G13.1
BillableOther systemic atrophy primarily affecting central nervous system in neoplastic disease
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
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
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.
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
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