Crystal's Coder Hub🧬 ICD-10 CM G04.02 — Postimmunization Acute Disseminated Encephalitis, Myelitis And Encephalomyelitis
Billable Code Confirmed
ICD-10-CM G04.02 is a valid, billable 5-character diagnosis code for FY2026. The first three characters (G04) define the category of encephalitis/myelitis, the 4th character (0) specifies acute disseminated encephalomyelitis, and the 5th character (2) specifically denotes the postimmunization etiology. No additional characters are required.
Non-Billable Parent Codes — Never Submit These
- ❌
G04— 3-character header — Lacks specificity regarding the exact type of encephalitis or myelitis.- ❌
G04.0— 4-character header — Lacks etiology specificity (fails to distinguish postinfectious from post-immunization).Always submit G04.02 (all 5 characters) when acute disseminated encephalitis or myelitis is documented with a post-vaccinal etiology.
Clinical Context: Etiology and Additional Coding
ICD-10-CM G04.02 captures central nervous system inflammation that develops specifically as a reaction to a vaccine. ICD-10-CM guidelines require the use of an additional code to identify the specific vaccine causing the adverse effect (e.g., T50.A-, T50.B-, T50.Z- for adverse effects of vaccines).
Code Classification
ICD-10-CM Diagnosis Code — As a diagnosis code, wRVU, assistant payable, and global period fields are not applicable. See the CPT Procedural Crosswalk and ICD-10-PCS Crosswalk sections below for related procedures.
🔍 Code Description
ICD-10-CM G04.02 classifies Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis. This code represents an uncommon, acute inflammatory and demyelinating disease of the central nervous system that occurs following an immunization or vaccination.
The pathophysiology involves a transient autoimmune response triggered by vaccine antigens, which cross-react with myelin basic protein, leading to multifocal demyelination in the brain and spinal cord.^2 Clinically, patients present with rapid-onset encephalopathy (altered mental status, lethargy) combined with multifocal neurological deficits. Selecting this code is critical to distinguish a post-vaccinal etiology from the more common postinfectious ADEM.
🌳 Code Tree / Hierarchy
G04 Encephalitis, myelitis and encephalomyelitis ❌ Non-billable
│
├── G04.1 Tropical spastic paraplegia ✅ Billable
├── G04.2 Bacterial meningoencephalitis and meningomyelitis, NEC ✅ Billable
├── G04.3 Acute necrotizing hemorrhagic encephalopathy ❌ Non-billable
│
├── G04.0 Acute disseminated encephalitis and encephalomyelitis (ADEM) ❌ Non-billable
│ │
│ ├── G04.00 Acute disseminated encephalitis and encephalomyelitis, unspecified ✅ Billable
│ ├── G04.01 Postinfectious acute disseminated encephalitis and encephalomyelitis ✅ Billable
│ └── G04.02 Postimmunization acute disseminated encephalitis... ◀ THIS CODE ✅ Billable
│
└── G04.9 Encephalitis, myelitis and encephalomyelitis, unspecified ✅ Billable
Etiological Specificity
Selecting G04.02 over the
G04.00unspecified code is critical because it establishes an adverse reaction to a medical intervention (vaccination). This directly impacts public health tracking (e.g., VAERS reporting) and clarifies the medical necessity for immunomodulatory treatments.
✅ Includes
The following clinical terms and scenarios map to G04.02 when documented:
- encephalitis, post immunization
- encephalomyelitis, post immunization
- myelitis following immunization procedures
- ADEM triggered by recent vaccination
❌ Excludes
Excludes 1 — Cannot Be Coded Simultaneously with CODE
| Code | Description | Note |
|---|---|---|
| G93.40 | Encephalopathy, unspecified | Mutually exclusive as G04.02 provides the specific etiology and type of encephalopathy. |
| G04.3- | Acute necrotizing hemorrhagic encephalopathy | Represents a distinct, far more fulminant hemorrhagic condition that is classified separately. |
| G04.81 | Other noninfectious acute disseminated encephalomyelitis | Mutually exclusive; G04.02 should be used specifically when the etiology is postimmunization. |
Excludes 1 Violation Risk
A coder might mistakenly try to report G93.40 (Encephalopathy) for a patient’s altered mental status alongside G04.02. Because encephalopathy is integral to the definition of ADEM, do not code G93.40 separately.
Excludes 2 — May Be Coded in Addition if Separately Present
| Code | Description | Note |
|---|---|---|
| G37.3 | Acute transverse myelitis | May be coded simultaneously if the patient has a distinct, separately identifiable episode of acute transverse myelitis not linked to the ADEM presentation. |
| G35.- | Multiple sclerosis | Can be coded if the patient has a confirmed underlying diagnosis of multiple sclerosis in addition to the acute ADEM event. |
| G92.8 | Toxic encephalitis | Appropriate if there is a separate toxic exposure causing encephalitis in addition to the postimmunization reaction. |
📋 Clinical Overview
Etiology Distinction
Accurate code selection under the G04.0 subcategory depends entirely on the triggering event documented by the provider.
| Feature | G04.02 — Postimmunization | G04.01 — Postinfectious | G04.00 — Unspecified |
|---|---|---|---|
| Triggering Event | Recent vaccination / immunization | Recent viral or bacterial infection | Unknown or undocumented |
| Clinical Presentation | Encephalopathy + multifocal deficits | Encephalopathy + multifocal deficits | Encephalopathy + multifocal deficits |
| Additional Coding | Requires T50.- code for the vaccine | May require B95-B97 to identify organism | N/A |
CDI Query Trigger — Clarification of ADEM Etiology
If a provider documents “Acute disseminated encephalomyelitis (ADEM)” and notes both recent vaccines and recent viral illnesses without explicitly linking one as the primary trigger, query the provider to clarify the specific etiology (postinfectious vs. post-immunization) to ensure accurate code selection.
Common Diagnoses / Clinical Indications
Relevant manifestations and common presenting symptoms associated with postimmunization ADEM:
- Encephalopathy: Acute alteration in mental status, lethargy, confusion, or coma.
- Multifocal Neurological Deficits: Paresis, cranial nerve palsies, ataxia, or sensory levels (from myelitis).
- Seizures: Acute symptomatic seizures secondary to cortical inflammation.
Coding Manifestations
Always code the required external cause codes and any distinct manifestations to fully capture the patient’s complexity:
- T50.B95A — Adverse effect of other viral vaccines, initial encounter (Required addition)
- R40.20 — Unspecified coma (if documented)
- R56.9 — Unspecified convulsions (if seizures occur)
💰 HCC Risk Adjustment (CMS-HCC v28)
| Field | Detail |
|---|---|
| CMS-HCC Model Version | v28 (2024-2025 Implementation) |
| HCC Assignment | ✅ Mapped — HCC 76 / 72 |
| HCC Category | HCC 76 (Neurological Conditions) / HCC 72 (Spinal Cord Disorders) |
| RAF Coefficient | ~0.35 - 0.45 (varies by demographic/status) |
G04.02 maps to an HCC reflecting complex neurological disorders and contributes to the RAF score under CMS-HCC v28.
Capture Annually
Acute conditions like ADEM are primarily coded during the active phase of treatment. Once the acute phase has resolved, any ongoing or permanent residual deficits (e.g., hemiplegia, cognitive deficits) should be coded as sequelae for appropriate annual risk adjustment.
🏥 DRG Assignment
MDC 01 — Diseases and Disorders of the Nervous System
| DRG | Title | Est. Relative Weight* |
|---|---|---|
| DRG 097 | Non-Bacterial Infections of Nervous System Except Viral Meningitis with MCC | ~2.10 - 2.30 |
| DRG 098 | Non-Bacterial Infections of Nervous System Except Viral Meningitis with CC | ~1.20 - 1.40 |
| DRG 099 | Non-Bacterial Infections of Nervous System Except Viral Meningitis without CC/MCC | ~0.80 - 0.90 |
Approximate. Verify against IPPS FY2026 Final Rule tables.
Sequencing and Complications
🔗 Related ICD-10-CM Codes
Etiology Variants
| Code | Description |
|---|---|
| G04.02 | Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis ← This Code |
| G04.01 | Postinfectious acute disseminated encephalitis and encephalomyelitis |
| G04.00 | Acute disseminated encephalitis and encephalomyelitis, unspecified |
External Cause / Adverse Effect Variants (Required Secondary Codes)
| Code | Description |
|---|---|
| T50.A15A | Adverse effect of pertussis vaccine, including combinations with a pertussis component, initial encounter |
| T50.B95A | Adverse effect of other viral vaccines, initial encounter |
| T50.Z95A | Adverse effect of other vaccines and biological substances, initial encounter |
🛠️ Commonly Associated CPT Codes (Neurology / Inpatient)
Inpatient and Profee Setting Context
These CPT codes represent standard diagnostic evaluation services for patients presenting with suspected ADEM in an ED or inpatient setting.
| CPT Code | Description | Profee Coding Notes (Modifier 26) |
|---|---|---|
| 62270 | Spinal puncture, lumbar, diagnostic | Do not report with fluoroscopic guidance (77003) if bundled by payer policy. |
| 70553 | Magnetic resonance (e.g., proton) imaging, brain; without contrast material, followed by contrast material(s) and further sequences | Modifier -26 required for the professional component if the physician interprets the scan in a facility setting. |
| 95816 | Electroencephalogram (EEG); including recording awake and drowsy | Modifier -26 required for professional interpretation. |
NCCI Bundling Considerations
- CPT 62270 billed on the same day as an initial hospital care E/M code (e.g., CPT 99223) requires a Modifier -25 on the E/M code to indicate a significant, separately identifiable evaluation was performed distinct from the lumbar puncture procedure.
🔬 ICD-10-CM Diagnosis Crosswalk
When G04.02 is an inpatient diagnosis, these ICD-10-PCS codes are relevant for the associated inpatient procedures.
| PCS Section | Body System | Root Operation | Clinical Application |
|---|---|---|---|
| 3 (Administration) | E (Physiological Systems and Anatomical Regions) | 0 (Introduction) | Administration of IV high-dose corticosteroids or IVIG: 3E033VZ (Introduction of other therapeutic substance into peripheral vein, percutaneous approach). |
| 0 (Central Nervous System) | 0 (Brain and Meninges) | 9 (Drainage) | Diagnostic lumbar puncture to evaluate CSF for pleocytosis and rule out active infection: 009T3ZX (Drainage of spinal canal, percutaneous approach, diagnostic). |
| 6 (Extracorporeal Therapies) | A (Physiological Systems) | 5 (Pheresis) | Therapeutic plasma exchange (Plasmapheresis) for steroid-refractory ADEM: 6A550Z3 (Pheresis of Plasma, Single). |
💊 Coding Scenarios and Examples
Scenario 1 — Inpatient: Acute Post-Vaccinal ADEM
Clinical Vignette: A 24-year-old male presents to the ED with severe lethargy, ataxia, and left-sided weakness. He received a viral booster vaccine 10 days prior. MRI of the brain and spine reveals multiple hyperintense demyelinating lesions. CSF analysis is negative for active viral infection. The attending neurologist diagnoses postimmunization acute disseminated encephalomyelitis (ADEM) and initiates high-dose IV methylprednisolone.
Principal Diagnosis:
- G04.02 — Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis (Reason for admission)
Secondary Diagnoses:
- T50.B95A — Adverse effect of other viral vaccines, initial encounter (Required to identify the cause)
- R27.0 — Ataxia, unspecified (Manifestation)
- G81.94 — Hemiplegia, unspecified affecting left non-dominant side (Manifestation)
MS-DRG Assignment: Groups to DRG 099 (Non-Bacterial Infections of Nervous System Except Viral Meningitis without CC/MCC), assuming the manifestations do not act as MCC/CCs based on specific grouper logic.
Scenario 2 — CDI Query: Vague Etiology Documentation
Clinical Vignette: A patient is admitted with altered mental status and focal neurological deficits. The discharge summary lists the principal diagnosis as “Acute disseminated encephalomyelitis.” In the H&P, the physician noted, “Patient had a mild URI 3 weeks ago and also received a flu shot 2 weeks ago; unclear which event triggered the ADEM.”
Action / Outcome: This documentation creates coding uncertainty because ADEM has distinct specific codes for postinfectious (G04.01) versus postimmunization (G04.02) etiologies. A CDI query should be sent presenting the clinical indicators and asking the provider to state, if known, the most likely trigger of the ADEM, or if it remains clinically undetermined.
Query Response: Provider updates the addendum to confirm: “ADEM, secondary to recent influenza vaccination.”
Corrected ICD-10-CM Coding:
- G04.02 — Postimmunization acute disseminated encephalitis, myelitis and encephalomyelitis
- T50.B95A — Adverse effect of other viral vaccines, initial encounter
⚠️ Coding Pitfalls and Tips
| Pitfall or Tip | |
|---|---|
| ❌ | Missing Vaccine Cause Code. Failing to assign the appropriate T50.- code (Adverse effect of vaccines and biological substances) along with G04.02 violates ICD-10-CM coding guidelines for adverse effects. |
| ❌ | Incorrect Sequencing of T-Code. Do not sequence the T-code for an adverse effect as the principal diagnosis. The manifestation (ADEM, G04.02) must be sequenced first. |
| ✅ | Query for Underlying Trigger. Always look for documentation of a recent viral infection or vaccination in patients diagnosed with ADEM, and query the provider if the link is documented vaguely. |
| ✅ | Code Associated Neurological Deficits. Capture the full clinical picture by coding any documented hemiplegia, seizures, or coma in addition to the primary ADEM diagnosis. |
📚 Sources
1. CMS/NCHS. ICD-10-CM Official Guidelines for Coding and Reporting, FY2026. (Guideline I.C.19.e for adverse effects requires assigning the manifestation first, followed by the T-code).2. American Academy of Neurology (AAN). Clinical consensus and review of Acute Disseminated Encephalomyelitis.
3. Tenembaum S., et al. (2007). Acute disseminated encephalomyelitis. Neurology, 68(16 Suppl 2), S23-S36. (Source for clinical presentation and etiology definitions).
4. CMS. 2025-2026 Medicare Advantage Risk Adjustment — CMS-HCC Model v28 ICD-10-CM Mappings.
5. CMS. IPPS Final Rule FY2026 — MS-DRG Definitions Manual v43. MDC 01 logic tables.
6. AMA. CPT Professional Edition 2026. Surgery / Nervous System.
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