G04.91 ICD-10-CM Code: Myelitis, unspecified

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for G04.91 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on G04.91 itself; “inherited from” names the category or block whose note applies here.

Coder workflow for G04.91

MedCoder structured workflow — derived from this code’s own official record

Before you code G04.91

1. Unspecified does not mean incorrect. When the record gives no greater specificity, G04.91 may be the appropriate code. Check the record for detail that supports a more specific sibling. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

   See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with G04.91. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

   See the official tabular notes

Choose the right path

1. Does the record document the detail a more specific sibling code needs?
   Yes → Review the specific siblings in this subcategory.
   No → Continue — G04.91 is appropriate when the documentation goes no further.
2. Does the documentation support a condition named in G04.91’s Excludes1 note?
   Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
   No → Continue.

   ReviewG93.40

Consider G04.91. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Any detail beyond this code’s title

What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

• Excludes1 — check before selecting G04.91(1 note)

  Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with G04.91: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

  CompareG93.40

  See the official tabular notes · Guidelines I.A.12.a

• Excludes2 — not part of G04.91(7 notes)

  Coding workflow: The conditions named in this note are not included in G04.91. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

  CompareG37.3, G31.2, G35, G93.32, G37.4, G92.8

  See the official tabular notes · Guidelines I.A.12.b

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (3)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Infection, infected, infective (opportunistic), spinal cord NOS
• Myelitis (acute) (ascending) (childhood) (chronic) (descending) (diffuse) (disseminated) (idiopathic) (pressure) (progressive) (spinal cord) (subacute)
• Myeloradiculitis

Don’t confuse G04.91 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of G04.91 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• G93.40— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of encephalopathy NOS moves the case to G93.40; the two are not reported together unless documented as unrelated.

  Full comparison: G04.91 vs G93.40 →

• G37.3— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of acute transverse myelitis adds G37.3; it is not part of this code and both may be reported when both are documented.

  Full comparison: G04.91 vs G37.3 →

• G31.2— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of alcoholic encephalopathy adds G31.2; it is not part of this code and both may be reported when both are documented.

  Full comparison: G04.91 vs G31.2 →

• G35— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of multiple sclerosis adds G35; it is not part of this code and both may be reported when both are documented.

  Full comparison: G04.91 vs G35 →

• G93.32— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of myalgic encephalomyelitis adds G93.32; it is not part of this code and both may be reported when both are documented.

  Full comparison: G04.91 vs G93.32 →

• G37.4— Excludes2 relationship (Informational)

  Distinct conditions — both may be reported.

  A distinct condition — both may be reported when both are documented.

  What moves you there: Documentation of subacute necrotizing myelitis adds G37.4; it is not part of this code and both may be reported when both are documented.

  Full comparison: G04.91 vs G37.4 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Verify Before Coding

• MCC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name G04.91 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes1 note: G03 — Meningitis due to other and unspecified causes (via G04.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

Referenced by 1 Excludes2 note: G95 — Other and unspecified diseases of spinal cord (via G04.-).

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: MCC — Major Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 191 clinically related codes on its CMS exclusion list. Does not count toward severity when the stay groups to DRGs 974-976 (HIV with Major Related Condition).

Named in the grouper logic of 8 MS-DRGs: DRG 023 (MDC 01), DRG 024 (MDC 01), DRG 097 (MDC 01), DRG 098 (MDC 01), DRG 099 (MDC 01), DRG 974 (MDC 25), DRG 975 (MDC 25), DRG 976 (MDC 25).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):NVS002 — Encephalitis (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as MCC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

B54 — Unspecified malaria, B58.2 — Toxoplasma meningoencephalitis, B75 — Trichinellosis, G04.00 — Acute disseminated encephalitis and encephalomyelitis, unspecified, G04.01 — Postinfectious acute disseminated encephalitis and encephalomyelitis (postinfectious ADEM), G04.31 — Postinfectious acute necrotizing hemorrhagic encephalopathy, G04.81 — Other encephalitis and encephalomyelitis, G04.82 — Acute flaccid myelitis, G04.89 — Other myelitis, G04.90 — Encephalitis and encephalomyelitis, unspecified, G37.0 — Diffuse sclerosis of central nervous system, G37.4 — Subacute necrotizing myelitis of central nervous system, G37.5 — Concentric sclerosis [Balo] of central nervous system, H66.40 — Suppurative otitis media, unspecified, unspecified ear, H66.41 — Suppurative otitis media, unspecified, right ear, H66.42 — Suppurative otitis media, unspecified, left ear, H66.43 — Suppurative otitis media, unspecified, bilateral, J09.X3 — Influenza due to identified novel influenza A virus with gastrointestinal manifestations, J09.X9 — Influenza due to identified novel influenza A virus with other manifestations, J10.2 — Influenza due to other identified influenza virus with gastrointestinal manifestations, +170 more

Same CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Spinal Cord Disorders/Injuries) for risk-adjusted payment.

B00.82 — Herpes simplex myelitis, B01.12 — Varicella myelitis, B02.24 — Postherpetic myelitis, G04.1 — Tropical spastic paraplegia, G04.82 — Acute flaccid myelitis, G04.89 — Other myelitis, G05.4 — Myelitis in diseases classified elsewhere, G14 — Postpolio syndrome, G32.0 — Subacute combined degeneration of spinal cord in diseases classified elsewhere, G32.81 — Cerebellar ataxia in diseases classified elsewhere, G37.3 — Acute transverse myelitis in demyelinating disease of central nervous system, G37.4 — Subacute necrotizing myelitis of central nervous system, G83.4 — Cauda equina syndrome, G90.1 — Familial dysautonomia [Riley-Day], G95.0 — Syringomyelia and syringobulbia, G95.11 — Acute infarction of spinal cord (embolic) (nonembolic), G95.19 — Other vascular myelopathies, G95.20 — Unspecified cord compression, G95.29 — Other cord compression, G95.81 — Conus medullaris syndrome, +230 more

Related risk categories

These categories interact through CMS's HCC hierarchy — one can suppress the other's risk-adjustment weight when both are present on a claim.

Quadriplegia, Paraplegia

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Encephalitis).

B10.09 — Other human herpesvirus encephalitis, B26.2 — Mumps encephalitis, B40.81 — Blastomycotic meningoencephalitis, B57.42 — Meningoencephalitis in Chagas' disease, B58.2 — Toxoplasma meningoencephalitis, B60.11 — Meningoencephalitis due to Acanthamoeba (culbertsoni), G04.00 — Acute disseminated encephalitis and encephalomyelitis, unspecified, G04.01 — Postinfectious acute disseminated encephalitis and encephalomyelitis (postinfectious ADEM), G04.02 — Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis, G04.2 — Bacterial meningoencephalitis and meningomyelitis, not elsewhere classified, G04.30 — Acute necrotizing hemorrhagic encephalopathy, unspecified, G04.31 — Postinfectious acute necrotizing hemorrhagic encephalopathy, G04.32 — Postimmunization acute necrotizing hemorrhagic encephalopathy, G04.39 — Other acute necrotizing hemorrhagic encephalopathy, G04.81 — Other encephalitis and encephalomyelitis, G04.82 — Acute flaccid myelitis, G04.89 — Other myelitis, G04.90 — Encephalitis and encephalomyelitis, unspecified, G05.3 — Encephalitis and encephalomyelitis in diseases classified elsewhere, G05.4 — Myelitis in diseases classified elsewhere, +35 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Myelitis”; these codes share that main term but sit in a different category of the Tabular List.

A17.82 — Tuberculous meningoencephalitis (tuberculous), A52.14 — Late syphilitic encephalitis (syphilitic), B00.82 — Herpes simplex myelitis (herpes simplex), B01.12 — Varicella myelitis (varicella), B02.24 — Postherpetic myelitis (postherpetic), G05.4 — Myelitis in diseases classified elsewhere (in diseases classified elsewhere), G36.0 — Neuromyelitis optica [Devic] (optic neuritis in), G37.3 — Acute transverse myelitis in demyelinating disease of central nervous system (transverse), G37.4 — Subacute necrotizing myelitis of central nervous system (necrotizing, subacute), G92.9 — Unspecified toxic encephalopathy (toxic)

Contextual Map

Every relationship of G04.91 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run G04.91 with these 2 related codes in Claim Check

Hierarchy

• G00-G99 — Chapter 6: Diseases of the Nervous System (G00-G99) (G00-G99)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes1 notes

• G03 — Meningitis due to other and unspecified causes[Excludes1](via G04.-): “meningoencephalitis (G04.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Referenced by Excludes2 notes

• G95 — Other and unspecified diseases of spinal cord[Excludes2](via G04.-): “myelitis (G04.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Clinical classification (CCSR)

• NVS002 — Encephalitis[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

Risk adjustment (CMS-HCC)

• HCC 182 — Spinal Cord Disorders/Injuries [CMS-HCC]— CMS-HCC V28 · 2026

MS-DRG Grouper

• MCC — Major Complication or Comorbidity [MS-DRG severity]: “As a secondary diagnosis this code can raise the stay's MS-DRG severity tier (MCC). Not counted when the stay groups to DRGs 974-976 (HIV with Major Related Condition).”— CMS MS-DRG Definitions Manual (Appendix C) · FY2026
• DRG 023 — CRANIOTOMY WITH MAJOR DEVICE IMPLANT OR ACUTE COMPLEX CNS PRINCIPAL DIAGNOSIS WITH MCC OR ANTINEOPLASTIC IMPLANT OR EPILEPSY WITH NEUROSTIMULATOR[MS-DRG]: “CRANIOTOMY WITH MAJOR DEVICE IMPLANT OR ACUTE COMPLEX CNS PRINCIPAL DIAGNOSIS WITH MCC OR ANTINEOPLASTIC IMPLANT OR EPILEPSY WITH NEUROSTIMULATOR (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 024 — CRANIOTOMY WITH MAJOR DEVICE IMPLANT OR ACUTE COMPLEX CNS PRINCIPAL DIAGNOSIS WITHOUT MCC[MS-DRG]: “CRANIOTOMY WITH MAJOR DEVICE IMPLANT OR ACUTE COMPLEX CNS PRINCIPAL DIAGNOSIS WITHOUT MCC (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 097 — NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITH MCC[MS-DRG]: “NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITH MCC (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 098 — NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITH CC[MS-DRG]: “NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITH CC (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 099 — NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITHOUT CC/MCC[MS-DRG]: “NON-BACTERIAL INFECTION OF NERVOUS SYSTEM EXCEPT VIRAL MENINGITIS WITHOUT CC/MCC (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 974 — HIV WITH MAJOR RELATED CONDITION WITH MCC[MS-DRG]: “HIV WITH MAJOR RELATED CONDITION WITH MCC (MDC 25)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 975 — HIV WITH MAJOR RELATED CONDITION WITH CC[MS-DRG]: “HIV WITH MAJOR RELATED CONDITION WITH CC (MDC 25)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• and 1 more

MDC crossing

• MDC 01 — Diseases and Disorders of the Nervous System[MDC crossing]: “Diseases and Disorders of the Nervous System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 8,892 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026
• MDC 25 — Human Immunodeficiency Virus Infections[MDC crossing]: “Human Immunodeficiency Virus Infections — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

• Infection, infected, infective (opportunistic), spinal cord NOS[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Myelitis (acute) (ascending) (childhood) (chronic) (descending) (diffuse) (disseminated) (idiopathic) (pressure) (progressive) (spinal cord) (subacute)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
• Myeloradiculitis[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (18)

• G04 — Encephalitis, myelitis and encephalomyelitis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.0 — Acute disseminated encephalitis and encephalomyelitis (ADEM)[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.00 — Acute disseminated encephalitis and encephalomyelitis, unspecified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.01 — Postinfectious acute disseminated encephalitis and encephalomyelitis (postinfectious ADEM)[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.02 — Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.1 — Tropical spastic paraplegia[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.2 — Bacterial meningoencephalitis and meningomyelitis, not elsewhere classified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G04.3 — Acute necrotizing hemorrhagic encephalopathy[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• and 10 more

Change history

• FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Risk adjustment Official source data

2026 Mid-Year Final ICD-10 Mappings + Model Software (cms.gov/files/zip/2026-midyear-final-icd-10-mappings.zip, 2026-midyear-final-model-software.zip) — CMS-HCC V28, PY2026 mid-year final release Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to G04.91 in its code family, with their registry titles.

• G04.30 — Acute necrotizing hemorrhagic encephalopathy, unspecified
• G04.31 — Postinfectious acute necrotizing hemorrhagic encephalopathy
• G04.32 — Postimmunization acute necrotizing hemorrhagic encephalopathy
• G04.39 — Other acute necrotizing hemorrhagic encephalopathy
• G04.8 — Other encephalitis, myelitis and encephalomyelitis
• G04.81 — Other encephalitis and encephalomyelitis
• G04.82 — Acute flaccid myelitis
• G04.89 — Other myelitis
• G04.9 — Encephalitis, myelitis and encephalomyelitis, unspecified
• G04.90 — Encephalitis and encephalomyelitis, unspecified

View all codes in the G04 family