2027 ICD-10-CM Diagnosis Code G31.9Degenerative disease of nervous system, unspecified

ICD-10-CM Codes›G00–G99›G30-G32›G31

ICD-10-CM G31.9
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

G31.9 is a billable ICD-10-CM diagnosis code for degenerative disease of nervous system, unspecified. 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. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Neurocognitive disorders and Other nervous system disorders (often hereditary or degenerative).

G31.9 no longer risk-adjusts for Medicare Advantage: it mapped to HCC 52 under the retired CMS-HCC V24 model through payment year 2025 but maps to no category in the live V28 model. It still risk-adjusts in the ESRD (V21) category 52, ESRD (V24) category 52, and RxHCC Part D (V08) category 112 for payment year 2027.

Code Identity

ICD-10-CM Code
G31.9
Billable Status
Yes — Valid for Submission
Code Describes
Degenerative disease of nervous system, unspecified
Short Description
Degenerative disease of nervous system, unspecified
Same as the full description in the CMS dataset.
Parent Code
G31
Other degenerative diseases of nervous system, not elsewhere classified

Code Classification

ChapterG00–G99Diseases of the nervous system
SectionG30-G32Other degenerative diseases of the nervous system
CategoryG31Other degenerative diseases of nervous system, not elsewhere classified
This CodeG31.9Degenerative disease of nervous system, unspecified

Medicare Risk Adjustment (HCC)Billing

G31.9 no longer risk-adjusts for Medicare Advantage: it maps to no payment category in the live CMS-HCC V28 model, although it still risk-adjusts in the other CMS models shown below.

CMS-HCC V28 (Medicare Advantage Payment Model)
Not mapped
Dropped in V28 see all codes that no longer risk-adjust
Prior Model (CMS-HCC V24)
HCC 52
V24 retired last contributed to a Medicare Advantage risk score in payment year 2025
Other CMS Models
ESRD (V21): HCC 52 · ESRD (V24): HCC 52
ESRD V21 weights: 0.045 dialysis, 0.000–0.328 functioning graft · ESRD V24 weights: 0.042 dialysis, 0.000–0.399 functioning graft
Part D (RxHCC)
RxHCC 112 — Dementia, Except Alzheimer's Disease
also risk-adjusts in the Part D prescription drug model (V08)

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.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Acquired cerebral atrophy
  • Atrophy of brainstem
  • Atrophy of caudate nucleus
  • Atrophy of cerebellar vermis
  • Atrophy of cortex of frontal and temporal lobe
  • Atrophy of cortex of frontal lobe
  • Atrophy of cortex of occipital lobe
  • Atrophy of cortex of parietal lobe
  • Atrophy of cortex of temporal lobe
  • Atrophy of hilum of dentate nucleus
  • Atrophy of hippocampus
  • Atrophy of hypothalamus
  • Atrophy of inner granular layer of cerebellar cortex
  • Atrophy of pons and cerebellum
  • Atrophy of Purkinje cells of cerebellar cortex
  • Atrophy of pyramidal tract
  • Atrophy of spinocerebellar tract
  • Autonomic nervous system disorder co-occurrent and due to neurodegenerative disorder
  • Cerebellar degeneration
  • Cerebral atrophy
  • Cerebral degeneration
  • Cerebral degeneration due to Creutzfeldt-Jakob disease
  • Cerebral degeneration due to Parkinson's disease
  • Cerebral degeneration presenting primarily with dementia
  • Chorea due to heredodegenerative disorder
  • Circumscribed atrophy of brain
  • Congenital blindness
  • Degenerative brain disorder
  • Degenerative disease of the central nervous system
  • Diffuse atrophy of cerebellum
  • Diffuse atrophy of cerebrum
  • Epilepsy co-occurrent and due to degenerative brain disorder
  • Fibrosis, neurodegeneration, cerebral angiomatosis syndrome
  • Frontal lobe degeneration
  • Frontal lobe degeneration with motor neurone disease
  • Frontotemporal degeneration
  • Gemignani syndrome
  • Global brain atrophy
  • Hemiatrophy of cerebellar cortex
  • Hemiatrophy of cerebral cortex
  • Hydrocephalus ex vacuo
  • Hydrocephalus ex vacuo due to degenerative brain disorder
  • Microphthalmia with brain atrophy syndrome
  • Parkinsonian pyramidal syndrome
  • Parkinsonism due to heredodegenerative disorder
  • PF-ILD-progressive fibrosing interstitial lung disease
  • Posterior cortical atrophy syndrome
  • Prieto Badia Mulas syndrome
  • Progressive focal cortical atrophy
  • Secondary cerebellar degeneration
  • Structural abnormality of caudate nucleus
  • Structural abnormality of hippocampus

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to G31.9: its own notes plus those printed at G31 and Chapter 6. A note printed at a category, block or chapter applies to every code under it.

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.

From G31 Other degenerative diseases of nervous system, not elsewhere classified applies to 14 codes
From Chapter 6 (G00-G99) Diseases of the nervous system applies to 700 codes
  • 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)

Use Additional Code

Add a second code, after this one, to fully describe the condition.

From G31 Other degenerative diseases of nervous system, not elsewhere classified applies to 14 codes

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name G31.9, its category, or a range that includes it.

Excludes1 2

These codes carry an Excludes1 note naming G31.9: they are not reported together with it, unless the two conditions are unrelated.

  • F06.7 Mild neurocognitive disorder due to known physiological condition
    cerebral degeneration (G31.9)
  • G31.84 Mild cognitive impairment of uncertain or unknown etiology
    cerebral degeneration (G31.9)

Use Additional Code 1

These codes say to use an additional code from the note, and G31.9 is one of the codes named.

  • G31 Other degenerative diseases of nervous system, not elsewhere classified
    code, if applicable, for codes G31.0-G31.83, G31.85-G31.9, to identify: names G31.85-G31.9, which includes this code

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NVS011
Neurocognitive disorders
Default principal diagnosis: inpatient Yes · outpatient Yes
CCSR NVS006
Other nervous system disorders (often hereditary or degenerative)
Default principal diagnosis: inpatient No · outpatient No

Patient EducationClinical

Degenerative Nerve Diseases

Degenerative nerve diseases affect many of your body's activities, such as balance, movement, talking, breathing, and heart function. Many of these diseases are genetic. Sometimes the cause is a medical condition such as alcoholism, a tumor, or a stroke. Other causes may include toxins, chemicals, and viruses.

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert G31.9 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
330.9 Cereb degen in child NOS
Approximate The match is approximate rather than exact.
ICD-9-CM
331.9 Cereb degeneration NOS
Approximate The match is approximate rather than exact.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About G31.9Overview

What is the ICD-10 code for degenerative disease of nervous system, unspecified?

The ICD-10-CM code for degenerative disease of nervous system, unspecified is G31.9 (sometimes written as G319). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is G31.9 a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report degenerative disease of nervous system, unspecified on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does G31.9 group to?

When degenerative disease of nervous system, unspecified 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.

Does G31.9 need an additional code?

When the condition applies, yes. The Use Additional Code note at G31, which applies to G31.9, reads: code, if applicable, for codes G31.0-G31.83, G31.85-G31.9, to identify: dementia with anxiety (F02.84, F02.A4, F02.B4, F02.C4); dementia with behavioral disturbance (F02.81-, F02.A1-, F02.B1-, F02.C1-); dementia with mood disturbance (F02.83, F02.A3, F02.B3, F02.C3); dementia with psychotic disturbance (F02.82, F02.A2, F02.B2, F02.C2); dementia without behavioral disturbance (F02.80, F02.A0, F02.B0, F02.C0); and 1 more listed in the Instructional Notes section. The additional code is reported after G31.9.

What is the ICD-9 equivalent of G31.9?

Under the General Equivalence Mappings, degenerative disease of nervous system, unspecified converts to ICD-9-CM 330.9 (cereb degen in child NOS) and 331.9 (cereb degeneration NOS). The mapping is approximate, so confirm the match fits the documentation.

Does G31.9 risk-adjust for Medicare Advantage payment?

Not for Medicare Advantage. G31.9 mapped to HCC 52 in the retired CMS-HCC V24 model, which last determined payment in 2025, but it maps to no category in the live V28 model; see all codes that no longer risk-adjust. It still risk-adjusts in the ESRD (V21) category 52 (Dementia Without Complication), ESRD (V24) category 52 (Dementia Without Complication), and RxHCC Part D (V08) category 112 (Dementia, Except Alzheimer's Disease).