2026 ICD-10-CM Diagnosis Code G04.02Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis

ICD-10-CM Codes›G00–G99›G00-G09›G04

ICD-10-CM G04.02
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

G04.02 is a billable ICD-10-CM diagnosis code for postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026). In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Encephalitis; and Exposure, encounters, screening or contact with infectious disease.

Code Identity

ICD-10-CM Code
G04.02
Billable Status
Yes — Valid for Submission
Code Describes
Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis
Short Description
Postimmun ac dissem encphlts, myelitis and encephalomyelitis
Parent Code
G04.0
Acute disseminated encephalitis and encephalomyelitis (ADEM)

Code Classification

ChapterG00–G99Diseases of the nervous system
SectionG00-G09Inflammatory diseases of the central nervous system
CategoryG04Encephalitis, myelitis and encephalomyelitis
This CodeG04.02Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Adverse reaction to cholera vaccine
  • Adverse reaction to hepatitis A vaccine
  • Adverse reaction to hepatitis B vaccine
  • Adverse reaction to influenza virus vaccine
  • Adverse reaction to measles vaccine
  • Adverse reaction to mumps vaccine
  • Adverse reaction to pertussis vaccine
  • Adverse reaction to plague vaccine
  • Adverse reaction to poliomyelitis vaccine
  • Adverse reaction to rabies vaccine
  • Adverse reaction to rubella vaccine
  • Adverse reaction to smallpox vaccine
  • Adverse reaction to tuberculosis vaccine
  • Adverse reaction to typhoid vaccine
  • Adverse reaction to typhus vaccine
  • Adverse reaction to yellow fever vaccine
  • Encephalitis caused by bacillus Calmette-Guerin vaccine
  • Encephalitis caused by cholera vaccine
  • Encephalitis caused by diphtheria vaccine
  • Encephalitis caused by hepatitis A vaccine
  • Encephalitis caused by hepatitis B vaccine
  • Encephalitis caused by influenza vaccine
  • Encephalitis caused by measles vaccine
  • Encephalitis caused by mixed vaccine
  • Encephalitis caused by mumps vaccine
  • Encephalitis caused by pertussis vaccine
  • Encephalitis caused by plague vaccine
  • Encephalitis caused by polio vaccine
  • Encephalitis caused by rabies vaccine
  • Encephalitis caused by rubella vaccine
  • Encephalitis caused by smallpox vaccine
  • Encephalitis caused by tetanus vaccine
  • Encephalitis caused by typhoid vaccine
  • Encephalitis caused by typhus vaccine
  • Encephalitis caused by yellow fever vaccine
  • Encephalomyelitis caused by vaccine

Tabular List NotesGuidance

Coding notes and annotation back-references applicable to this code.

Inclusion Terms

  • Encephalitis, post immunization
  • Encephalomyelitis, post immunization

Use Additional Code

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Complication(s) (from) (of)
      • vaccination
        • encephalitis or encephalomyelitis
    • Complication(s) (from) (of)
      • vaccination
        • myelitis
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • acute
        • disseminated
          • postimmunization (postvaccination)
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • due to
        • smallpox inoculation
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • following vaccination or other immunization procedure
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • postimmunization
    • Encephalitis(chronic) (hemorrhagic) (idiopathic) (nonepidemic) (spurious) (subacute)
      • postvaccinal
    • Encephalomyelitis
      • acute disseminated
        • postimmunization
    • Encephalomyelitis
      • postvaccinal
    • Encephalopathy(acute)
      • in (due to) (with)
        • vaccination
    • Leukoencephalitis
      • acute (subacute) hemorrhagic
        • postimmunization or postvaccinal
    • Leukoencephalopathy
      • postimmunization and postvaccinal
    • Meningoencephalomyelitis
      • acute NEC (viral)
        • disseminated
          • postimmunization or postvaccination
    • Meningoencephalomyelitis
      • postimmunization or postvaccination
    • Myelitis(acute) (ascending) (childhood) (chronic) (descending) (diffuse) (disseminated) (idiopathic) (pressure) (progressive) (spinal cord) (subacute)
      • postimmunization
    • Myelitis(acute) (ascending) (childhood) (chronic) (descending) (diffuse) (disseminated) (idiopathic) (pressure) (progressive) (spinal cord) (subacute)
      • postvaccinal

Clinical ClassificationClinical

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

CCSR NVS002
Encephalitis
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR FAC016
Exposure, encounters, screening or contact with infectious disease
Default principal diagnosis: inpatient No · outpatient No

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 G04.02 to ICD-9-CMHistory

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

ICD-9-CM
323.51 Enceph/myel folwg immune
Approximate The match is approximate rather than exact.
ICD-9-CM
323.52 Myelitis follwg immune
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 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About G04.02Overview

Is G04.02 (Acute disseminated encephalitis and encephalomyelitis (ADEM)) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What is the ICD-9 equivalent of G04.02?

Under the General Equivalence Mappings, postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis converts to ICD-9-CM 323.51 (enceph/myel folwg immune) and 323.52 (myelitis follwg immune). The mapping is approximate, so confirm the match fits the documentation.

Footnotes

[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.