What is ICD-10 code G91.3?

G91.3 is the ICD-10-CM diagnosis code for Post-traumatic hydrocephalus, unspecified. Post-traumatic hydrocephalus is excess cerebrospinal fluid accumulation in the brain that develops after a head injury, when the specific timing or severity details are not specified. G91.3 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. G91.3 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. G91.3 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. G91.3 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 G91.3?

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. G91.3 is a payment HCC under V28 (Dementia, Mild or Unspecified) but was not retained as a payment HCC under V24. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting G91.3?

Link this code to the original head injury code for complete clinical picture Document the temporal relationship between the head trauma and hydrocephalus development Because G91.3 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.

G91.3 ICD-10 Code: Post-traumatic hydrocephalus

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupG91.3Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Other disorders of the nervous system (G89-G99)

G91.3

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

Post-traumatic hydrocephalus, unspecified

Post-traumatic hydrocephalus is excess cerebrospinal fluid accumulation in the brain that develops after a head injury, when the specific timing or severity details are not specified.

Buddy Insight

CMS-HCC V28

Mapped

HCC 127

Dementia, Mild or Unspecified

RAF 0.464

CMS-HCC V24

Not risk-adjusted for this model/period.

RAF 0

ACA/HHS

Not risk-adjusted for this model/period.

RAF 0

ESRD/PACE

Not risk-adjusted for this model/period.

RAF 0

RXHCC

Not risk-adjusted for this model/period.

RAF 0

Code Trumping

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Code Book Path

Official

G91Hydrocephalus

G91.3Post-traumatic hydrocephalus, unspecified

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for G91.3 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for G91.3 in this effective period.

Related Child Codes

Official

G91.0Communicating hydrocephalus

G91.1Obstructive hydrocephalus

G91.2(Idiopathic) normal pressure hydrocephalus

G91.4Hydrocephalus in diseases classified elsewhere

G91.8Other hydrocephalus

Includes

Official

acquired hydrocephalus

Excludes 1

Official

Arnold-Chiari syndrome with hydrocephalus (Q07.-)
congenital hydrocephalus (Q03.-)
spina bifida with hydrocephalus (Q05.-)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for G91.3 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for G91.3 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for G91.3 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

History of antecedent traumatic brain injury with documentation of temporal relationship

Imaging demonstrating progressive ventriculomegaly after traumatic brain injury

Clinical deterioration or failure to improve as expected post-trauma (worsening cognition, gait, or incontinence)

Differentiation from ex-vacuo ventriculomegaly (brain atrophy) which does not represent true hydrocephalus

MEAT Support

HCC Buddy guidance

History of antecedent traumatic brain injury with documentation of temporal relationship

Imaging demonstrating progressive ventriculomegaly after traumatic brain injury

Clinical deterioration or failure to improve as expected post-trauma (worsening cognition, gait, or incontinence)

Differentiation from ex-vacuo ventriculomegaly (brain atrophy) which does not represent true hydrocephalus

Audit Caution

HCC Buddy guidance

Failing to use the post-traumatic specific code when the hydrocephalus clearly resulted from a documented head injury

Not coding the original traumatic brain injury as a sequela code alongside the hydrocephalus

Confusing post-traumatic ventriculomegaly from brain atrophy with true hydrocephalus requiring treatment

Missing the diagnosis entirely in trauma patients who fail to improve neurologically as expected

Common Mistakes

HCC Buddy guidance

G91.0 — Communicating hydrocephalus: non-traumatic etiology; use G91.3 when trauma is the documented cause

G91.1 — Obstructive hydrocephalus: use when post-traumatic swelling or hemorrhage causes focal obstruction

S06.9X0A — Unspecified intracranial injury, initial encounter: the original injury code, not the hydrocephalus complication

G93.1 — Anoxic brain damage: may coexist with post-traumatic hydrocephalus but represents a different pathology

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 G91.3 an HCC code?

Yes. G91.3 maps to Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model.

HCC Category Mapping

V28HCC 127, Dementia, Mild or Unspecified

0.464

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 G91.3

For G91.3to 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 G91.3 during that encounter, not just copy-forwarded from a problem list.

What This Code Means

G91.3 is the ICD-10-CM diagnosis code for post-traumatic hydrocephalus, unspecified. Post-traumatic hydrocephalus is excess cerebrospinal fluid accumulation in the brain that develops after a head injury, when the specific timing or severity details are not specified. G91.3 sits in the ICD-10-CM chapter for diseases of the nervous system (g00-g99), within the section covering other disorders of the nervous system (g89-g99).

Under the CMS-HCC V28 risk adjustment model, G91.3 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.464. G91.3 was not retained as a payment HCC under the older V24 model, so V28 introduced or recategorized it during the 2024–2026 phase-in. 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.

Link this code to the original head injury code for complete clinical picture. Because G91.3 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 G91.3 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Link this code to the original head injury code for complete clinical picture
•Document the temporal relationship between the head trauma and hydrocephalus development

Clinical Significance

Post-traumatic hydrocephalus develops as a complication of traumatic brain injury, typically weeks to months after the initial injury, due to impaired cerebrospinal fluid absorption from blood products or scarring in the subarachnoid space. It is an important complication to identify because it is surgically treatable and can significantly improve neurological recovery after brain injury. This code captures the ongoing resource utilization of trauma patients with this complication.

Documentation Requirements

✓History of antecedent traumatic brain injury with documentation of temporal relationship
✓Imaging demonstrating progressive ventriculomegaly after traumatic brain injury
✓Clinical deterioration or failure to improve as expected post-trauma (worsening cognition, gait, or incontinence)
✓Differentiation from ex-vacuo ventriculomegaly (brain atrophy) which does not represent true hydrocephalus
✓Treatment plan or surgical intervention documentation (shunt placement or endoscopic procedure)

Commonly Confused Codes

•G91.0: Communicating hydrocephalus: non-traumatic etiology; use G91.3 when trauma is the documented cause
•G91.1: Obstructive hydrocephalus: use when post-traumatic swelling or hemorrhage causes focal obstruction
•S06.9X0A: Unspecified intracranial injury, initial encounter: the original injury code, not the hydrocephalus complication
•G93.1: Anoxic brain damage: may coexist with post-traumatic hydrocephalus but represents a different pathology