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G13.1 ICD-10-CM Code: Other systemic atrophy primarily affecting central nervous system in neoplastic disease

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FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Systemic atrophies primarily affecting the central nervous system (G10-G14)

G13.1

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

Other systemic atrophy primarily affecting central nervous system in neoplastic disease

Wasting and degeneration of the brain and spinal cord that develops as a secondary effect of cancer.

Buddy the Bee presenting code insight

Buddy Insight

Other systemic atrophy primarily affecting the central nervous system in neoplastic disease captures central nervous system degeneration caused by cancer, including paraneoplastic cerebellar degeneration and other central paraneoplastic syndromes.

CMS-HCC V28

0

0

RAF 0

CMS-HCC V24

HCC 75

RAF 0.425

ACA/HHS

0

0

RAF 0

ESRD/PACE

HCC 75

RAF 0.0

RXHCC

HCC 158

RAF 0.0

Code Trumping

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Code Book Path

Official
G13Systemic atrophies primarily affecting central nervous system in diseases classified elsewhere
G13.1Other systemic atrophy primarily affecting central nervous system in neoplastic disease

Inclusion Terms

Official
  • Paraneoplastic limbic encephalopathy

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for G13.1 in this effective period.

Related Child Codes

Official
G13.0Paraneoplastic neuromyopathy and neuropathy
G13.2Systemic atrophy primarily affecting the central nervous system in myxedema
G13.8Systemic atrophy primarily affecting central nervous system in other diseases classified elsewhere

Includes

Official

ICD-10-CM does not list Includes notes for G13.1 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for G13.1 in this effective period.

Code First

Official
  • underlying neoplasm (C00-D49)

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for G13.1 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for G13.1 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance
Documented central nervous system degeneration or atrophy attributed to neoplastic disease
Underlying malignancy identified, documented, and coded first
Paraneoplastic antibody testing results if performed
Brain MRI or CT showing central nervous system atrophy or degeneration

MEAT Support

HCC Buddy guidance
Documented central nervous system degeneration or atrophy attributed to neoplastic disease
Underlying malignancy identified, documented, and coded first
Paraneoplastic antibody testing results if performed
Brain MRI or CT showing central nervous system atrophy or degeneration

Audit Caution

HCC Buddy guidance
Confusing paraneoplastic central nervous system degeneration with brain metastases — they are fundamentally different processes
Failing to code the underlying malignancy as the first-listed diagnosis
Using generic neurodegeneration codes when the cancer-associated etiology is documented
Not distinguishing between central nervous system (G13.1) and peripheral nervous system (G13.0) paraneoplastic involvement

Common Mistakes

HCC Buddy guidance
G13.0 — Paraneoplastic neuromyopathy and neuropathy: for peripheral nervous system involvement, not central
C79.31 — Secondary malignant neoplasm of brain: for direct metastatic disease, not paraneoplastic effect
G31.9 — Degenerative disease of nervous system, unspecified: does not capture the neoplastic etiology
G93.1 — Anoxic brain damage: different mechanism of brain injury

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 G13.1 an HCC code?

Yes. G13.1 maps to Myasthenia Gravis/Myoneural Conditions and Guillain-Barre Syndrome under the V24 model but is not retained in V28.

HCC Category Mapping

V24HCC 75, Myasthenia Gravis/Myoneural Conditions and Guillain-Barre Syndrome
0.425
ESRDHCC 75, Myasthenia Gravis/Myoneural Conditions and Guillain-Barre Syndrome
0.000
RxHCCHCC 158, Guillain-Barre Syndrome and Other Polyneuropathies
0.000

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 G13.1

For G13.1to 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 G13.1 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

G13.1 is the ICD-10-CM diagnosis code for other systemic atrophy primarily affecting central nervous system in neoplastic disease. Wasting and degeneration of the brain and spinal cord that develops as a secondary effect of cancer. G13.1 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering systemic atrophies primarily affecting the central nervous system (g10-g14).

Under the older CMS-HCC V24 model, G13.1 maps to Myasthenia Gravis/Myoneural Conditions and Guillain-Barre Syndrome (HCC 75) with a community, non-dual, aged base RAF weight of 0.425. 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.

Always code the underlying malignancy in addition to this code. Because G13.1 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 G13.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

  • •Always code the underlying malignancy in addition to this code
  • •Document the specific type of cancer and its relationship to the neurological manifestation

Clinical Significance

Other systemic atrophy primarily affecting the central nervous system in neoplastic disease captures central nervous system degeneration caused by cancer, including paraneoplastic cerebellar degeneration and other central paraneoplastic syndromes. This diagnosis reflects severe neurological complications of malignancy requiring multidisciplinary oncology and neurology management.

Documentation Requirements

  • ✓Documented central nervous system degeneration or atrophy attributed to neoplastic disease
  • ✓Underlying malignancy identified, documented, and coded first
  • ✓Paraneoplastic antibody testing results if performed
  • ✓Brain MRI or CT showing central nervous system atrophy or degeneration
  • ✓Neurological examination documenting central nervous system deficits
  • ✓Clinical course showing temporal relationship to cancer diagnosis

Code First

Commonly Confused Codes

  • •G13.0: Paraneoplastic neuromyopathy and neuropathy: for peripheral nervous system involvement, not central
  • •C79.31: Secondary malignant neoplasm of brain: for direct metastatic disease, not paraneoplastic effect
  • •G31.9: Degenerative disease of nervous system, unspecified: does not capture the neoplastic etiology
  • •G93.1: Anoxic brain damage: different mechanism of brain injury

Child Codes

G13.0Paraneoplastic neuromyopathy and neuropathy
G13.1Other systemic atrophy primarily affecting central nervous system in neoplastic disease
G13.2Systemic atrophy primarily affecting the central nervous system in myxedema
G13.8Systemic atrophy primarily affecting central nervous system in other diseases classified elsewhere

Code Hierarchy

└G13Systemic atrophies primarily affecting central nervous system in diseases classified elsewhere└G13.1Other systemic atrophy primarily affecting central nervous system in neoplastic disease
└G13.1Other systemic atrophy primarily affecting central nervous system in neoplastic disease

Because G13.1 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.

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