2027 ICD-10-CM Diagnosis Code G04.90Encephalitis and encephalomyelitis, unspecified

ICD-10-CM Codes›G00–G99›G00-G09›G04

ICD-10-CM G04.90
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

G04.90 is a billable ICD-10-CM diagnosis code for encephalitis and encephalomyelitis, 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis.

Code Identity

ICD-10-CM Code
G04.90
Billable Status
Yes — Valid for Submission
Code Describes
Encephalitis and encephalomyelitis, unspecified
Short Description
Encephalitis and encephalomyelitis, 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 CodeG04.90Encephalitis and encephalomyelitis, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acute encephalitis
  • Autonomic disorder due to encephalitis
  • Bacterial ventriculitis
  • Brainstem encephalitis
  • Chorea due to and following encephalitis
  • Dystonia due to encephalitis
  • Encephalitis
  • Encephalitis, myelitis and encephalomyelitis
  • Encephalomyelitis
  • Encephalomyelitis caused by bacterium
  • Encephalomyelitis caused by Burkholderia
  • Ependymitis
  • Epilepsy due to infectious disease of central nervous system
  • Epilepsy due to infectious encephalitis
  • Focal encephalitis
  • Granular ependymitis
  • Infection caused by Mycoplasmoides pneumoniae
  • Infection causing encephalomyelitis
  • Infectious disease of brain
  • Infective encephalitis
  • Infective ventriculitis
  • Malignant ventriculitis
  • Meningoencephalitis
  • Meningoencephalomyelitis
  • Meningomyelitis
  • Mycoplasma pneumoniae encephalitis
  • Myelitis caused by bacterium
  • Primary encephalitis
  • Ventriculitis of the brain

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to G04.90: 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

Applicable To

Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.

  • Ventriculitis (cerebral) NOS

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 G04.90, its category, or a range that includes it.

Excludes1 2

These codes carry an Excludes1 note naming G04.90: 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 G04.90: 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 G04.90.

  • 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

  • Encephalitis

    inflammation of the brain due to infection, autoimmune processes, toxins, and other conditions. viral infections (see encephalitis, viral) are a relatively frequent cause of this condition.

  • Encephalomyelitis

    a general term indicating inflammation of the brain and spinal cord, often used to indicate an infectious process, but also applicable to a variety of autoimmune and toxic-metabolic conditions. there is significant overlap regarding the usage of this term and encephalitis in the literature.

  • Meningoencephalitis

    an inflammatory process involving the brain (encephalitis) and meninges (meningitis), most often produced by pathogenic organisms which invade the central nervous system, and occasionally by toxins, autoimmune disorders, and other conditions.

Patient EducationClinical

Encephalitis

Encephalitis is inflammation (swelling) of the brain. It can happen when an infection or medical condition that affects the brain activates your immune system. The inflammation can cause a wide range of symptoms. In extreme cases, it can lead to brain damage, stroke, or even death.

The full article covers:

  • What is encephalitis?
  • What causes encephalitis?
  • Who is more likely to get encephalitis?
  • What are the symptoms of encephalitis?
  • How is encephalitis diagnosed?
  • What are the treatments for encephalitis?
  • Can encephalitis be prevented?

Read the full article at MedlinePlus

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

Convert G04.90 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 G04.90Overview

What is the ICD-10 code for encephalitis and encephalomyelitis, unspecified?

The ICD-10-CM code for encephalitis and encephalomyelitis, unspecified is G04.90 (sometimes written as G0490). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is G04.90 (Encephalitis, myelitis and encephalomyelitis, unspecified) a billable code?

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

What MS-DRG does G04.90 group to?

When encephalitis and encephalomyelitis, 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 G04.90 a CC or MCC?

CMS lists G04.90 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 G04.90 be reported with G93.40?

Not as a rule. The Excludes1 note at G04, which applies to G04.90, 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).