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

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for G91.8 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on G91.8 itself; “inherited from” names the category or block whose note applies here.

Coder workflow for G91.8

MedCoder structured workflow — derived from this code’s own official record

Before you code G91.8

1. “Other” (NEC) means the condition is specified in the record but no dedicated code captures it. Confirm the documented form is not one a sibling code names before settling on G91.8; if the record states no specifics at all, the unspecified sibling applies instead. “Other” codes are for documented conditions the classification gives no specific code; “unspecified” codes are for records lacking the detail (Guidelines I.A.9.a, I.A.9.b).

   ReviewG91.0, G91.1, G91.2, G91.4

   See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with G91.8. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

   See the official tabular notes

Choose the right path

1. Does the documentation support a condition named in G91.8’s Excludes1 note?
   Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
   No → Continue.

   ReviewQ07, Q03, Q05

Consider G91.8. Then confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement

Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).

Any detail beyond this code’s title

What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

• Excludes1 — check before selecting G91.8(3 notes)

  Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with G91.8: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

  CompareQ07, Q03, Q05

  See the official tabular notes · Guidelines I.A.12.a

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Indexed Clinical Terms (1)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

• Hydrocephalus (acquired) (external) (internal) (malignant) (recurrent), specified NEC

Don’t confuse G91.8 with…

MedCoder analysis — pairs from this code’s official notes, distinctions written by MedCoder

The codes most likely to be picked instead of G91.8 — named in its own official notes, or sharing its laterality or encounter family — with what separates them and which documentation moves the case. Each links to the full comparison.

• Q07— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of Arnold-Chiari syndrome with hydrocephalus moves the case to Q07; the two are not reported together unless documented as unrelated.

  Full comparison: G91.8 vs Q07 →

• Q03— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of congenital hydrocephalus moves the case to Q03; the two are not reported together unless documented as unrelated.

  Full comparison: G91.8 vs Q03 →

• Q05— Excludes1 conflict (Excludes1 — generally not reported together)

  Not coded here — the tabular list excludes this pair for the same encounter.

  Not reported together with this code, except where the two conditions are documented as unrelated.

  What moves you there: Documentation of spina bifida with hydrocephalus moves the case to Q05; the two are not reported together unless documented as unrelated.

  Full comparison: G91.8 vs Q05 →

• G91.3— Specificity relationship (Review)

  Overlapping diagnoses in the same category.

  Overlapping codes in the same category — the documentation decides which is the more specific fit.

  What moves you there: A record that documents no more than “Post-traumatic hydrocephalus, unspecified” takes G91.3; specificity in the record moves it back here.

  Full comparison: G91.8 vs G91.3 →

Relationships are read from this code's own official notes, its laterality siblings and its encounter family; the wording of each distinction is MedCoder editorial, and the documentation named in each “what moves you there” line is the official note phrase or sibling title itself. Source: CMS ICD-10-CM tabular instructional notes; ICD-10-CM Official Guidelines, Section I.A.12.a.

Verify Before Coding

• CC as a secondary diagnosis (FY2026). Can raise the stay's MS-DRG severity tier.

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name G91.8 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes1 note: Q03 — Congenital hydrocephalus (via G91.-).

These codes’ tabular lists mark this diagnosis as mutually exclusive — not reported together unless the conditions are documented as unrelated.

MS-DRG Grouper Relationships (FY2026)

Potential MS-DRG participation — not a DRG assignment.

FY2026 MS-DRG: CC — Complication or Comorbidity. Reported as a secondary diagnosis, this code raises the stay's MS-DRG severity tier — except when the principal diagnosis is one of 108 clinically related codes on its CMS exclusion list.

Named in the grouper logic of 2 MS-DRGs: DRG 056 (MDC 01), DRG 057 (MDC 01).

From the CMS MS-DRG Definitions Manual (Appendices B and C). Actual DRG assignment depends on the complete claim.

Clinical classification (AHRQ CCSR):NVS020 — Other nervous system disorders (neither hereditary nor degenerative) (default).

Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Healthcare Cost and Utilization Project (HCUP), Agency for Healthcare Research and Quality.

Related Codes

Same clinical process (MS-DRG)

Acts as CC — raises the severity of other admissions. CMS groups these diagnoses into one MS-DRG principal-diagnosis exclusion process — a CC/MCC on this list never raises severity when the principal diagnosis is also on it.

G31.85 — Corticobasal degeneration, G31.86 — Alexander disease, G31.87 — Primary progressive apraxia of speech, G31.89 — Other specified degenerative diseases of nervous system, G31.9 — Degenerative disease of nervous system, unspecified, G90.1 — Familial dysautonomia [Riley-Day], G91.0 — Communicating hydrocephalus, G91.1 — Obstructive hydrocephalus, G91.2 — (Idiopathic) normal pressure hydrocephalus, G91.3 — Post-traumatic hydrocephalus, unspecified, G91.9 — Hydrocephalus, unspecified, G93.81 — Temporal sclerosis, G93.89 — Other specified disorders of brain, G93.9 — Disorder of brain, unspecified, Q03.0 — Malformations of aqueduct of Sylvius, Q03.1 — Atresia of foramina of Magendie and Luschka, Q03.8 — Other congenital hydrocephalus, Q03.9 — Congenital hydrocephalus, unspecified, Q04.4 — Septo-optic dysplasia of brain, Q04.5 — Megalencephaly, +87 more

Same CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Dementia, Mild or Unspecified) for risk-adjusted payment.

F03.A3 — Unspecified dementia, mild, with mood disturbance, F03.A4 — Unspecified dementia, mild, with anxiety, G30.0 — Alzheimer's disease with early onset, G30.1 — Alzheimer's disease with late onset, G30.8 — Other Alzheimer's disease, G30.9 — Alzheimer's disease, unspecified, G31.01 — Pick's disease, G31.09 — Other frontotemporal neurocognitive disorder, G31.81 — Alpers disease, G31.82 — Leigh's disease, G31.83 — Neurocognitive disorder with Lewy bodies, G31.86 — Alexander disease, G91.0 — Communicating hydrocephalus, G91.1 — Obstructive hydrocephalus, G91.2 — (Idiopathic) normal pressure hydrocephalus, G91.3 — Post-traumatic hydrocephalus, unspecified, G91.4 — Hydrocephalus in diseases classified elsewhere, G91.9 — Hydrocephalus, unspecified, G93.7 — Reye's syndrome, I67.3 — Progressive vascular leukoencephalopathy, +44 more

Related risk categories

These categories interact through CMS's HCC hierarchy — one can suppress the other's risk-adjustment weight when both are present on a claim.

Dementia, Severe, Dementia, Moderate

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Other nervous system disorders (neither hereditary nor degenerative)).

G90.8 — Other disorders of autonomic nervous system, G90.81 — Serotonin syndrome, G90.89 — Other disorders of autonomic nervous system, G90.9 — Disorder of the autonomic nervous system, unspecified, G90.A — Postural orthostatic tachycardia syndrome [POTS], G91.0 — Communicating hydrocephalus, G91.1 — Obstructive hydrocephalus, G91.2 — (Idiopathic) normal pressure hydrocephalus, G91.3 — Post-traumatic hydrocephalus, unspecified, G91.4 — Hydrocephalus in diseases classified elsewhere, G91.9 — Hydrocephalus, unspecified, G92 — Toxic encephalopathy, G92.00 — Immune effector cell-associated neurotoxicity syndrome, grade unspecified, G92.01 — Immune effector cell-associated neurotoxicity syndrome, grade 1, G92.02 — Immune effector cell-associated neurotoxicity syndrome, grade 2, G92.03 — Immune effector cell-associated neurotoxicity syndrome, grade 3, G92.04 — Immune effector cell-associated neurotoxicity syndrome, grade 4, G92.05 — Immune effector cell-associated neurotoxicity syndrome, grade 5, G92.8 — Other toxic encephalopathy, G92.9 — Unspecified toxic encephalopathy, +97 more

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main term “Hydrocephalus”; these codes share that main term but sit in a different category of the Tabular List.

A50.49 — Other late congenital neurosyphilis (syphilitic, congenital), B89 — Unspecified parasitic disease (in, parasitic disease), G93.2 — Benign intracranial hypertension (otitic), O33.6 — Maternal care for disproportion due to hydrocephalic fetus (causing disproportion), O66.3 — Obstructed labor due to other abnormalities of fetus (causing disproportion, with obstructed labor), P37.1 — Congenital toxoplasmosis (due to toxoplasmosis), Q03.0 — Malformations of aqueduct of Sylvius (aqueduct Sylvius stricture), Q03.1 — Atresia of foramina of Magendie and Luschka (foramen Magendie block, congenital), Q03.8 — Other congenital hydrocephalus (congenital, specified NEC), Q03.9 — Congenital hydrocephalus, unspecified (newborn), Q05.0 — Cervical spina bifida with hydrocephalus (congenital, with spina bifida, cervical), Q05.1 — Thoracic spina bifida with hydrocephalus (congenital, with spina bifida, dorsal), Q05.2 — Lumbar spina bifida with hydrocephalus (congenital, with spina bifida, lumbar), Q05.3 — Sacral spina bifida with hydrocephalus (congenital, with spina bifida, sacral), Q05.4 — Unspecified spina bifida with hydrocephalus (congenital, with spina bifida)

Contextual Map

Every relationship of G91.8 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run G91.8 with this related code in Claim Check

Hierarchy

• G00-G99 — Chapter 6: Diseases of the Nervous System (G00-G99) (G00-G99)[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G89-G99 — Other disorders of the nervous system[Hierarchy]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Referenced by Excludes1 notes

• Q03 — Congenital hydrocephalus[Excludes1](via G91.-): “acquired hydrocephalus (G91.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2026

Clinical classification (CCSR)

• NVS020 — Other nervous system disorders (neither hereditary nor degenerative)[CCSR]— AHRQ CCSR for ICD-10-CM Diagnoses (HCUP)

Risk adjustment (CMS-HCC)

• HCC 127 — Dementia, Mild or Unspecified [CMS-HCC]— CMS-HCC V28 · 2026

MS-DRG Grouper

• CC — Complication or Comorbidity [MS-DRG severity]: “As a secondary diagnosis this code can raise the stay's MS-DRG severity tier (CC).”— CMS MS-DRG Definitions Manual (Appendix C) · FY2026
• DRG 056 — DEGENERATIVE NERVOUS SYSTEM DISORDERS WITH MCC[MS-DRG]: “DEGENERATIVE NERVOUS SYSTEM DISORDERS WITH MCC (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026
• DRG 057 — DEGENERATIVE NERVOUS SYSTEM DISORDERS WITHOUT MCC[MS-DRG]: “DEGENERATIVE NERVOUS SYSTEM DISORDERS WITHOUT MCC (MDC 01)”— CMS MS-DRG Definitions Manual (Appendix B) · FY2026

MDC crossing

• MDC 01 — Diseases and Disorders of the Nervous System[MDC crossing]: “Diseases and Disorders of the Nervous System — the grouper's crossing between diagnoses and procedures: a principal diagnosis sets the MDC, and same-MDC procedures move the stay to its surgical DRGs. 8,892 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2026

Index entries

• Hydrocephalus (acquired) (external) (internal) (malignant) (recurrent), specified NEC[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes

• G91 — Hydrocephalus[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G91.0 — Communicating hydrocephalus[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G91.1 — Obstructive hydrocephalus[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G91.2 — (Idiopathic) normal pressure hydrocephalus[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G91.3 — Post-traumatic hydrocephalus, unspecified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G91.4 — Hydrocephalus in diseases classified elsewhere[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026
• G91.9 — Hydrocephalus, unspecified[Sibling]— CMS ICD-10-CM tabular structure · icd10cm-fy2026

Change history

• FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data

CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page

Claim edits Official source data

CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page

Inpatient payment groups Official source data

CMS MS-DRG Definitions Manual (incl. Appendix B diagnosis index, Appendix C CC/MCC list, Appendix E procedure index) and IPPS Final Rule tables — v43 Release, file and checksum · Publisher’s page

Risk adjustment Official source data

2026 Mid-Year Final ICD-10 Mappings + Model Software (cms.gov/files/zip/2026-midyear-final-icd-10-mappings.zip, 2026-midyear-final-model-software.zip) — CMS-HCC V28, PY2026 mid-year final release Release, file and checksum · Publisher’s page

Change history and date-of-service validity Official source data

CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page

Comparisons, relationships and the contextual map MedCoder structured relationship

Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears

Summary and FAQ answers MedCoder explanation

Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Nearest Codes in This Family

Official ICD-10-CM classifications closest to G91.8 in its code family, with their registry titles.

• G91 — Hydrocephalus
• G91.0 — Communicating hydrocephalus
• G91.1 — Obstructive hydrocephalus
• G91.2 — (Idiopathic) normal pressure hydrocephalus
• G91.3 — Post-traumatic hydrocephalus, unspecified
• G91.4 — Hydrocephalus in diseases classified elsewhere
• G91.9 — Hydrocephalus, unspecified

View all codes in the G91 family