G40.509
BillableEpileptic seizures related to external causes, not intractable, without status epilepticus
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is G40.509 an HCC code?
Yes. G40.509 maps to Seizure Disorders and Convulsions under the CMS-HCC V28 risk adjustment model (and Seizure Disorders and Convulsions under V24).
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 G40.509
For G40.509 to 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 G40.509 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
G40.509 is the ICD-10-CM diagnosis code for epileptic seizures related to external causes, not intractable, without status epilepticus. Seizures triggered by external causes that can be controlled with medication, not currently in a prolonged seizure state. G40.509 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering episodic and paroxysmal disorders (g40-g47).
Under the CMS-HCC V28 risk adjustment model, G40.509 maps to Seizure Disorders and Convulsions (HCC 201) with a community, non-dual, aged base RAF weight of 0.262. Under the older V24 model, G40.509 mapped to the same category but with a base RAF weight of 0.244 — V28 recalibrated weights across the entire model. 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 is used when the patient is not experiencing status epilepticus at the time of encounter. Because G40.509 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 G40.509 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code is used when the patient is not experiencing status epilepticus at the time of encounter
- •Document the specific external trigger (environmental, emotional, or physiological) when possible
Clinical Significance
Epileptic seizures related to external causes without status epilepticus captures patients with seizures triggered by identifiable external factors who are not in a prolonged seizure state. These patients may have seizures provoked by stress, sleep deprivation, substance use, or metabolic disturbances. Identifying and addressing the external trigger is key to preventing recurrence and guides treatment decisions.
Documentation Requirements
- ✓Specific external cause documented (e.g., alcohol, drug exposure, sleep deprivation, metabolic disturbance)
- ✓Confirmation that seizures are not intractable (medication-responsive)
- ✓Confirmation that status epilepticus is NOT present
- ✓Seizure description including type and duration
- ✓External cause code assignment
- ✓Plan for trigger avoidance and follow-up
Commonly Confused Codes
- •G40.501 — Same condition but WITH status epilepticus; requires prolonged seizure documentation
- •G40.409 — Other generalized epilepsy without status epilepticus; use when no external cause identified
- •R56.9 — Unspecified convulsions; for isolated seizure without established epilepsy diagnosis
- •G40.909 — Unspecified epilepsy; less specific and does not capture the external cause relationship
- •F10.231 — Alcohol dependence with withdrawal seizures; more specific when alcohol withdrawal is the cause