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ICD-10-CM G40.509: Epileptic seizures related to external causes, not intractable, without status epilepticus

G40.509 is the billable ICD-10-CM code for epileptic seizures related to external causes, not intractable, without status epilepticus, in category G40 (Epilepsy and recurrent seizures) of the Episodic and paroxysmal disorders section. As a principal diagnosis it groups to MS-DRG 023 Craniotomy with Major Device Implant or Acute Complex CNS Principal Diagnosis with MCC or Antineoplastic Implant or Epilepsy with Neurostimulator, MS-DRG 100 Seizures with MCC, MS-DRG 101 Seizures without MCC (MDC 01 MDC 01: Diseases and Disorders of the Nervous System). It maps to HCC 201 (Seizure Disorders and Convulsions) in CMS-HCC V28. As a secondary diagnosis it is a CC, excluded with 89 principal diagnoses. 16 LCD billing and coding articles list it as covered.

Compiled from CMS files by the QuickIntell RCM Editorial Team · Data effective · Method: reference methodology · Report a data correction

Data currency: ICD-10-CM code descriptions: FY2027 (effective October 1, 2026); IPPS Table 5 (MS-DRG weights): v44 FY2027 (final rule + CN3) (effective October 1, 2026); ICD-10-CM to HCC mapping: PY2027 initial ICD-10-CM mappings (dates of service 2026) (payment year 2027); Medicare Coverage Database billing and coding articles: MCD release October 8, 2026 (effective October 4, 2026). Next CMS release: FY2028 code files, effective October 1, 2027 for the ICD-10-CM code descriptions; FY2028 final rule (August 2027), effective October 1, 2027; an April 1 interim grouper update when CMS issues one for the IPPS Table 5 (MS-DRG weights); PY2028 advance notice (February 2027) and the PY2027 midyear final mapping (spring 2027) for the ICD-10-CM to HCC mapping; weekly, every Thursday for the Medicare Coverage Database billing and coding articles.

Code
G40.509
billable FY2027 ICD-10-CM code
Category
G40 Epilepsy and recurrent seizures
Chapter
6: Diseases of the nervous system (G00-G99)
CC/MCC (MS-DRG v44)
CC
CMS-HCC V28
HCC 201
POA reporting
Required

Tabular list notes for G40.509

The tabular list has no notes on G40.509 itself; the notes below apply from its parent headings.

Inherited from G40 (Epilepsy and recurrent seizures)

Excludes1 (never coded together)
  • conversion disorder with seizures (F44.5)
  • convulsions NOS (R56.9)
  • post traumatic seizures (R56.1)
  • seizure (convulsive) NOS (R56.9)
  • seizure of newborn (P90)
Excludes2 (not part of this condition, may be coded together)
  • hippocampal sclerosis (G93.81)
  • mesial temporal sclerosis (G93.81)
  • temporal sclerosis (G93.81)
  • Todd's paralysis (G83.84)
Notes
  • the following terms are to be considered equivalent to intractable: pharmacoresistant (pharmacologically resistant), treatment resistant, refractory (medically) and poorly controlled

Inherited from G40.5 (Epileptic seizures related to external causes)

Inclusion terms
  • Epileptic seizures related to alcohol
  • Epileptic seizures related to drugs
  • Epileptic seizures related to hormonal changes
  • Epileptic seizures related to sleep deprivation
  • Epileptic seizures related to stress
Use additional code
  • code for adverse effect, if applicable, to identify drug (T36-T50 with fifth or sixth character 5)
Code also
  • , if applicable, associated epilepsy and recurrent seizures (G40.-)

MS-DRG grouping when G40.509 is the principal diagnosis

MS-DRG v44 groups for G40.509 as principal diagnosis
MDCMS-DRGs
01 MDC 01: Diseases and Disorders of the Nervous System023100101

The final group also depends on procedures, secondary diagnoses and discharge status; the MS-DRG lookup for G40.509 shows the full grouping logic.

CC/MCC status and inpatient edits

As a secondary diagnosis, G40.509 is a CC under MS-DRG v44, unless the principal diagnosis is one of the 89 codes in its exclusion list. Check G40.509 against a principal diagnosis.

  • Present on admission: a POA indicator is required on inpatient claims.

CMS-HCC V28 risk adjustment

In the PY2027 mapping, G40.509 maps to HCC 201 (Seizure Disorders and Convulsions). The HCC lookup shows the RAF weight and the hierarchy.

LCD billing and coding articles that list G40.509

16 current Medicare billing and coding articles list G40.509 among the diagnoses that support medical necessity for the services they cover.

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Frequently asked questions

What does ICD-10-CM G40.509 mean?

G40.509 means epileptic seizures related to external causes, not intractable, without status epilepticus. It is one of 25 billable codes under G40, epilepsy and recurrent seizures.

Which MS-DRG does G40.509 group to?

As the principal diagnosis, G40.509 groups to MS-DRG 023 Craniotomy with Major Device Implant or Acute Complex CNS Principal Diagnosis with MCC or Antineoplastic Implant or Epilepsy with Neurostimulator, MS-DRG 100 Seizures with MCC, MS-DRG 101 Seizures without MCC (MDC 01 MDC 01: Diseases and Disorders of the Nervous System) under MS-DRG v44, before procedures and secondary diagnoses are applied.

Does G40.509 map to an HCC?

Yes: HCC 201, Seizure Disorders and Convulsions, in the PY2027 CMS-HCC V28 mapping.

Is G40.509 a CC or MCC?

G40.509 is a CC under MS-DRG v44; 89 principal diagnoses exclude it.

Sources

Every figure on this page is taken from the CMS publications below, as released by the Centers for Medicare & Medicaid Services. Projection built 2026-10-10. Verify against the primary file before billing or contracting decisions. The reference methodology explains how each figure is computed from these files.

Disclaimer

The descriptor and notes are reproduced from the FY2027 ICD-10-CM files; MS-DRG, CC/MCC and HCC facts come from the CMS FY2027 and PY2027 tables. Code from the medical record and the ICD-10-CM Official Guidelines. Not legal, clinical or billing advice.

Found a figure that does not match the CMS file? Report a data correction with the CMS file and the value you expected; the request reaches the editorial team through the contact form.