G23.3
BillableHypomyelination with atrophy of the basal ganglia and cerebellum
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G23.3 an HCC code?
Yes. G23.3 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 G23.3
For G23.3to 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 G23.3 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G23.3 is the ICD-10-CM diagnosis code for hypomyelination with atrophy of the basal ganglia and cerebellum. A rare genetic disorder characterized by underdevelopment of nerve fiber insulation in the brain, affecting the basal ganglia and cerebellum, leading to movement and coordination problems. G23.3 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, G23.3 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, G23.3 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.
This is a rare condition; ensure genetic testing or imaging findings are documented to confirm diagnosis. Because G23.3 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 G23.3 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a rare condition; ensure genetic testing or imaging findings are documented to confirm diagnosis
- •Document associated symptoms including developmental delay, ataxia, or dystonia
Clinical Significance
Hypomyelination with atrophy of the basal ganglia and cerebellum is a rare genetic neurodegenerative disorder that significantly impacts motor function and development. It carries high resource utilization due to lifelong multidisciplinary care needs including neurology, physical therapy, and genetic counseling. Accurate capture is essential for risk adjustment as it reflects a complex neurological condition requiring ongoing management.
Documentation Requirements
- ✓Confirmed diagnosis via MRI showing hypomyelination pattern with basal ganglia and cerebellar atrophy
- ✓Genetic testing results confirming specific gene mutation (e.g., TUBB4A)
- ✓Detailed neurological examination findings including motor deficits, ataxia, or dystonia
- ✓Documentation of developmental milestones and any regression
- ✓Current treatment plan and specialist involvement
Commonly Confused Codes
- •G23.8 — Other specified degenerative diseases of basal ganglia; use when a different basal ganglia degeneration is specified but not hypomyelination
- •G23.9 — Degenerative disease of basal ganglia, unspecified; use only when the specific type cannot be determined
- •G31.80 — Leukodystrophy, unspecified; broader category of white matter diseases without the specific basal ganglia/cerebellar pattern
- •G11.1 — Early-onset cerebellar ataxia; involves cerebellum but without the hypomyelination component