G23.8
BillableOther specified degenerative diseases of basal ganglia
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G23.8 an HCC code?
Yes. G23.8 maps to Parkinson and Huntington Diseases 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 G23.8
For G23.8 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 G23.8 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G23.8 is the ICD-10-CM diagnosis code for other specified degenerative diseases of basal ganglia. Other rare degenerative conditions affecting the basal ganglia (deep brain structures controlling movement) not classified elsewhere. G23.8 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, G23.8 maps to Parkinson and Huntington Diseases (HCC 199) with a community, non-dual, aged base RAF weight of 0.000. Under the older CMS-HCC V24 model, G23.8 maps to Parkinson's and Huntington's Diseases (HCC 78) with a community, non-dual, aged base RAF weight of 0.584. 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 basal ganglia degeneration does not fit other G23 categories. Because G23.8 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 G23.8 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 basal ganglia degeneration does not fit other G23 categories
- •Provide detailed clinical documentation describing the type of degeneration and affected structures
Clinical Significance
Other specified degenerative diseases of basal ganglia captures rare progressive neurological conditions affecting deep brain structures that control movement. These conditions carry significant morbidity and require specialized neurological care with ongoing monitoring. Proper coding ensures appropriate risk adjustment for patients with complex movement disorder management needs.
Documentation Requirements
- ✓Specific identification of the type of basal ganglia degeneration that does not fit other G23 categories
- ✓Neuroimaging results (MRI/CT) demonstrating basal ganglia involvement
- ✓Neurological examination documenting movement disorder symptoms (rigidity, dystonia, chorea)
- ✓Documentation that more specific codes (G23.0-G23.3) were considered and ruled out
- ✓Treatment plan including medications and therapy referrals
Commonly Confused Codes
- •G23.3 — Hypomyelination with atrophy of basal ganglia and cerebellum; more specific diagnosis with hypomyelination pattern
- •G23.9 — Degenerative disease of basal ganglia, unspecified; use only when the specific type cannot be identified at all
- •G20 — Parkinson's disease; a common basal ganglia disorder with its own specific code
- •G23.1 — Progressive supranuclear ophthalmoplegia; specific basal ganglia degeneration with characteristic eye movement abnormalities