G23.9
BillableDegenerative disease of basal ganglia, unspecified
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G23.9 an HCC code?
Yes. G23.9 maps to Parkinson and Other Degenerative Disease of Basal Ganglia under the CMS-HCC V28 risk adjustment model (and Parkinson's and Huntington's Diseases 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 G23.9
For G23.9to 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 G23.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G23.9 is the ICD-10-CM diagnosis code for degenerative disease of basal ganglia, unspecified. This code describes a progressive disease affecting the basal ganglia (deep brain structures that control movement) when the specific type of degeneration cannot be determined. It is used when a patient has symptoms of basal ganglia degeneration but the exact diagnosis has not been specified. G23.9 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering extrapyramidal and movement disorders (g20-g26).
Under the CMS-HCC V28 risk adjustment model, G23.9 maps to Parkinson and Other Degenerative Disease of Basal Ganglia (HCC 199) with a community, non-dual, aged base RAF weight of 0.615. Under the older CMS-HCC V24 model, G23.9 maps to Parkinson's and Huntington's Diseases (HCC 78) with a community, non-dual, aged base RAF weight of 0.606. 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.
Use this code only when the specific type of basal ganglia degeneration (such as Parkinson's disease, Huntington's disease, or progressive supranuclear palsy) cannot be identified from the medical record. Because G23.9 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 G23.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Use this code only when the specific type of basal ganglia degeneration (such as Parkinson's disease, Huntington's disease, or progressive supranuclear palsy) cannot be identified from the medical record
- •Query the physician if more specific diagnostic information is available, as more specific codes (G23.0-G23.8) provide better clinical detail and may affect treatment planning
Clinical Significance
Degenerative disease of basal ganglia, unspecified, is used when a patient has documented basal ganglia degeneration but the specific etiology has not been determined. While this captures the condition for risk adjustment, it represents an opportunity for improved documentation specificity. These patients typically require ongoing neurological management and monitoring.
Documentation Requirements
- ✓Documentation of neurological symptoms consistent with basal ganglia dysfunction (tremor, rigidity, bradykinesia)
- ✓Neuroimaging showing basal ganglia changes
- ✓Statement that specific type of degeneration has not been determined or is pending workup
- ✓Evidence that provider evaluation has occurred and more specific diagnosis is not yet possible
- ✓Current medication list and treatment plan
Commonly Confused Codes
- •G20 — Parkinson's disease; should be used instead if documentation supports this specific diagnosis
- •G23.8 — Other specified degenerative diseases of basal ganglia; use when the type is identified but doesn't fit standard categories
- •G23.1 — Progressive supranuclear ophthalmoplegia; a specific basal ganglia degeneration
- •G25.9 — Extrapyramidal and movement disorder, unspecified; broader category not limited to degenerative etiology