What is ICD-10 code G31.85?

G31.85 is the ICD-10-CM diagnosis code for Corticobasal degeneration. It belongs to the ICD-10-CM chapter for diseases of the nervous system (g00-g99). G31.85 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. G31.85 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. G31.85 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. G31.85 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for G31.85?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. G31.85 was a payment HCC under V24 (Dementia Without Complication) but the V28 recalibration removed it from the risk adjustment model. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting G31.85?

This code represents a neurodegenerative disease; ensure documentation clearly indicates corticobasal degeneration diagnosis rather than similar conditions like Parkinson's disease or progressive supranuclear palsy Code G31.85 is specific to corticobasal degeneration; do not use more general codes like G31.83 (Dementia with Lewy bodies) unless the patient has a confirmed dual diagnosis Because G31.85 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM G31.85: Corticobasal degeneration

ICD-10-CM Code View

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Code lookupG31.85Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Other degenerative diseases of the nervous system (G30-G32)

G31.85

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Corticobasal degeneration

Corticobasal degeneration is a rare progressive brain disorder that causes gradual loss of nerve cells in specific areas of the brain, leading to problems with movement, balance, and cognitive function. Patients typically experience stiffness, jerky movements, and difficulty with coordination that worsens over time.

Buddy Insight

Corticobasal degeneration is a rare progressive neurodegenerative disorder affecting the cerebral cortex and basal ganglia.

CMS-HCC V28

N/A

—

Not risk-adjusted for this model/period.

Not mapped

CMS-HCC V24

Mapped

HCC 52

Dementia Without Complication

RAF 0.346

ACA/HHS

N/A

—

Not risk-adjusted for this model/period.

Not mapped

ESRD/PACE

Mapped

HCC 52

Dementia Without Complication

RAF 0.042

RXHCC

Mapped

HCC 112

Dementia and Other Specified ...

Not separately weighted

Code Book Path

Official

G31Other degenerative diseases of nervous system, not elsewhere classified

G31.8Other specified degenerative diseases of nervous system

G31.85Corticobasal degeneration

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for G31.85 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for G31.85 in this effective period.

Related Child Codes

Official

G31.80Leukodystrophy, unspecified

G31.81Alpers disease

G31.82Leigh's disease

G31.83Neurocognitive disorder with Lewy bodies

G31.84Mild cognitive impairment of uncertain or unknown etiology

Includes

Official

ICD-10-CM does not list Includes notes for G31.85 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for G31.85 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for G31.85 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for G31.85 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for G31.85 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Provider documentation explicitly stating corticobasal degeneration or corticobasal syndrome

Neurological examination documenting asymmetric motor findings (rigidity, dystonia, myoclonus)

Neuroimaging (MRI) showing asymmetric cortical atrophy

Documentation of characteristic features: alien limb phenomenon, apraxia, cortical sensory loss

MEAT Support

HCC Buddy guidance

Provider documentation explicitly stating corticobasal degeneration or corticobasal syndrome

Neurological examination documenting asymmetric motor findings (rigidity, dystonia, myoclonus)

Neuroimaging (MRI) showing asymmetric cortical atrophy

Documentation of characteristic features: alien limb phenomenon, apraxia, cortical sensory loss

Audit Caution

HCC Buddy guidance

Misdiagnosing as Parkinson's disease — corticobasal degeneration does not typically respond to levodopa and has asymmetric cortical features

Using G31.89 (other specified) instead of the specific G31.85 when corticobasal degeneration is documented

Confusing corticobasal degeneration with progressive supranuclear palsy — both are tauopathies but have distinct clinical presentations

Not recognizing the cognitive components alongside the motor symptoms

Common Mistakes

HCC Buddy guidance

G20 — Parkinson's disease; more symmetric presentation, tremor-predominant, responsive to levodopa

G23.1 — Progressive supranuclear ophthalmoplegia; has vertical gaze palsy and early falls rather than asymmetric limb dysfunction

G31.83 — Neurocognitive disorder with Lewy bodies; has visual hallucinations and fluctuating cognition

G31.89 — Other specified degenerative diseases of nervous system; use only if corticobasal degeneration cannot be specifically confirmed

Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.

Is G31.85 an HCC code?

Yes. G31.85 maps to Dementia Without Complication under the V24 model but is not retained in V28.

Code: G31.85
Description: Corticobasal degeneration
HCC (V28): No CMS-HCC V28 mapping
RAF: —
Billable: Yes
Payment year: 2026

HCC Category Mapping

V24HCC 52, Dementia Without Complication

0.346

ESRDHCC 52, Dementia Without Complication

0.042

RxHCCHCC 112, Dementia and Other Specified Brain Disorders

Not separately weighted

Each model's RAF is its CMS base weight for that model's standard population, so 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 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.

Work G31.85 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for G31.85

For G31.85to 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 G31.85 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

G31.85 is the ICD-10-CM diagnosis code for corticobasal degeneration. Corticobasal degeneration is a rare progressive brain disorder that causes gradual loss of nerve cells in specific areas of the brain, leading to problems with movement, balance, and cognitive function. Patients typically experience stiffness, jerky movements, and difficulty with coordination that worsens over time. G31.85 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering other degenerative diseases of the nervous system (g30-g32).

Under the older CMS-HCC V24 model, G31.85 maps to Dementia Without Complication (HCC 52) with a community, non-dual, aged base RAF weight of 0.346. 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 code represents a neurodegenerative disease; ensure documentation clearly indicates corticobasal degeneration diagnosis rather than similar conditions like Parkinson's disease or progressive supranuclear palsy. Because G31.85 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 G31.85 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•This code represents a neurodegenerative disease; ensure documentation clearly indicates corticobasal degeneration diagnosis rather than similar conditions like Parkinson's disease or progressive supranuclear palsy
•Code G31.85 is specific to corticobasal degeneration; do not use more general codes like G31.83 (Dementia with Lewy bodies) unless the patient has a confirmed dual diagnosis

Clinical Significance

Corticobasal degeneration is a rare progressive neurodegenerative disorder affecting the cerebral cortex and basal ganglia. It causes asymmetric motor dysfunction, cognitive decline, and distinctive features like alien limb phenomenon. Due to its rarity and complex presentation, it is frequently misdiagnosed, making documentation specificity critical for accurate risk adjustment.

Documentation Requirements

✓Provider documentation explicitly stating corticobasal degeneration or corticobasal syndrome
✓Neurological examination documenting asymmetric motor findings (rigidity, dystonia, myoclonus)
✓Neuroimaging (MRI) showing asymmetric cortical atrophy
✓Documentation of characteristic features: alien limb phenomenon, apraxia, cortical sensory loss
✓Evidence that other conditions (Parkinson's, progressive supranuclear palsy) have been considered and ruled out
✓Current treatment plan addressing motor and cognitive symptoms

Commonly Confused Codes

•G20: Parkinson's disease; more symmetric presentation, tremor-predominant, responsive to levodopa
•G23.1: Progressive supranuclear ophthalmoplegia; has vertical gaze palsy and early falls rather than asymmetric limb dysfunction
•G31.83: Neurocognitive disorder with Lewy bodies; has visual hallucinations and fluctuating cognition
•G31.89: Other specified degenerative diseases of nervous system; use only if corticobasal degeneration cannot be specifically confirmed
•G31.09: Other frontotemporal neurocognitive disorder; overlapping tau pathology but different clinical presentation