Infective myositis

Disease Description (Causes and Symptoms)

• Definition: Infective myositis is a condition in which bacteria, viruses, parasites, or, rarely, fungi invade muscle tissue and cause inflammation. It can range from localized inflammation to abscess formation and systemic infection.
• Main causes: Direct invasion (wounds or surgical sites), contamination related to surgery or injections, and hematogenous spread (spread of systemic infection through the bloodstream). Common pathogens include Staphylococcus aureus, group A Streptococcus, gram-negative bacteria, Clostridium, Mycobacterium tuberculosis, viruses such as influenza and enteroviruses, and parasites such as Trichinella.
• Pathophysiology summary: When pathogens enter the muscle, a local inflammatory response (neutrophil and macrophage infiltration, vasodilation, and edema) occurs. In severe cases, this can lead to necrosis, abscesses, and muscle fiber damage, resulting in decreased muscle strength and loss of function. In some cases, there is a risk of a systemic toxic response (sepsis) or rhabdomyolysis due to muscle destruction (→ acute kidney injury).
• Main symptoms: Localized pain and tenderness, redness, warmth, and swelling; localized skin inflammation or rash; systemic fever and chills; and restricted movement or weakness of the affected muscle. If an abscess develops, localized pus discharge or palpable tenderness may occur.

Treatment Methods

• Initial evaluation and classification: Mild (localized, without systemic symptoms) vs. severe (high fever, systemic toxicity, abscess, muscle weakness, or suspected hematogenous infection).
• Medication treatment:
  • If bacterial infection is suspected: Administer empirical antibiotics (selected according to severity, source of infection, and local resistance patterns; for example, including coverage for Staphylococcus aureus and Streptococcus). Adjust antibiotics based on blood and drainage culture results (targeted therapy). Intravenous (IV) antibiotics are recommended for patients with severe disease, systemic symptoms, or immunosuppression.
  • Viral myositis: Generally conservative treatment (rest, hydration, and pain control). Antiviral medication may be considered depending on the causative virus (e.g., influenza).
  • Special pathogens such as parasites and tuberculosis require long-term medication tailored to the specific pathogen.
• Procedures/Surgery:
  • If an abscess is confirmed, image-guided drainage (invasive aspiration or catheter placement) or surgical drainage is required. Surgical removal (debridement) may be necessary if necrotic tissue is present.
• Conservative/Supportive treatment:
  • Analgesics/anti-inflammatory drugs, adequate rest and gradual rehabilitation (physical therapy), and fluid and electrolyte management (aggressive fluid therapy if rhabdomyolysis is suspected, with monitoring of uric acid and creatine kinase).
• Follow-up: Monitor symptom improvement, blood tests (CRP, WBC), conversion to negative cultures, and reduction of the abscess on imaging to determine the treatment duration and route.
• Precautions: Immunosuppressed patients, patients with diabetes, and the presence of longstanding lesions increase the risk of treatment failure and recurrence — consider early cultures, appropriate drainage, and long-term antibiotic therapy.

Checklist Before an Outpatient Visit (Including Example Questions)

• Items to prepare (bring to the appointment):
  • A list of currently used medications (including prescriptions, OTC medications, and supplements) and allergy history
  • Medical history (especially recent surgery, trauma, injections, or infections), and whether diabetes or immunosuppression is present
  • Recent test results (blood tests, imaging tests, etc.) and photos already taken (of the redness, swelling, or wound area)
  • Records of when symptoms began, changes in body temperature, pain intensity (on a numerical scale), and the degree of movement restriction
• Items to check before the outpatient visit:
  • Presence of fever or systemic symptoms (if present, immediate emergency evaluation is recommended)
  • Drainage, foul odor, or rapid worsening at a wound or surgical site
  • Symptoms suggesting systemic complications, such as decreased urine output, muscle weakness, or dizziness
• Example questions (to use as a note for the doctor):
  • “Could any of the medications I am currently taking cause a problem?”
  • “Do I need to visit the emergency room given my current situation?”

General Example Questions to Ask the Doctor

• Is this antibiotic appropriate for my pathogen? What side effects should I expect?
• Is my condition severe enough to require hospitalization, or can it be treated with oral medication as an outpatient?
• How long will treatment take? How high is the risk of recurrence?
• When can I resume daily activities and exercise? When should I start physical therapy?

Examples of Disease-Specific Questions

• Are my symptoms (e.g., localized pain, fever, and a wound) suggestive enough of an abscess to warrant concern? When should I undergo imaging (ultrasound/CT/MRI)?
• If my blood test shows an elevated CPK (creatine kinase) level, is there a risk of rhabdomyolysis or kidney injury? In this case, what additional tests/treatment are needed?

(Additional guidance) If you have emergency signs (rapidly worsening pain, high fever or low blood pressure, impaired consciousness, decreased urination, or brown urine, etc.), visit the emergency room immediately.