What is ICD-10 code G91.8?

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 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. G91.8 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. G91.8 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. G91.8 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.

Which HCC does G91.8 map to under V28?

G91.8 maps to CMS-HCC V28 HCC 127 (Dementia, Mild or Unspecified). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

2026 ICD-10-CM G91.8: Other hydrocephalus

G91.8 maps to CMS-HCC V28 127. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools

ICD-10-CM Code View

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Code lookupG91.8

Effective 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)

G91.8

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

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.

Buddy Insight

Other hydrocephalus captures specified forms of hydrocephalus that do not fit into the communicating, obstructive, normal pressure, or post-traumatic categories.

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 127

Dementia, Mild or Unspecified

Code-level coefficient reference

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 121

HHS-HCC 121 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 51

Dementia With Complications

Code-level coefficient reference

RXHCC

N/A

—

Not risk-adjusted for this model/period.

Not mapped

Code Book Path

Official

G91Hydrocephalus G91.8Other hydrocephalus

Inclusion Terms

Official

No inclusion terms are included in this display for G91.8. Check the code and parent instructions in the Code Book.

Excludes 2

Official

certain conditions originating in the perinatal period (P04-P96)Inherited from G00-G99
certain infectious and parasitic diseases (A00-B99)Inherited from G00-G99
complications of pregnancy, childbirth and the puerperium (O00-O9A)Inherited from G00-G99
congenital malformations, deformations, and chromosomal abnormalities (Q00-Q99)Inherited from G00-G99
endocrine, nutritional and metabolic diseases (E00-E88)Inherited from G00-G99
injury, poisoning and certain other consequences of external causes (S00-T88)Inherited from G00-G99
neoplasms (C00-D49)Inherited from G00-G99
symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified (R00-R94)Inherited from G00-G99

Related Codes

Official

G91.0Communicating hydrocephalus G91.1Obstructive hydrocephalus G91.2(Idiopathic) normal pressure hydrocephalus G91.3Post-traumatic hydrocephalus, unspecified G91.4Hydrocephalus in diseases classified elsewhere

Includes

Official

acquired hydrocephalusInherited from G91

Excludes 1

Official

Arnold-Chiari syndrome with hydrocephalus (Q07.-)Inherited from G91
congenital hydrocephalus (Q03.-)Inherited from G91
spina bifida with hydrocephalus (Q05.-)Inherited from G91

Code First

Official

No Code First sequencing instructions are included in this display for G91.8. Check the code and parent instructions in the Code Book.

Use Additional

Official

No Use Additional Code instructions are included in this display for G91.8. Check the code and parent instructions in the Code Book.

Code Also

Official

No Code Also instructions are included in this display for G91.8. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

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

MEAT Support

HCC Buddy guidance

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

Audit Caution

HCC Buddy guidance

Using G91.8 as a catch-all when a more specific hydrocephalus code is actually appropriate

Defaulting to G91.9 (unspecified) when the documentation supports G91.8 (other specified)

Not querying the provider to determine if a more specific type of hydrocephalus can be identified

Failing to document what makes this hydrocephalus 'other' rather than one of the specified types

Common Mistakes

HCC Buddy guidance

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

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is G91.8 an HCC code?

Yes. G91.8 (Other hydrocephalus) maps to HCC 127, Dementia, Mild or Unspecified under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.341. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: G91.8 is billable and maps to V28 HCC 127, Dementia, Mild or Unspecified. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: G91.8
Description: Other hydrocephalus
HCC (V28): HCC 127 — Dementia, Mild or Unspecified
RAF reference coefficient: 0.341
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 127, Dementia, Mild or Unspecified

0.341

ESRDHCC 51, Dementia With Complications

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work G91.8 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for G91.8

For G91.8, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.

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

Coder workflow notes

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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 source-labeled community, non-dual, aged reference coefficient of 0.341. No V24 mapping is shown for G91.8; use the applicable model and payment year when reviewing the V28 mapping. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

Use this code when hydrocephalus is present but doesn't fit into the communicating or obstructive categories. For G91.8, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, 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