G11.9
BillableHereditary ataxia, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G11.9 an HCC code?
Yes. G11.9 maps to Cerebellar Ataxia and Other Degenerative Diseases of Nervous System 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 G11.9
For G11.9 to 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 G11.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G11.9 is the ICD-10-CM diagnosis code for hereditary ataxia, unspecified. An inherited disorder affecting coordination and balance when the specific type of hereditary ataxia cannot be determined or is not specified. G11.9 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering systemic atrophies primarily affecting the central nervous system (g10-g14).
Under the CMS-HCC V28 risk adjustment model, G11.9 maps to Cerebellar Ataxia and Other Degenerative Diseases of Nervous System (HCC 200) with a community, non-dual, aged base RAF weight of 0.262. Under the older CMS-HCC V24 model, G11.9 maps to Spinal Cord Disorders/Injuries (HCC 72) with a community, non-dual, aged base RAF weight of 0.464. 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 is a default code used when the specific hereditary ataxia type is unknown; query the provider for more specific diagnosis if available. Because G11.9 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 G11.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a default code used when the specific hereditary ataxia type is unknown; query the provider for more specific diagnosis if available
- •Avoid using this code if any specific ataxia subtype can be identified from documentation
Clinical Significance
Hereditary ataxia, unspecified is used when a patient has a confirmed hereditary coordination disorder but the specific subtype has not been determined. While this captures disease burden, it represents a missed opportunity for specificity and may indicate incomplete diagnostic workup. Coders should query providers whenever possible to obtain a more specific diagnosis.
Documentation Requirements
- ✓Documentation confirming the ataxia is hereditary in nature
- ✓Family history of ataxia or documented genetic basis
- ✓Neurological examination findings consistent with cerebellar dysfunction
- ✓Explanation of why specific subtype cannot be determined (pending testing, inconclusive results)
- ✓Current functional status and care plan
- ✓Any prior or planned genetic testing