G04.91 ICD-10-CM Code: Myelitis, unspecified
HCC Buddy Code Card
Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.
FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Inflammatory diseases of the central nervous system (G00-G09)
G04.91
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMyelitis, unspecified
Inflammation of the spinal cord when the specific cause or type is not identified.
Buddy Insight
Myelitis, unspecified represents spinal cord inflammation without a determined etiology, which still signals a serious neurological condition requiring extensive workup and management.
CMS-HCC V28
MappedHCC 182
RAF 0.282
CMS-HCC V24
MappedHCC 72
RAF 0.464
ACA/HHS
00
RAF 0
ESRD/PACE
MappedHCC 72
RAF 0.0
RXHCC
MappedHCC 155
RAF 0.0
Code Trumping
Basket needed
Code Book Path
Inclusion Terms
OfficialICD-10-CM does not list inclusion terms for G04.91 in this effective period.
Excludes 2
OfficialICD-10-CM does not list Excludes 2 notes for G04.91 in this effective period.
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for G04.91 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for G04.91 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for G04.91 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for G04.91 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for G04.91 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is G04.91 an HCC code?
Yes. G04.91 maps to Spinal Cord Disorders/Injuries under the CMS-HCC V28 risk adjustment model (and Spinal Cord Disorders/Injuries under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for G04.91
For G04.91to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed G04.91 during that encounter, not just copy-forwarded from a problem list.
What This Code Means
G04.91 is the ICD-10-CM diagnosis code for myelitis, unspecified. Inflammation of the spinal cord when the specific cause or type is not identified. G04.91 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering inflammatory diseases of the central nervous system (g00-g09).
Under the CMS-HCC V28 risk adjustment model, G04.91 maps to Spinal Cord Disorders/Injuries (HCC 182) with a community, non-dual, aged base RAF weight of 0.282. Under the older V24 model, G04.91 mapped to the same category but with a base RAF weight of 0.464, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This unspecified code should be used only when documentation doesn't support a more specific myelitis diagnosis. Because G04.91 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for G04.91 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This unspecified code should be used only when documentation doesn't support a more specific myelitis diagnosis
- •Consider querying the provider if additional diagnostic workup results become available
Clinical Significance
Myelitis, unspecified represents spinal cord inflammation without a determined etiology, which still signals a serious neurological condition requiring extensive workup and management. While less specific than other myelitis codes, it captures the disease burden of an inflammatory spinal cord process that typically demands high-cost diagnostic imaging, specialist referrals, and potential long-term therapy.
Documentation Requirements
- ✓Clinical evidence of spinal cord inflammation (motor, sensory, or autonomic dysfunction)
- ✓Statement that myelitis is present even if etiology is undetermined
- ✓Any diagnostic workup performed or pending
- ✓Current functional limitations and neurological exam findings
- ✓Documentation explaining why a more specific diagnosis cannot be determined
- ✓Treatment plan including immunotherapy or rehabilitation if applicable
Commonly Confused Codes
- •G04.89 — Other myelitis: use when the myelitis type is identified but does not fit specific subcategories
- •G04.82 — Acute flaccid myelitis: a specific subtype with distinct clinical criteria
- •G37.3 — Acute transverse myelitis in demyelinating disease: specific to demyelinating etiology
- •G95.19 — Other vascular myelopathies: for myelopathy caused by vascular insufficiency, not inflammation