ICD-10 code M35.4: Diffuse (eosinophilic) fasciitis

Key Takeaways

ICD-10 code M35.4 is the billable diagnosis code for diffuse (eosinophilic) fasciitis, also known as Shulman syndrome

M35.4 remains valid and billable for FY2026 (October 1, 2025 through September 30, 2026) and has carried this description since FY2016, unchanged this year

Documentation must show peripheral eosinophilia and fascial inflammation. Use additional codes for associated manifestations

Practice management software like Pabau helps clinicians document and submit M35.4 accurately, reducing claim errors

Diffuse (eosinophilic) fasciitis rarely shows up in a coder’s queue, and that’s part of the problem. ICD-10 code M35.4 is the specific, billable code for this fascia disorder, sitting under the broader M35 connective tissue category. Because so few practices submit it, missing documentation or a wrong secondary code can hold up a claim longer than it should.

Here’s what the code covers, how to document it, and where billing usually goes wrong.

ICD-10 code M35.4: Quick reference and code details

ICD-10 code M35.4 identifies diffuse (eosinophilic) fasciitis, a rare inflammatory condition affecting the fascial layer beneath the skin. The code is billable under the ICD-10-CM system and valid for fiscal year 2026 reimbursement.

Coders and clinicians use this code when documentation confirms eosinophilic fasciitis or its alternate name, Shulman syndrome, a condition that sits within the broader M35 connective tissue group.

According to the Centers for Medicare and Medicaid Services (CMS), M35.4 remains valid and billable for FY2026, the code set running October 1, 2025 through September 30, 2026. The code has been part of ICD-10-CM since FY2016 and carries no changes for FY2026.

The table below summarizes the core code attributes coders need at a glance.

What is diffuse (eosinophilic) fasciitis?

Diffuse (eosinophilic) fasciitis is a rare connective tissue disorder. It causes inflammation of the deep fascia along with marked peripheral eosinophilia. The condition brings progressive skin thickening, usually on the arms and legs, and it shares some clinical overlap with scleroderma.

Even so, the two are distinct diagnoses with separate ICD-10 codes. ICD-10 code M35.4 captures the eosinophilic fasciitis presentation specifically, setting it apart from other connective tissue syndromes in the M35 group.

These clinical findings support an M35.4 diagnosis:

• Peripheral eosinophilia on lab testing
• Fascial inflammation confirmed on biopsy
• Characteristic skin thickening on physical exam

Clinicians at dermatology practices see this code most often, alongside rheumatology teams working from the same biopsy and eosinophil-count workup. Physical therapy practices often step in once skin thickening limits joint movement, and their functional notes can support ongoing medical necessity. Eosinophilic fasciitis is rare, so payers scrutinize these claims more closely than common diagnoses.

What are the synonyms and alternate names for M35.4?

Only one term is formally indexed for this code. Recognizing the informal eponym matters too, since coders often see it in referral letters instead of the descriptor.

• Eosinophilic fasciitis – the term listed in the ICD-10-CM alphabetic index, under “Fasciitis, diffuse (eosinophilic)”
• Shulman syndrome – a widely used clinical eponym, after Dr. L.E. Shulman who first described the condition in 1974; not a formally indexed synonym, but common in referral letters and older literature

When a clinician’s notes reference Shulman syndrome rather than the full condition name, M35.4 is still the correct code to assign. The CDC/NCHS ICD-10-CM index tool confirms eosinophilic fasciitis as the indexed term, so coders can map the eponym to it with confidence. Don’t default to an unspecified connective tissue code just because the note uses the eponym instead of the descriptor.

Where M35.4 sits in the M35 connective tissue code family

ICD-10 code M35.4 sits within the M35 subcategory, other systemic involvement of connective tissue, itself part of the M30-M36 block for systemic connective tissue disorders. Knowing the sibling codes helps coders pick the most specific, accurate code when documentation describes overlapping features or a differential diagnosis comes up.

M35.4 is the only specific code for eosinophilic fasciitis. When documentation doesn’t confirm the diagnosis clearly, M35.9 (unspecified) becomes the fallback. Query the clinician before defaulting to the unspecified code. M35.4 gives payers clearer clinical detail for claim adjudication.

Related codes coders sometimes confuse with M35.4

Several codes outside the M35 group come up often alongside ICD-10 code M35.4. Coders should know which ones work as additional codes, and which represent distinct conditions needing a separate query.

The WHO ICD-10 browser shows the parent classification hierarchy for these codes, useful for understanding how eosinophilic fasciitis fits into international coding standards. The ICD-10-CM version used in the US differs from the international ICD-10 in code-level detail, so confirm compliance-aligned documentation workflows apply the correct version for payer submissions.

What documentation does an M35.4 claim need?

Assigning ICD-10 code M35.4 accurately requires specific clinical evidence in the medical record. Because eosinophilic fasciitis is rare, claims submitted with this code face closer audit scrutiny.

The ICD-10-CM Official Guidelines for Coding and Reporting come from CMS and the National Center for Health Statistics (NCHS). They require the treating provider to confirm the diagnosis before a coder assigns it.

The documentation elements below support a clean M35.4 claim. Missing any of them creates audit exposure, particularly for Medicare and commercial payers.

• Peripheral eosinophilia: lab report showing elevated eosinophil count (typically above 1,500 cells/microlitre); document the actual value, not just “elevated WBC”
• Fascial biopsy result: pathology report confirming eosinophilic infiltration of the deep fascia; the biopsy site and depth should be specified in the report
• Skin induration findings: clinical exam notes describing skin thickening, woody texture, or restricted movement, so the physical exam reflects the diagnosis
• Provider attestation: the treating clinician must have documented the confirmed diagnosis; coders cannot assign M35.4 from labs or imaging alone without a provider statement
• Additional code use: if eosinophilia is separately documented as a distinct finding, D72.10 may be added as a secondary code per ICD-10-CM guidelines

Digital intake forms capture the documentation fields rare autoimmune conditions need at the point of care, instead of reconstructing them later. Practices managing rheumatology or dermatology patients benefit from templates that prompt for the specific findings behind codes like M35.4.

Pabau’s client record system lets clinicians attach biopsy results, lab values, and exam findings directly to the encounter, building the paper trail payers require.

Reduce claim errors on rare diagnostic codes

Pabau helps dermatology and rheumatology practices document the specific clinical findings that support codes like ICD-10 code M35.4, keeping claims clean and reducing audit risk.

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Billing and reimbursement rules for M35.4

ICD-10 code M35.4 is a billable, specific code. It’s valid for claim submission and reimbursement without needing a more specific fourth or fifth character.

The code remains unchanged for FY2026, the annual update cycle running October 1, 2025 through September 30, 2026, and it has carried this description since FY2016. Coders should still verify currency each October, since other codes do change annually.

The billing considerations below matter most for M35.4 claims.

• Payer prior authorization: some commercial payers require prior authorization for rare connective tissue disorder workups; verify payer-specific requirements before scheduling biopsy or specialized testing
• Medical necessity documentation: claims may be denied if the record lacks objective findings; the documentation checklist in the section above directly maps to medical necessity requirements
• Outpatient vs. inpatient: M35.4 may be used in both settings; for inpatient, it can serve as principal diagnosis if it is the condition chiefly responsible for admission
• Inherited Excludes1 note: M35.4 has no code-specific Excludes note, but it inherits the M35 category Excludes1: don’t code reactive perforating collagenosis (L87.1) alongside M35.4

Practices using integrated claims management software can flag encounters where rare autoimmune codes get selected, then prompt coders to confirm documentation before submission. This matters most for low-volume specialists who might submit M35.4 only a handful of times a year.

The AAPC ICD-10-CM code lookup adds crosswalk references and coding notes that supplement the official CMS guidelines.

How Pabau supports accurate ICD-10 code M35.4 documentation

Rare connective tissue disorder codes demand more documentation precision than common diagnoses. A single missing lab value or absent provider attestation can stall a claim for weeks.

Pabau’s practice management platform supports clinicians with HIPAA-compliant clinical documentation workflows, so the structured findings that codes like M35.4 need get captured consistently.

Clinicians can build encounter templates that prompt for peripheral eosinophilia values, biopsy references, and physical exam findings. Everything stays stored within the patient record and ready at the point of coding. The platform’s claims workflows let practices attach supporting lab documents directly to the encounter before submission, cutting down the back-and-forth with payers on rare autoimmune claims.

For practices managing ongoing connective tissue conditions, secure data management keeps longitudinal records accessible for re-audit or appeals without manual file retrieval.

Getting M35.4 documentation and billing right

ICD-10 code M35.4 is a specific, billable code for diffuse (eosinophilic) fasciitis. It remains valid for FY2026 claims and has stayed part of ICD-10-CM since FY2016, with no changes to the M35 family this year.

The two most common coding errors are missing Shulman syndrome as an alternate name and submitting without complete lab and biopsy documentation. Both risks shrink once clinical workflows are set up to capture the right evidence at the point of care.

Pabau’s clinical documentation and claims management tools help practices handling rare autoimmune diagnoses build the paper trail that supports clean M35.4 submissions.

To see how the platform handles structured documentation for specialist conditions, book a demo.

Continue your research

Looking up another musculoskeletal code? ICD-10 code M16.9 covers unspecified osteoarthritis of the hip, a common differential when joint mobility is affected.

Need a soft-tissue procedure code for a biopsy? CPT code 11006 covers debridement of skin, subcutaneous tissue, and fascia, relevant when fascial biopsy leads to further debridement.

Coding a drug-induced connective tissue presentation? ICD-10 code M34.2 covers systemic sclerosis induced by drugs and chemicals, a related differential in the same code family.

Frequently asked questions