ICD-10 M45.1: Upper cervical ankylosing spondylitis

Key Takeaways

ICD-10 code M45.1 is a billable diagnosis code for ankylosing spondylitis of the occipito-atlanto-axial region, valid for claims with service dates on or after October 1, 2015.

Documentation must specify the anatomical region (occipito-atlanto-axial) and include imaging findings, HLA-B27 status, and clinical criteria to support the code.

M45.1 is one of the 10 ankylosing spondylitis subcodes within M45, organized by spinal region; selecting the wrong subcode is the most common reason for claim specificity denials. (M45 also includes M45.A0-M45.AB for non-radiographic axial spondyloarthritis, a related but distinct diagnosis.)

Once M45.1 is documented in the chart, Pabau’s claims management software submits, tracks, and processes billing for the claim with insurers, so the specificity captured at the point of care carries through to reimbursement.

ICD-10 code M45.1 is a billable diagnosis code for ankylosing spondylitis of the occipito-atlanto-axial region, where the base of the skull meets the top two cervical vertebrae (C1 and C2). Coders assign it when the clinical note names this specific joint region, not a general ankylosing spondylitis diagnosis.

Practice management software like Pabau won’t select the diagnosis code for you. But once M45.1 is documented in the chart, its claims management software submits and tracks the claim with insurers and handles the billing that follows, carrying that specificity through to reimbursement.

The occipito-atlanto-axial region represents the uppermost cervical spine, where the skull articulates with C1 (atlas) and C2 (axis). Ankylosing spondylitis at this location causes inflammation, pain, and progressive fusion that can compromise neck rotation and, in severe cases, atlantoaxial stability.

Rheumatology and musculoskeletal practices coding AS visits need to match the clinical documentation to the correct M45 subcode every time.

ICD-10 code M45.1: Definition, billable status, and effective date

Clinical overview: Ankylosing spondylitis of the occipito-atlanto-axial region

Ankylosing spondylitis (AS) is a chronic seronegative inflammatory arthritis that primarily targets the axial skeleton, beginning at the sacroiliac joints and ascending through the lumbar, thoracic, and cervical spine.

When AS reaches the occipito-atlanto-axial region, it affects the three joints connecting the occiput (skull base), C1, and C2, the most mechanically complex part of the cervical spine.

Inflammation here leads to pain, restricted neck rotation, and, over years, bony fusion. Physical therapy teams and rehabilitation specialists treating upper cervical AS need accurate coding to support claims and demonstrate medical necessity.

AS falls within the broader category of seronegative spondyloarthropathies. The condition is strongly associated with HLA-B27 positivity, present in roughly 90% of AS patients. According to the National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIAMS data shows AS typically begins before age 45 and affects men more commonly than women.

Sacroiliitis on imaging, combined with characteristic inflammatory back pain, anchors the diagnosis. That pain pattern includes morning stiffness lasting more than 30 minutes and improvement with movement.

For coding purposes, the clinician must specify the spinal region involved. A general diagnosis of “ankylosing spondylitis” without a regional qualifier routes to M45.0 (multiple sites) or M45.9 (site unspecified) – neither of which provides the specificity that supports M45.1 on a claim.

Synonyms for M45.1

Several clinical terms map to ICD-10 code M45.1. Knowing them helps coders recognize the correct code when physician notes use varied terminology. Digital intake forms and clinical note templates can be structured to capture these terms at the point of care.

• Ankylosing spondylitis of the occipito-atlanto-axial region
• Ankylosing spondylitis of the upper cervical spine
• AS of the C0-C2 region
• Bamboo spine – informal clinical descriptor for the radiographic appearance of fused vertebrae in advanced AS, classically described in the thoracolumbar spine but used loosely for fusion at any spinal level, including the upper cervical spine
• Marie-Strümpell disease of the occipito-atlanto-axial region
• Rheumatoid spondylitis, upper cervical (as approximate synonym, not a direct clinical equivalent)

M45 subcodes: The 10 ankylosing spondylitis codes by spinal region

M45 is the parent code for ankylosing spondylitis, and the 10 subcodes below (M45.0 through M45.9) organize AS diagnoses by spinal region. Selecting the correct subcode requires the physician’s note to specify the location of active disease.

M45 also includes M45.A0 through M45.AB, which cover non-radiographic axial spondyloarthritis (nr-axSpA) by spinal region. This is a related but clinically distinct diagnosis that sits earlier on the axial spondyloarthritis spectrum, before the structural changes seen in AS appear on imaging. The AAPC Codify ICD-10-CM lookup lists all M45 subcodes with their full descriptions.

M45.1 is highlighted because it represents a clinically distinct presentation. Disease in the occipito-atlanto-axial region carries different functional implications and treatment priorities than lumbar or thoracic AS. Coding it correctly communicates that distinction to payers and downstream care teams. Coders working across the M45 family can compare this guide with the adjacent cervical region subcode, M45.2.

Documentation requirements for M45.1

Insufficient documentation is the leading cause of M45-family code denials. Payers and auditors expect the clinical note to support not just the AS diagnosis but the specific spinal region. Structured client records help rheumatology and musculoskeletal teams capture the details that justify M45.1 at every visit.

The following documentation elements reduce denial risk:

• Diagnosis statement: The clinical note must explicitly name the condition (ankylosing spondylitis) and the affected region (occipito-atlanto-axial, upper cervical, or C0-C2). A note that reads “inflammatory back pain, neck” is insufficient.
• Imaging findings: Cervical spine X-ray, MRI, or CT showing sacroiliitis, syndesmophytes, or atlantoaxial joint changes supports the diagnosis. Document the imaging modality and relevant findings.
• Clinical criteria: Modified New York Criteria or ASAS axial spondyloarthritis criteria should be documented or referenced. Morning stiffness duration, response to NSAIDs, and HLA-B27 positivity all contribute.
• HLA-B27 status: Document whether HLA-B27 testing was performed and the result. Positive status supports the diagnosis and may be coded separately (see HLA-B27 section below).
• Duration and chronicity: Specify that the condition is chronic and ongoing, not an acute episode, to differentiate from acute inflammatory conditions.
• Functional impact: Note limitations in cervical range of motion, activities of daily living, or safety concerns (atlantoaxial instability) to establish medical necessity.

DRG groupings for M45.1

For inpatient claims, ICD-10 code M45.1 maps to MS-DRG (Medicare Severity Diagnosis Related Groups) groupings that determine hospital reimbursement. The CMS ICD-10 codes page publishes annual MS-DRG updates. The MS-DRG assignments below reflect standard FY2026 groupings; verify against the current CMS MS-DRG Definitions Manual for the fiscal year of service.

M45.1 falls under the bone diseases and arthropathies Major Diagnostic Category, the same MDC as recently coded diagnoses such as M95.2. The presence or absence of a major complication or comorbidity (MCC) drives the DRG assignment between 553 and 554.

Comorbidities such as uveitis, sacroiliitis, or atlantoaxial instability can affect both the DRG assignment and the relative weight used to calculate payment when coded alongside M45.1. Each concurrent condition must be supported by its own clinical documentation, the same standard used for differential diagnoses like M05.9.

Related and differential ICD-10 codes for ankylosing spondylitis

Coders working with M45.1 regularly encounter codes for differential diagnoses, comorbidities, and concurrent conditions. Using the wrong code from this group is a common audit trigger, similar to distinguishing inflammatory spondylitis from degenerative disease coded as M47.9. CMS’s ICD-10 coding guidelines address sequencing rules for inflammatory arthropathies.

HLA-B27 and coding ankylosing spondylitis

HLA-B27 positivity is a diagnostic marker, not a standalone condition. Most competitors’ code reference pages list it without explaining how and when to code it alongside M45.1. Several also mix it up with the similarly named Z14.8 (Genetic carrier of other disease), a different code entirely, used for asymptomatic genetic carrier status rather than a susceptibility marker like HLA-B27.

When HLA-B27 positivity is documented, tested, and relevant to managing the patient’s AS, it may be coded as a secondary diagnosis using Z15.89 (Genetic susceptibility to other disease).

Do not apply Z15.89 routinely. It requires the clinician to document that the HLA-B27 status influenced clinical decision-making during the encounter. Code M45.1 as the principal diagnosis. Z15.89 is secondary only when the note supports it.

It remains uncertain whether coding guidelines specifically mandate Z15.89 for HLA-B27 in all AS encounters. The AHA Coding Clinic is the authoritative source for sequencing questions like this, so verify current guidance before applying Z15.89 routinely.

When HLA-B27 testing is the reason for the encounter, rather than established AS treatment, that diagnostic workup drives the principal code selection instead. Sports medicine practices treating inflammatory conditions that overlap with AS face the same sequencing questions.

Get clean claims out the door once your codes are documented

Once M45.1 and any concurrent codes are documented in the chart, Pabau's claims management software submits and tracks the claim with insurers, flags issues before they turn into denials, and handles the billing that follows. See how it works for rheumatology and musculoskeletal practices.

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Coding M45.1 in your EHR and practice management software

Most AS coding errors happen between the clinical note and the billing system. A physician documents “ankylosing spondylitis, upper cervical region” clearly in the chart. Then a coder, or an EHR auto-suggest tool, maps it to M45.9 (unspecified) because the system does not recognize “upper cervical” as the occipito-atlanto-axial region.

The specificity was in the note. The error happened at the lookup step. AI-assisted clinical documentation can close that disconnect by structuring notes to include region-specific language at the time of the encounter, rather than relying on post-hoc coding reconciliation.

When selecting M45.1 in a practice management system, also consider:

• Concurrent codes: Select additional codes for concurrent conditions (sacroiliitis M46.1, uveitis H20.x) at the time of encounter, not as a billing afterthought.
• Modifier awareness: Modifiers apply to the CPT/HCPCS procedure codes billed alongside M45.1 – for example, imaging interpretations or injection codes – not to the diagnosis code itself. Confirm modifier requirements with the payer whenever the encounter involves a bilateral structure or a separately identifiable service.
• Claims scrubbing: Build M45.1 into your pre-submission claim scrub rules. Pair it with compliant CPT codes for the service rendered (rheumatology E/M codes, injection codes, imaging interpretations).
• Prior authorization documentation: For biologics used in AS treatment (TNF inhibitors, IL-17 inhibitors), the M45 subcode in the patient’s record must match the code submitted for the authorization request. A mismatch between M45.1 in the chart and M45.9 in the PA request is a common prior-authorization denial trigger.

Practices managing high volumes of AS patients benefit from automated workflows that trigger documentation reminders when an AS diagnosis code is selected, prompting clinicians to confirm the spinal region before the note is finalized. This prevents the “unspecified” default from propagating through to claims.

HIPAA compliance requirements for medical offices also apply to how diagnosis codes are stored and transmitted in practice management systems, whether the record concerns M45.1 or an unrelated diagnosis such as M27.2.

ICD-9 to ICD-10 conversion for M45.1

Before ICD-10-CM was implemented in the United States on October 1, 2015, ankylosing spondylitis was coded under ICD-9-CM code 720.0 (Ankylosing spondylitis). The crosswalk from 720.0 to M45.1 is approximate.

ICD-9 did not distinguish between spinal regions for AS, so the entire M45 family maps from a single ICD-9 code. The ResDAC ICD codes resource on Medicare files explains the ICD-9 to ICD-10 transition in the context of Medicare claims data.

The one-to-many nature of this crosswalk is why specificity matters. Any retrospective data analysis comparing AS coding before and after October 2015 must account for the fact that ICD-9 code 720.0 represents all spinal regions, while ICD-10-CM M45.1 through M45.9 represent distinct anatomical sites.

Practice management platforms that maintain long-term patient records should flag encounters coded under the old 720.0 structure as needing review, if the patient continues to receive AS treatment under ICD-10 codes.

Conclusion

Claim denials for ICD-10 code M45.1 are almost always preventable. The code is billable, well-established, and unambiguous – the problem is upstream, in documentation that skips the regional qualifier. When the clinical note names the occipito-atlanto-axial region and the supporting imaging and clinical criteria are recorded, M45.1 submits cleanly.

Pabau’s claims management software takes over once M45.1 and any concurrent codes are documented in the chart. It submits and tracks the claim with insurers, reconciles payments as they come in, and handles the billing along the way, so specificity captured at the point of care doesn’t get lost in a manual claims process.

If your rheumatology or musculoskeletal practice is seeing M45-family denials, book a demo to see how Pabau handles claims from documentation to submission.

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