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M45.1 ICD-10-CM Code: Ankylosing spondylitis of occipito-atlanto-axial region

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

Coding at a Glance

Tabular directives
2 Excludes1 · 1 Excludes2
Risk adjustment
CMS-HCC V28: 1 category · CMS-HCC V22 category 40 · RxHCC V08 category 83

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 545 — CONNECTIVE TISSUE DISORDERS WITH MCC (MDC 08)
  • MS-DRG 546 — CONNECTIVE TISSUE DISORDERS WITH CC (MDC 08)
  • MS-DRG 547 — CONNECTIVE TISSUE DISORDERS WITHOUT CC/MCC (MDC 08)

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.

Risk Adjustment (CMS-HCC)

Payment categories this diagnosis maps to under CMS-HCC V28, payment year 2026.

  • HCC 93 — Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders (supersedes HCC 94)

Other models: CMS-HCC V22 HCC 40 · RxHCC V08 HCC 83

Risk scores depend on the enrollee's full accepted diagnosis set and segment; a category mapping alone does not determine payment.

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for M45.1 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 M45.1 itself; “inherited from” names the category or block whose note applies here.

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).

Source: inherited from M45

Excludes2 — Not Included Here

Conditions not covered by this code, but which may be reported alongside it when both are present.

Source: inherited from M45

Coder workflow for M45.1

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

Before you code M45.1

  1. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with M45.1. 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 M45.1’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.

    ReviewM02.3, M08.1

Consider M45.1. 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).

Official instructions as workflow

  • Excludes1 — check before selecting M45.1(2 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with M45.1: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareM02.3, M08.1

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

  • Excludes2 — not part of M45.1(1 note)

    Coding workflow: The conditions named in this note are not included in M45.1. When the record documents both, both may be reported; the note is a boundary, not a prohibition.

    CompareM35.2

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

Coding decision scenarios

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

Documentation: Both the condition M45.1 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).

ReviewM02.3, M08.1

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 M45.1 with…

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

The codes most likely to be picked instead of M45.1 — 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.

  • M02.3— 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 arthropathy in Reiter's disease moves the case to M02.3; the two are not reported together unless documented as unrelated.

    Full comparison: M45.1 vs M02.3 →

  • M08.1— 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 juvenile (ankylosing) spondylitis moves the case to M08.1; the two are not reported together unless documented as unrelated.

    Full comparison: M45.1 vs M08.1 →

  • M45.9— 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 “Ankylosing spondylitis of unspecified sites in spine” takes M45.9; specificity in the record moves it back here.

    Full comparison: M45.1 vs M45.9 →

  • M35.2— Excludes2 relationship (Informational)

    Distinct conditions — both may be reported.

    A distinct condition — both may be reported when both are documented.

    What moves you there: Documentation of Behçet's disease adds M35.2; it is not part of this code and both may be reported when both are documented.

    Full comparison: M45.1 vs M35.2 →

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.

Code Overview

Ankylosing spondylitis of occipito-atlanto-axial region is a billable ICD-10-CM diagnosis code (M45.1).

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

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.

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 structured relationships — computed from published CMS and AHRQ datasets

Other codes that name M45.1 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 2 Excludes1 notes across 2 chapters: G55 — Nerve root and plexus compressions in diseases classified elsewhere (via M45.-), M05 — Rheumatoid arthritis with rheumatoid factor (via M45.-).

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

Referenced by 7 Excludes2 notes: M20 — Acquired deformities of fingers and toes (via M45.-), M20-M25 — Other joint disorders (M20-M25) (via M45.-), M21 — Other acquired deformities of limbs (via M45.-), M22 — Disorder of patella (via M45.-), M23 — Internal derangement of knee (via M45.-), M24 — Other specific joint derangements (via M45.-), M25 — Other joint disorder, not elsewhere classified (via M45.-).

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

Referenced by 1 Code First instruction: J99 — Respiratory disorders in diseases classified elsewhere (via M45.-).

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 3 MS-DRGs: DRG 545 (MDC 08), DRG 546 (MDC 08), DRG 547 (MDC 08).

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

Clinical classification (AHRQ CCSR):MUS011 — Spondylopathies/spondyloarthropathy (including infective) (default); MUS003 — Rheumatoid arthritis and related disease.

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 CMS-HCC risk category (V28)

CMS maps these diagnoses to the same Hierarchical Condition Category (Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders) for risk-adjusted payment.

M33.99 — Dermatopolymyositis, unspecified with other organ involvement, M34.0 — Progressive systemic sclerosis, M34.1 — CR(E)ST syndrome, M34.81 — Systemic sclerosis with lung involvement, M34.82 — Systemic sclerosis with myopathy, M34.83 — Systemic sclerosis with polyneuropathy, M34.89 — Other systemic sclerosis, M34.9 — Systemic sclerosis, unspecified, M36.0 — Dermato(poly)myositis in neoplastic disease, M45.0 — Ankylosing spondylitis of multiple sites in spine, M45.2 — Ankylosing spondylitis of cervical region, M45.3 — Ankylosing spondylitis of cervicothoracic region, M45.4 — Ankylosing spondylitis of thoracic region, M45.5 — Ankylosing spondylitis of thoracolumbar region, M45.6 — Ankylosing spondylitis lumbar region, M45.7 — Ankylosing spondylitis of lumbosacral region, M45.8 — Ankylosing spondylitis sacral and sacrococcygeal region, M45.9 — Ankylosing spondylitis of unspecified sites in spine, M45.A0 — Non-radiographic axial spondyloarthritis of unspecified sites in spine, M45.A1 — Non-radiographic axial spondyloarthritis of occipito-atlanto-axial region, +487 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.

Systemic Lupus Erythematosus and Other Specified Systemic Connective Tissue Disorders

Same clinical category (CCSR)

AHRQ's Clinical Classifications Software groups these diagnoses under the same clinical categories (Spondylopathies/spondyloarthropathy (including infective), Rheumatoid arthritis and related disease).

+673 more

Same Index main term, other category

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

A01.05 — Typhoid osteomyelitis (typhosa), A04.9 — Bacterial intestinal infection, unspecified (in, enterobacteria), A18.01 — Tuberculosis of spine (tuberculous), A23.9 — Brucellosis, unspecified (in, brucellosis), A54.41 — Gonococcal spondylopathy (gonococcal), L40.53 — Psoriatic spondylitis (psoriatic), M08.1 — Juvenile ankylosing spondylitis (juvenile ankylosing), M10.08 — Idiopathic gout, vertebrae (gouty), M46.1 — Sacroiliitis, not elsewhere classified (sacroiliac NEC)

Contextual Map

Every relationship of M45.1 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 M45.1 with these 10 related codes in Claim Check

Hierarchy

Referenced by Excludes1 notes

Referenced by Excludes2 notes

Referenced by Code First instructions

Clinical classification (CCSR)

Risk adjustment (CMS-HCC)

  • HCC 93 — Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders [CMS-HCC]: “Rheumatoid Arthritis and Other Specified Inflammatory Rheumatic Disorders — supersedes HCC 94 (Systemic Lupus Erythematosus and Other Specified Systemic Connective Tissue Disorders)”— CMS-HCC V28 · 2026

MS-DRG Grouper

MDC crossing

  • MDC 08 — Diseases and Disorders of the Musculoskeletal System and Connective Tissue[MDC crossing]: “Diseases and Disorders of the Musculoskeletal System and Connective Tissue — 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. 18,439 same-MDC procedures group here; browse them on the MDC page.”— CMS MS-DRG Definitions Manual · FY2027

Index entries

  • Spondylitis (chronic), ankylosing (chronic), occipito-atlanto-axial region[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes (21)

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 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
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

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. "M45.1 — Ankylosing spondylitis of occipito-atlanto-axial region." ICD-10-CM FY2027. https://medcoder.ai/icd10/code/m45.1-ankylosing-spondylitis-of-occipito-atlanto-axial-region

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Ankylosing spondylitis of occipito-atlanto-axial region

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 M45.1 in its code family, with their registry titles.

View all codes in the M45 family