2027 ICD-10-CM Diagnosis Code G23.3Hypomyelination with atrophy of the basal ganglia and cerebellum
ICD-10-CM Codes›G00–G99›G20-G26›G23
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 199
- Chronic Condition
G23.3 is a billable ICD-10-CM diagnosis code for hypomyelination with atrophy of the basal ganglia and cerebellum. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 56 through 57. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 21 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Other nervous system disorders (often hereditary or degenerative).
For Medicare Advantage risk adjustment, G23.3 maps to CMS-HCC Category 199 (Parkinson and Other Degenerative Disease of Basal Ganglia) under the V28 model, adding a risk factor of about 0.615 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
G23.3 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to G23.3: its own notes plus those printed at G23 and Chapter 6. A note printed at a category, block or chapter applies to every code under it.
Applicable To
Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.
- H-ABC
- TUBB4A-related neurologic disorders
Excludes2
Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.
- multi-system degeneration of the autonomic nervous system (G90.3)
- certain conditions originating in the perinatal period (P04-P96)
- certain infectious and parasitic diseases (A00-B99)
- complications of pregnancy, childbirth and the puerperium (O00-O9A)
- congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)
- endocrine, nutritional and metabolic diseases (E00-E88)
- injury, poisoning and certain other consequences of external causes (S00-T88)
- neoplasms (C00-D49)
- symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Disorder (of) See Also: Disease;
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Patient EducationClinical
Brain Diseases
Your brain is the control center of your body. It controls your thoughts, memory, speech, and movement. It regulates the function of many organs. It's part of your nervous system, which also includes your spinal cord and peripheral nerves. The nervous system sends signals between your brain and the rest of the body.
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Code History & ChangesHistory
Replacement G23.3 replaces the following previously assigned code(s):
- G23.8 - Other specified degenerative diseases of basal ganglia
Questions About G23.3Overview
What is the ICD-10 code for hypomyelination with atrophy of the basal ganglia and cerebellum?
The ICD-10-CM code for hypomyelination with atrophy of the basal ganglia and cerebellum is G23.3 (sometimes written as G233). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is G23.3 (Other degenerative diseases of basal ganglia) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report hypomyelination with atrophy of the basal ganglia and cerebellum on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does G23.3 group to?
When hypomyelination with atrophy of the basal ganglia and cerebellum is the principal diagnosis on an inpatient stay, it groups to MS-DRG 57, 56, with relative weights from 1.2835 to 2.2649 depending on complications. Higher weights mean higher Medicare reimbursement.
Is G23.3 a CC or MCC?
CMS lists G23.3 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 21 closely related codes in its exclusion list.
What HCC is G23.3?
G23.3 (hypomyelination with atrophy of the basal ganglia and cerebellum) maps to CMS-HCC Category 199 (Parkinson and Other Degenerative Disease of Basal Ganglia), commonly written as HCC 199, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 78 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 161.
Does G23.3 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, G23.3 adds a risk adjustment factor of about 0.615 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.219 to 0.634 depending on the payment segment). HCC 199 sits at the top of its hierarchy, so no other condition category supersedes it. See the full factor table on the HCC 199 category page.
