myositis

DEFINITION of myositis

myositis is inflammation of skeletal muscle tissue, producing muscle weakness, tenderness, and swelling, and is distinguished from simple myalgia (muscle pain without documented inflammation) and from rhabdomyolysis (acute muscle breakdown releasing myoglobin, which may or may not be inflammatory in origin). The underlying mechanism involves infiltration of muscle fibers by inflammatory cells (neutrophils in infective forms, lymphocytes/macrophages in autoimmune forms), leading to fiber necrosis and, over time, fibrosis or atrophy if untreated. myositis can be infective (bacterial pyomyositis, viral, parasitic), autoimmune/idiopathic (polymyositis, dermatomyositis), traumatic, or drug-induced (statin-induced myositis). Clinically it presents with proximal muscle weakness, elevated creatine kinase (CK), and, in autoimmune forms, may be accompanied by characteristic skin findings (heliotrope rash, Gottron’s papules in dermatomyositis) or systemic/pulmonary involvement.

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ETYMOLOGY of myositis

greek From Greek mys (μῦς, genitive myos), meaning “muscle” (literally “mouse,” from the resemblance of a flexing muscle to a small animal moving under the skin) + -itis (Greek “-ῖτις”), meaning “inflammation.” The combining form o- (also seen as myos-) appears throughout muscular terminology: myopathy (myo- + -pathy, “muscle disease”), myalgia (myo- + -algia, “muscle pain”), and myoma (myo- + -oma, “muscle tumor”).

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ALIASES / ALTERNATE TERMS

Term	Relationship
Polymyositis	Autoimmune subtype affecting multiple muscle groups without skin involvement (M33.20-M33.22)
Dermatomyositis	Autoimmune subtype with characteristic skin rash plus muscle inflammation (M33.10-M33.12)
Pyomyositis	Bacterial (often S. aureus) infective myositis, frequently abscess-forming; coded as infective myositis (M60.00-M60.09)
Interstitial myositis	Chronic, low-grade inflammatory infiltrate between muscle fibers, not localized to an abscess (M60.10-M60.19)

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RELATED TERMS

• myopathy — broader umbrella term for any muscle disease, not necessarily inflammatory (e.g., metabolic or genetic myopathies).
• myalgia — muscle pain without documented inflammation; coded separately (M79.1) and should not be used interchangeably with myositis.
• rhabdomyolysis — acute breakdown of muscle tissue releasing myoglobin; may occur secondary to severe myositis but is a distinct process requiring its own code.
• fasciitis — inflammation of the fascia surrounding muscle rather than the muscle fibers themselves.
• dermatomyositis — autoimmune myositis with cutaneous involvement.
• polymyositis — autoimmune myositis without skin involvement.
• Statin-induced myopathy/myositis — drug-induced form; requires an associated T-code for the causative substance.

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CODING CORNER

🏥 ICD-10-CM CODES

Myositis, Unspecified/Other (M60.8-, M60.9 — Site and Laterality Required)

Code	Description
M60.9	Myositis, unspecified
M60.80	Other myositis, unspecified site
M60.811	Other myositis, right shoulder
M60.812	Other myositis, left shoulder
M60.821	Other myositis, right upper arm
M60.822	Other myositis, left upper arm
M60.851	Other myositis, right thigh
M60.852	Other myositis, left thigh
M60.861	Other myositis, right lower leg
M60.862	Other myositis, left lower leg
M60.88	Other myositis, other site
M60.89	Other myositis, multiple sites

Infective Myositis (M60.0- — e.g., Pyomyositis)

Code	Description
M60.00	Infective myositis, unspecified site
M60.011	Infective myositis, right shoulder
M60.012	Infective myositis, left shoulder
M60.051	Infective myositis, right thigh
M60.052	Infective myositis, left thigh
M60.061	Infective myositis, right lower leg
M60.062	Infective myositis, left lower leg
M60.08	Infective myositis, other site
M60.09	Infective myositis, multiple sites

Interstitial Myositis (M60.1-)

Code	Description
M60.10	Interstitial myositis, unspecified site
M60.19	Interstitial myositis, multiple sites

Autoimmune Myopathies (Polymyositis/Dermatomyositis — Organ Involvement Required)

Code	Description
M33.20	Polymyositis, organ involvement unspecified
M33.21	Polymyositis with respiratory involvement
M33.22	Polymyositis with myopathy
M33.10	Other dermatomyositis, organ involvement unspecified
M33.12	Other dermatomyositis with myopathy
M33.90	Dermatopolymyositis, unspecified, organ involvement unspecified

Companion Codes (When Applicable)

Code	Description
B95.61	MRSA infection as the cause of disease classified elsewhere (use with infective myositis if organism identified)
B95.62	MSSA infection as the cause of disease classified elsewhere
M79.1	Myalgia (use instead of myositis when only pain, no inflammation, is documented)

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🔧 COMMON CPT CODES

CPT Code	Description
20200	Biopsy, muscle; superficial
20205	Biopsy, muscle; deep
20206	Biopsy, muscle, percutaneous needle
20005	Incision and drainage of soft tissue abscess, subfascial (deep), intramuscular (used for pyomyositis abscess)
95860	Needle electromyography; one extremity with or without related paraspinal areas
95861	Needle electromyography; two extremities with or without related paraspinal areas
82550	Creatine kinase (CK), (CPK); total (supports diagnosis/monitoring)
85652	Sedimentation rate, erythrocyte; automated
86140	C-reactive protein

Modifiers: -LT/-RT (laterality), -50 (bilateral procedure), -59 (distinct procedural service).

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⚠️ Coding Note: M60.0-, M60.1-, and M60.8- categories require full site and laterality specificity (“myositis” or “muscle pain” alone in the documentation is insufficient — query for laterality if not stated, defaulting to M60.9 only when truly unspecified). When an infective organism is identified for pyomyositis, sequence the myositis code first followed by the applicable B95-B96 organism code as an additional diagnosis. Distinguish autoimmune myositis (M33 polymyositis/dermatomyositis, which require a 5th-character organ-involvement digit) from infective or traumatic myositis (M60), as these map to different clinical contexts and different HCC/MDC groupings. Watch for “generalized weakness” or “muscle pain” documented without a stated inflammatory process — this should prompt a physician query rather than automatic assignment of a myositis code, since M79.1- (myalgia) is the correct default absent documented inflammation. If pyomyositis progresses to sepsis, follow sequencing guidelines for sepsis due to a localized infection (sepsis code first, followed by the localized infective myositis code, per current guidelines).

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DERIVATIONS of myositis

TABLE definition AS Definition
FROM #medterm 
WHERE length(filter(roots, (word) => econtains([[myositis]].roots, word))) > 0 AND file.name != [[myositis]].file.name
SORT file.name ASC

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Query functionality

TABLE definition AS Definition
FROM #medterm 
WHERE file.name != this.file.name
AND any(contains(this.definition, definition))

Med terms dictionary Appendix A Prefixes Appendix B Combining Forms Appendix C Suffixes Appendix D Suffix forms