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I66.01 ICD-10-CM Code: Occlusion and stenosis of right middle cerebral artery

Compare with another codeCheck this code on a claim

Billing Status: YES. This is a valid, specific, and billable ICD-10-CM reference.

Coding at a Glance

Tabular directives
4 inclusion terms · 2 Excludes1 · 8 use-additional codes

Billable · FY2027A valid, specific ICD-10-CM code, reportable for dates of service in FY2027.

What you need to know

Source: CMS/NCHS Official ICD-10-CM tabular notes, quoted. From the CMS/NCHS tabular list for the release in force. A note the category or block publishes applies to this code too; the Instructions section marks which is which.

Use additional codeReport with this code when documented
  • code to identify presence of:
  • alcohol abuse and dependence (F10.-)
  • exposure to environmental tobacco smoke (Z77.22)
  • history of tobacco dependence (Z87.891)

+4 more in Instructions

Excludes1Never report with this code
  • Occlusion and stenosis of cerebral artery causing cerebral infarction (I63.3-I63.5)
  • traumatic intracranial hemorrhage (S06.-)
IncludesWhat this code covers
  • embolism of cerebral artery
  • narrowing of cerebral artery
  • obstruction (complete) (partial) of cerebral artery
  • thrombosis of cerebral artery

Most relevant related codes MedCoder-derived

Read off the official notes above and this code’s own position in the tabular list. Which to report is a documentation question; Compare shows the two side by side.

CompareCheck ClaimView Related Codes

Inpatient Payment Groups (MS-DRG)

Potential MS-DRG participation — not a DRG assignment.

MS-DRGs this diagnosis helps define, as principal or secondary, per the CMS ICD-10-CM/PCS MS-DRG Definitions Manual v44, Appendix B.

  • MS-DRG 061 — ISCHEMIC STROKE, PRECEREBRAL OCCLUSION OR TRANSIENT ISCHEMIA WITH THROMBOLYTIC AGENT WITH MCC (MDC 01)
  • MS-DRG 062 — ISCHEMIC STROKE, PRECEREBRAL OCCLUSION OR TRANSIENT ISCHEMIA WITH THROMBOLYTIC AGENT WITH CC (MDC 01)
  • MS-DRG 063 — ISCHEMIC STROKE, PRECEREBRAL OCCLUSION OR TRANSIENT ISCHEMIA WITH THROMBOLYTIC AGENT WITHOUT CC/MCC (MDC 01)
  • MS-DRG 067 — PRECEREBRAL OCCLUSION WITHOUT INFARCTION WITH MCC (MDC 01)
  • MS-DRG 068 — PRECEREBRAL OCCLUSION WITHOUT INFARCTION WITHOUT MCC (MDC 01)
  • MS-DRG 791 — PREMATURITY WITH MAJOR PROBLEMS (MDC 15)
  • MS-DRG 793 — FULL TERM NEONATE WITH MAJOR PROBLEMS (MDC 15)

A diagnosis appearing in a group's logic does not by itself determine the DRG assigned to a stay; the grouper uses the full claim.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for I66.01 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2027Effective: October 1, 2026

Trace:FY2027 changesChange historyRelease, file and checksum

Notes without a marker are published on I66.01 itself; “inherited from” names the category or block whose note applies here.

Includes

Conditions the official ICD-10-CM tabular list includes under this code.

  • embolism of cerebral artery
  • narrowing of cerebral artery
  • obstruction (complete) (partial) of cerebral artery
  • thrombosis of cerebral artery

Source: inherited from I66

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Use Additional Code

Supplementary codes the tabular list directs you to add.

  • Use additional code to identify presence of:
  • alcohol abuse and dependence (F10.-)
  • exposure to environmental tobacco smoke (Z77.22)
  • history of tobacco dependence (Z87.891)
  • hypertension (I10-I1A)
  • occupational exposure to environmental tobacco smoke (Z57.31)
  • tobacco dependence (F17.-)
  • tobacco use (Z72.0)

Source: inherited from I60-I69

Coder workflow for I66.01

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

Before you code I66.01

  1. Laterality is coded in this family. Confirm the side documented — right, left, or bilateral — and select the matching code (this page’s code: right). The unspecified-side code applies only when the record states no side. Laterality is assigned from the documented side; where a bilateral code exists and both sides are documented, it is used instead of two unilateral codes (Guidelines I.B.13).

    ReviewI66.02 · left, I66.09 · unspecified

    Guide: Laterality coding →

  2. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with I66.01. 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 I66.01’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.

    ReviewS06

  2. Is the side documented?
    Yes → Select the code for the documented side (or the bilateral code when both sides are documented and one exists).
    No → Use the unspecified-side code only when the record states no side; a query is the alternative.

    ReviewI66.02 · left, I66.09 · unspecified

Consider I66.01. Then work the Use Additional Code note, and 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).
Laterality
Right, left or bilateral as documented; unspecified only when the record states no side (Guidelines I.B.13).
The conditions the Use Additional Code note names
Reported with this code when documented; a conditional instruction (“if applicable”, “if known”) applies only when the record supports it.

Official instructions as workflow

  • Excludes1 — check before selecting I66.01(2 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with I66.01: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareS06

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

  • Use Additional Code — after identifying I66.01(8 notes)

    Coding workflow: Check whether the documentation supports the additional code(s) the note names, and report them with I66.01 when it does. Where the instruction is conditional (“if applicable”, “if known”), it applies only when the record documents the condition.

    ReviewF10, Z77.22, Z87.891, Z57.31, F17, Z72.0

    See the official tabular notes · Guidelines I.A.13

Coding decision scenarios

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

Documentation: Both the condition I66.01 describes and a condition named in its Excludes1 note are documented for the same encounter.

Coding question: Can both codes be reported?

Path: Review the Excludes1 note and the excluded code, and look for a provider statement on whether the two conditions are related.

Reason: Excludes1 means the two are not coded together; the exception is when the record shows the conditions are unrelated to each other (Guidelines I.A.12.a).

ReviewS06

Documentation: The record documents the condition on one side only.

Coding question: Which code in this family applies?

Path: Select the sibling code for the documented side.

Reason: Laterality is assigned from the documented side; the unspecified-side code is for records that state no side (Guidelines I.B.13).

ReviewI66.02 · left, I66.09 · unspecified

Documentation: A condition the Use Additional Code note names is documented.

Coding question: Is a second code reported with I66.01?

Path: Review the Use Additional Code note and the code it names.

Reason: The additional code is reported when the record documents the condition; a conditional instruction applies only when its condition is met (Guidelines I.A.13).

ReviewF10, Z77.22, Z87.891, Z57.31, F17, Z72.0

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.

Don’t confuse I66.01 with…

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

The codes most likely to be picked instead of I66.01 — 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.

  • I63.3— 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 Occlusion and stenosis of cerebral artery causing cerebral infarction moves the case to I63.3; the two are not reported together unless documented as unrelated.

    Full comparison: I66.01 vs I63.3 →

  • I63.5— 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 Occlusion and stenosis of cerebral artery causing cerebral infarction moves the case to I63.5; the two are not reported together unless documented as unrelated.

    Full comparison: I66.01 vs I63.5 →

  • S06— 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 traumatic intracranial hemorrhage moves the case to S06; the two are not reported together unless documented as unrelated.

    Full comparison: I66.01 vs S06 →

  • I66.09— 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 “Occlusion and stenosis of unspecified middle cerebral artery” takes I66.09; specificity in the record moves it back here.

    Full comparison: I66.01 vs I66.09 →

  • I66.02— Laterality relationship (Review)

    Same condition; these differ only in which side is documented.

    The same condition on the other side; the documented side decides, and a bilateral code, where one exists, replaces the pair when both sides are documented.

    What moves you there: A documented left side moves the case to I66.02.

    Full comparison: I66.01 vs I66.02 →

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.

Coding context

Guidelines, coding notes, decision aids, relationships (with MS-DRG and CCSR classification), hierarchy, HCC, coverage and the context map: what a coder reaches for after the core. Each section names whether it is official source data, a MedCoder-derived relationship or MedCoder editorial.

Code Overview

Occlusion and stenosis of right middle cerebral artery is a billable ICD-10-CM diagnosis code (I66.01).

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Decision Points

The directives on this code's own record, as a pre-claim checklist.

  1. Laterality is coded in this family. Confirm the documented side matches the code — the opposite-side siblings are in the relationships section. See the opposite-side sibling codes
  2. 8 Use Additional Code instructions — report the named additional code(s) when the documentation supports them. See the Use Additional Code notes
  3. 2 Excludes1 entries — codes named there are generally not reported together with this code (Guidelines I.A.12.a). See the Excludes1 notes

Checklist rows are derived from this code's own official directives; the wording of each check is MedCoder editorial. The official notes themselves are in the sections each row links to.

Verify Before Coding

  • No Medicare Code Editor or MS-DRG Definitions Manual restrictions apply to this code.

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-derived relationships — computed from published CMS and AHRQ datasets

Other codes that name I66.01 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: G31.84 — Mild cognitive impairment of uncertain or unknown etiology (via I66.-).

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

Referenced by 3 Excludes2 notes: I10 — Essential (primary) hypertension (via I66.-), I15 — Secondary hypertension (via I66.-), I74 — Arterial embolism and thrombosis (via I66.-).

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

Referenced by 1 Code First instruction: G46 — Vascular syndromes of brain in cerebrovascular diseases (via I66.-).

Each of these codes carries a Code First note naming this condition — when that code is reported, THIS code is sequenced first, ahead of it.

MS-DRG Grouper Relationships (FY2027)

Potential MS-DRG participation — not a DRG assignment.

FY2027 MS-DRG: not on the CMS CC/MCC list — as a secondary diagnosis this code does not change MS-DRG severity for that release.

Named in the grouper logic of 7 MS-DRGs: DRG 061 (MDC 01), DRG 062 (MDC 01), DRG 063 (MDC 01), DRG 067 (MDC 01), DRG 068 (MDC 01), DRG 791 (MDC 15), DRG 793 (MDC 15).

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

Clinical classification (AHRQ CCSR):CIR023 — Occlusion or stenosis of precerebral or cerebral arteries without infarction (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 condition, opposite side

Same category and title, differing only in which side of the body is affected.

I66.02 — Occlusion and stenosis of left middle cerebral artery (left)Compare I66.01 vs I66.02 →, I66.09 — Occlusion and stenosis of unspecified middle cerebral artery (unspecified)Compare I66.01 vs I66.09 →

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical category (Occlusion or stenosis of precerebral or cerebral arteries without infarction).

I65.02 — Occlusion and stenosis of left vertebral artery, I65.03 — Occlusion and stenosis of bilateral vertebral arteries, I65.09 — Occlusion and stenosis of unspecified vertebral artery, I65.1 — Occlusion and stenosis of basilar artery, I65.21 — Occlusion and stenosis of right carotid artery, I65.22 — Occlusion and stenosis of left carotid artery, I65.23 — Occlusion and stenosis of bilateral carotid arteries, I65.29 — Occlusion and stenosis of unspecified carotid artery, I65.8 — Occlusion and stenosis of other precerebral arteries, I65.9 — Occlusion and stenosis of unspecified precerebral artery, I66.02 — Occlusion and stenosis of left middle cerebral artery, I66.03 — Occlusion and stenosis of bilateral middle cerebral arteries, I66.09 — Occlusion and stenosis of unspecified middle cerebral artery, I66.11 — Occlusion and stenosis of right anterior cerebral artery, I66.12 — Occlusion and stenosis of left anterior cerebral artery, I66.13 — Occlusion and stenosis of bilateral anterior cerebral arteries, I66.19 — Occlusion and stenosis of unspecified anterior cerebral artery, I66.21 — Occlusion and stenosis of right posterior cerebral artery, I66.22 — Occlusion and stenosis of left posterior cerebral artery, I66.23 — Occlusion and stenosis of bilateral posterior cerebral arteries, +5 more

Same Index main term, other category

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

I63.51 — Cerebral infarction due to unspecified occlusion or stenosis of middle cerebral artery (artery, cerebral, middle, with infarction), I63.52 — Cerebral infarction due to unspecified occlusion or stenosis of anterior cerebral artery (artery, cerebral, anterior, with infarction), I63.53 — Cerebral infarction due to unspecified occlusion or stenosis of posterior cerebral artery (artery, cerebral, posterior, with infarction), I63.54 — Cerebral infarction due to unspecified occlusion or stenosis of cerebellar artery (artery, cerebellar, with infarction), I63.59 — Cerebral infarction due to unspecified occlusion or stenosis of other cerebral artery (artery, cerebral, specified NEC, with infarction), I65.0 — Occlusion and stenosis of vertebral artery (artery, vertebral), I65.1 — Occlusion and stenosis of basilar artery (artery, basilar), I65.2 — Occlusion and stenosis of carotid artery (artery, carotid), I65.8 — Occlusion and stenosis of other precerebral arteries (artery, auditory, internal), I65.9 — Occlusion and stenosis of unspecified precerebral artery (artery, precerebral), I70.9 — Other and unspecified atherosclerosis (artery), I70.92 — Chronic total occlusion of artery of the extremities (artery, total, extremities), I74.09 — Other arterial embolism and thrombosis of abdominal aorta (aortoiliac), I74.2 — Embolism and thrombosis of arteries of the upper extremities (peripheral arteries, upper extremity), I74.4 — Embolism and thrombosis of arteries of extremities, unspecified (artery, peripheral, thrombotic or embolic), I74.5 — Embolism and thrombosis of iliac artery (iliac artery), I77.1 — Stricture of artery (peripheral arteries, due to stricture or stenosis), I77.9 — Disorder of arteries and arterioles, unspecified (artery, peripheral), I89.0 — Lymphedema, not elsewhere classified (thoracic duct), I99.8 — Other disorder of circulatory system (vessel), +73 more

Lab tests where this diagnosis supports Medicare coverage (NCD)

Medicare's National Coverage Determination (NCD) program lists this diagnosis as medical justification for these lab tests.

Lipid Panel, Partial Thromboplastin Time (PTT) Test, Prothrombin Time Test and INR (PT/INR)

Contextual Map

Every relationship of I66.01 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 I66.01 with these 5 related codes in Claim Check

Hierarchy

Referenced by Excludes1 notes

Referenced by Excludes2 notes

  • I10 — Essential (primary) hypertension[Excludes2](via I66.-): “essential (primary) hypertension involving vessels of brain (I60-I69)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2027
  • I15 — Secondary hypertension[Excludes2](via I66.-): “secondary hypertension involving vessels of brain (I60-I69)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2027
  • I74 — Arterial embolism and thrombosis[Excludes2](via I66.-): “cerebral embolism and thrombosis (I63.3-I63.5, I66.-)”— CMS ICD-10-CM tabular instructional notes · icd10cm-fy2027

Referenced by Code First instructions

Clinical classification (CCSR)

MS-DRG Grouper

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. 8,919 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2027
  • MDC 15 — Newborns and Other Neonates with Conditions Originating in Perinatal Period[MDC crossing]: “Newborns and Other Neonates with Conditions Originating in Perinatal Period — 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.”— CMS MS-DRG Definitions Manual · FY2027

Nearest codes (18)

Change history

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

Reference

Index terms and tables, published questions and FAQ, every source behind this page with its release and checksum, the date-of-service check and the complete change history.

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 FY2027 tabular list, index and tables, effective October 1, 2026 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v44.0 (October 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 — v44 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-derived 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 editorial 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.

Current data releases:ICD-10-CM FY2027 · ICD-10-PCS FY2027 · HCPCS October 2026 · MS-DRG v44 · Medicare Code Editor v44.0 · NCCI PTP Q4 2026 · MUE Q4 2026 · NCD code lists 2026-01 · LCD export September 28, 2026 · All releases and sources

Labels on this page: Official source data · MedCoder-derived relationship · MedCoder editorial explanation. How to read the labels · All data sources and release dates · CMS coding rules

Cite this page

Reference this page in a research guide, syllabus or article. The release is included because code content changes each year.

MedCoder.ai. "I66.01 — Occlusion and stenosis of right middle cerebral artery." ICD-10-CM FY2027. https://medcoder.ai/icd10/code/i66.01-occlusion-and-stenosis-of-right-middle-cerebral-artery

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Occlusion and stenosis of right middle cerebral artery

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027, and none are recorded for this code. Note changes are tracked from FY2027 only.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to I66.01 in its code family, with their registry titles.

View all codes in the I66 family