2027 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 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

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 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 23 through 24, 97 through 99. 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 1 closely related codes. 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 vaccine
  • Encephalitis caused by yellow fever vaccine
  • Encephalomyelitis caused by vaccine
  • Myelitis caused by vaccine
  • Neurologic disorder caused by component of vaccine product containing Yellow fever virus antigen

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to G04.02: its own notes plus those printed at G04.0, 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.

  • Encephalitis, post immunization
  • Encephalomyelitis, post immunization

Excludes1

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

From G04.0 Acute disseminated encephalitis and encephalomyelitis (ADEM) applies to 3 codes
  • acute necrotizing hemorrhagic encephalopathy (G04.3-)
  • other noninfectious acute disseminated encephalomyelitis (noninfectious ADEM) (G04.81)
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)

Use Additional Code

Add a second code, after this one, to fully describe the condition.

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

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name G04.02, its category, or a range that includes it.

Excludes1 3

These codes carry an Excludes1 note naming G04.02: 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
  • G04.3 Acute necrotizing hemorrhagic encephalopathy
    acute disseminated encephalitis and encephalomyelitis (G04.0-) names G04.0-, which includes this code

Excludes2 2

These codes carry an Excludes2 note naming G04.02: its condition is not part of those codes, and both may be reported when the patient has both.

  • T88.1 Other complications following immunization, not elsewhere classified
    postimmunization encephalitis (G04.02)
  • G95 Other and unspecified diseases of spinal cord
    myelitis (G04.-) names G04.-, which includes this code

Notes 1

These notes mention G04.02.

  • 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
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 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

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, 2026 through September 30, 2027.

What MS-DRG does G04.02 group to?

When postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis is the principal diagnosis on an inpatient stay, it groups to MS-DRG 24, 98, 99, 23, 97, with relative weights from 1.5249 to 5.7422 depending on complications. Higher weights mean higher Medicare reimbursement.

Is G04.02 a CC or MCC?

CMS lists G04.02 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 1 closely related codes in its exclusion list.

Can G04.02 be reported with a code from G04.3?

Not as a rule. The Excludes1 note at G04.0, which applies to G04.02, lists "acute necrotizing hemorrhagic encephalopathy (G04.3-)". 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).

Does G04.02 need an additional code?

When the condition applies, yes. The Use Additional Code note reads: code to identify the vaccine (T50.A-, T50.B-, T50.Z-). The additional code is reported after G04.02.