the clinical presentations, pathogenesis, and treatment of the idiopathic inflammatory myopathies. JAMA. 2011;305(2):183-90. doi:10.1001/jama.2010.1977

Author Disclosure

Esther Quinones Budel, MD
Nothing to Disclose

Kayla Togneri, BA
Nothing to Disclose

Paola Gimenez, MD, MS
Nothing to Disclose

Ana Ortiz-Santiago, MD
Nothing to Disclose