G23.9 ICD-10-CM Code: Degenerative disease of basal ganglia, unspecified
G23.9 maps to CMS-HCC V28 199. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · free HCC coding tools
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FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Extrapyramidal and movement disorders (G20-G26)
G23.9
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceDegenerative 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.
Buddy Insight
Degenerative disease of basal ganglia, unspecified, is used when a patient has documented basal ganglia degeneration but the specific etiology has not been determined.
CMS-HCC V28
MappedHCC 199
Coefficient HCC 199: 0.615 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 119
Code-level coefficient reference
ESRD/PACE
MappedHCC 78
Code-level coefficient reference
RXHCC
MappedHCC 161
Code-level coefficient reference
Code Book Path
Inclusion Terms
OfficialNo inclusion terms are included in this display for G23.9. Check the code and parent instructions in the Code Book.
Excludes 2
Official- certain conditions originating in the perinatal period (P04-P96)Inherited from G00-G99, G23
- certain infectious and parasitic diseases (A00-B99)Inherited from G00-G99, G23
- complications of pregnancy, childbirth and the puerperium (O00-O9A)Inherited from G00-G99, G23
- congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)Inherited from G00-G99, G23
- endocrine, nutritional and metabolic diseases (E00-E88)Inherited from G00-G99, G23
- injury, poisoning and certain other consequences of external causes (S00-T88)Inherited from G00-G99, G23
- neoplasms (C00-D49)Inherited from G00-G99, G23
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)Inherited from G00-G99, G23
- multi-system degeneration of the autonomic nervous system (G90.3)Inherited from G00-G99, G23
Related Codes
Includes
OfficialNo Includes notes are included in this display for G23.9. Check the code and parent instructions in the Code Book.
Excludes 1
OfficialNo Excludes 1 notes are included in this display for G23.9. Check the code and parent instructions in the Code Book.
Code First
OfficialNo Code First sequencing instructions are included in this display for G23.9. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for G23.9. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for G23.9. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is G23.9 an HCC code?
Yes. G23.9 (Degenerative disease of basal ganglia, unspecified) maps to HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.615. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: G23.9 is billable and maps to V28 HCC 199, Parkinson and Other Degenerative Disease of Basal Ganglia. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- G23.9
- Description
- Degenerative disease of basal ganglia, unspecified
- HCC (V28)
- HCC 199 — Parkinson and Other Degenerative Disease of Basal Ganglia
- RAF reference coefficient
- 0.615
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work G23.9 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for G23.9
For G23.9, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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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 source-labeled community, non-dual, aged reference coefficient of 0.615. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
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. For G23.9, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, 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