2027 ICD-10-CM Diagnosis Code G04.91Myelitis, unspecified

ICD-10-CM Codes›G00–G99›G00-G09›G04

ICD-10-CM G04.91
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

G04.91 is a billable ICD-10-CM diagnosis code for myelitis, unspecified. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 23 through 24, 97 through 99, 974 through 976. As a secondary diagnosis, it counts as a major complication or comorbidity (MCC) and places an inpatient stay in the highest severity level of its MS-DRG family. It does not count, however, when the principal diagnosis is one of 191 closely related codes. Coders also document this condition as acute ascending myelitis. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis.

For Medicare Advantage risk adjustment, G04.91 maps to CMS-HCC Category 182 (Spinal Cord Disorders/Injuries) under the V28 model, adding a risk factor of about 0.478 for a community, non-dual, aged beneficiary in payment year 2027.

Code Identity

ICD-10-CM Code
G04.91
Billable Status
Yes — Valid for Submission
Code Describes
Myelitis, unspecified
Short Description
Myelitis, unspecified
Same as the full description in the CMS dataset.
Parent Code
G04.9
Encephalitis, myelitis and encephalomyelitis, unspecified

Code Classification

ChapterG00–G99Diseases of the nervous system
SectionG00-G09Inflammatory diseases of the central nervous system
CategoryG04Encephalitis, myelitis and encephalomyelitis
This CodeG04.91Myelitis, unspecified

Medicare Risk Adjustment (HCC)Billing

G04.91 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.

CMS-HCC V28 Category (Payment Model)
HCC 182— Spinal Cord Disorders/Injuries
Payment HCC · PY 2027 one of 251 ICD-10-CM codes in this category
Risk Adjustment Factor (RAF) Weight
+0.478
community, non-dual, aged · ranges 0.270–0.478 across segments
Hierarchy
Superseded by HCC 180, HCC 181, HCC 191, and HCC 192
a more severe related category takes the payment when both are reported
Prior Model (CMS-HCC V24)
HCC 72
V24 retired V28 pays 100% of MA risk scores since PY 2026
Other CMS Models
PACE (CMS-HCC V22): HCC 72 · ESRD (V21): HCC 72 · ESRD (V24): HCC 72
ESRD V21 weights: 0.102 dialysis, 0.256–0.456 functioning graft · ESRD V24 weights: 0.099 dialysis, 0.222–0.523 functioning graft
Part D (RxHCC)
RxHCC 155 — Spinal Cord Disorders
also risk-adjusts in the Part D prescription drug model (V08)

Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute ascending myelitis
  • Encephalitis, myelitis and encephalomyelitis
  • Infection causing myelitis
  • Myelitis
  • Myelitis caused by bacterium
  • Myelitis caused by virus

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to G04.91: its own notes plus those printed at G04 and Chapter 6. A note printed at a category, block or chapter applies to every code under it.

Includes

Further defines, or gives examples of, the content of the category.

From G04 Encephalitis, myelitis and encephalomyelitis applies to 14 codes
  • acute ascending myelitis
  • meningoencephalitis
  • meningomyelitis

Excludes1

Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.

From G04 Encephalitis, myelitis and encephalomyelitis applies to 14 codes

Excludes2

Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.

From G04 Encephalitis, myelitis and encephalomyelitis applies to 14 codes
  • acute transverse myelitis (G37.3)
  • alcoholic encephalopathy (G31.2)
  • multiple sclerosis (G35-)
  • myalgic encephalomyelitis (G93.32)
  • subacute necrotizing myelitis (G37.4)
  • toxic encephalitis (G92.8)
  • toxic encephalopathy (G92.8)
From Chapter 6 (G00-G99) Diseases of the nervous system applies to 700 codes
  • certain conditions originating in the perinatal period (P04-P96)
  • certain infectious and parasitic diseases (A00-B99)
  • complications of pregnancy, childbirth and the puerperium (O00-O9A)
  • congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
  • endocrine, nutritional and metabolic diseases (E00-E88)
  • injury, poisoning and certain other consequences of external causes (S00-T88)
  • neoplasms (C00-D49)
  • symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name G04.91, its category, or a range that includes it.

Excludes1 2

These codes carry an Excludes1 note naming G04.91: they are not reported together with it, unless the two conditions are unrelated.

  • G03 Meningitis due to other and unspecified causes
    meningoencephalitis (G04.-) names G04.-, which includes this code
  • G03 Meningitis due to other and unspecified causes
    meningomyelitis (G04.-) names G04.-, which includes this code

Excludes2 1

These codes carry an Excludes2 note naming G04.91: its condition is not part of those codes, and both may be reported when the patient has both.

  • G95 Other and unspecified diseases of spinal cord
    myelitis (G04.-) names G04.-, which includes this code

Notes 1

These notes mention G04.91.

  • G09 Sequelae of inflammatory diseases of central nervous system
    Category G09 is to be used to indicate conditions whose primary classification is to G00-G08 as the cause of sequelae, themselves classifiable elsewhere. The 'sequelae' include conditions specified as residuals. names G00-G08, which includes this code

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Myelitis

    inflammation of the spinal cord. relatively common etiologies include infections; autoimmune diseases; spinal cord; and ischemia (see also spinal cord vascular diseases). clinical features generally include weakness, sensory loss, localized pain, incontinence, and other signs of autonomic dysfunction.

Patient EducationClinical

Spinal Cord Diseases

Your spinal cord is a bundle of nerves that runs down the middle of your back. It carries signals back and forth between your body and your brain. It is protected by your vertebrae, which are the bone disks that make up your spine.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert G04.91 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
323.9 Encephalitis NOS
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About G04.91Overview

What is the ICD-10 code for myelitis, unspecified?

The ICD-10-CM code for myelitis, unspecified is G04.91 (sometimes written as G0491). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is G04.91 (Encephalitis, myelitis and encephalomyelitis, unspecified) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report myelitis, unspecified on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does G04.91 group to?

When myelitis, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 24, 98, 974, 976, 99, 975, 23, 97, with relative weights from 0.7879 to 5.7422 depending on complications. Higher weights mean higher Medicare reimbursement.

Is G04.91 a CC or MCC?

CMS lists G04.91 as an MCC (major complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it places the inpatient stay in the highest-weighted DRG of its severity family. It does not count when the principal diagnosis is one of the 191 closely related codes in its exclusion list.

Can G04.91 be reported with G93.40?

Not as a rule. The Excludes1 note at G04, which applies to G04.91, lists "encephalopathy NOS (G93.40)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).

What HCC is G04.91?

G04.91 (myelitis, unspecified) maps to CMS-HCC Category 182 (Spinal Cord Disorders/Injuries), commonly written as HCC 182, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 72 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 155.

Does G04.91 risk-adjust for Medicare Advantage payment?

Yes. When documented and reported on a Medicare Advantage encounter, G04.91 adds a risk adjustment factor of about 0.478 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.270 to 0.478 depending on the payment segment). A more severe related category (HCC 180, HCC 181, HCC 191, and HCC 192) supersedes it when both are reported. See the full factor table on the HCC 182 category page.