G91.8 ICD-10-CM Code: Other hydrocephalus
HCC Buddy Code Card
Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.
FY 2026 Apr update / Diseases of the nervous system (G00-G99) / Other disorders of the nervous system (G89-G99)
G91.8
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceOther hydrocephalus
A condition where excess fluid builds up in the brain due to various causes other than the more common types of hydrocephalus. This fluid accumulation can increase pressure inside the skull and affect brain function.
Buddy Insight
Other hydrocephalus captures specified forms of hydrocephalus that do not fit into the communicating, obstructive, normal pressure, or post-traumatic categories.
CMS-HCC V28
MappedHCC 127
RAF 0.464
CMS-HCC V24
00
RAF 0
ACA/HHS
00
RAF 0
ESRD/PACE
00
RAF 0
RXHCC
00
RAF 0
Code Trumping
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Code Book Path
Inclusion Terms
OfficialICD-10-CM does not list inclusion terms for G91.8 in this effective period.
Excludes 2
OfficialICD-10-CM does not list Excludes 2 notes for G91.8 in this effective period.
Related Child Codes
Includes
Official- acquired hydrocephalus
Excludes 1
Official- Arnold-Chiari syndrome with hydrocephalus (Q07.-)
- congenital hydrocephalus (Q03.-)
- spina bifida with hydrocephalus (Q05.-)
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for G91.8 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for G91.8 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for G91.8 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
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 G91.8 an HCC code?
Yes. G91.8 maps to Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model.
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 G91.8
For G91.8to 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 G91.8 during that encounter, not just copy-forwarded from a problem list.
What This Code Means
G91.8 is the ICD-10-CM diagnosis code for other hydrocephalus. A condition where excess fluid builds up in the brain due to various causes other than the more common types of hydrocephalus. This fluid accumulation can increase pressure inside the skull and affect brain function. G91.8 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, G91.8 maps to Dementia, Mild or Unspecified (HCC 127) with a community, non-dual, aged base RAF weight of 0.464. G91.8 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.
Use this code when hydrocephalus is present but doesn't fit into the communicating or obstructive categories. Because G91.8 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 G91.8 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Use this code when hydrocephalus is present but doesn't fit into the communicating or obstructive categories
- •Ensure documentation specifies the underlying cause or type of hydrocephalus to support medical necessity
Clinical Significance
Other hydrocephalus captures specified forms of hydrocephalus that do not fit into the communicating, obstructive, normal pressure, or post-traumatic categories. This includes conditions such as arrested hydrocephalus or other atypical presentations. Accurate use of this code prevents inappropriate use of the unspecified code when the type is documented but does not match other specific categories.
Documentation Requirements
- ✓Specification of the type of hydrocephalus that differentiates it from communicating, obstructive, normal pressure, or post-traumatic
- ✓Imaging evidence of ventricular enlargement or other hydrocephalus findings
- ✓Clinical symptoms and their relationship to the hydrocephalus
- ✓Treatment plan and any surgical interventions in place
- ✓Rationale for why more specific hydrocephalus codes do not apply
Commonly Confused Codes
- •G91.9: Hydrocephalus, unspecified: use only when no further detail is available; G91.8 is for specified-but-other types
- •G91.0: Communicating hydrocephalus: use when CSF absorption is impaired with open flow pathways
- •G91.2: Normal pressure hydrocephalus: use when the classic triad and normal CSF pressure are documented
- •G91.1: Obstructive hydrocephalus: use when a specific point of obstruction is identified