M45.1
BillableAnkylosing spondylitis of occipito-atlanto-axial region
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is M45.1 an HCC code?
Yes. M45.1 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under the CMS-HCC V28 risk adjustment model (and Rheumatoid Arthritis and Inflammatory Connective Tissue Disease under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for M45.1
For M45.1to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically — it has to be re-documented and supported each calendar year.
- 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
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed M45.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
M45.1 is the ICD-10-CM diagnosis code for ankylosing spondylitis of occipito-atlanto-axial region. Ankylosing spondylitis affecting the upper neck region where the skull connects to the spine (occipito-atlanto-axial area). M45.1 sits in the ICD-10-CM chapter for diseases of the musculoskeletal system and connective tissue (m00-m99), within the section covering spondylopathies (m45-m49).
Under the CMS-HCC V28 risk adjustment model, M45.1 maps to Rheumatoid Arthritis and Inflammatory Connective Tissue Disease (HCC 93) with a community, non-dual, aged base RAF weight of 0.175. Under the older V24 model, M45.1 mapped to the same category but with a base RAF weight of 0.307 — V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
Verify imaging or clinical documentation confirms involvement of the occipito-atlanto-axial region. Because M45.1 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. 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, RAF weights, and MEAT documentation criteria for M45.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify imaging or clinical documentation confirms involvement of the occipito-atlanto-axial region
- •This region-specific code indicates the inflammatory process is affecting the uppermost cervical spine
Clinical Significance
Ankylosing spondylitis of the occipito-atlanto-axial region represents high-risk spinal involvement that can cause severe neurological complications and atlantoaxial instability. This location requires careful monitoring for spinal cord compression and may necessitate surgical intervention.
Documentation Requirements
- ✓Documentation of ankylosing spondylitis with specific occipito-atlanto-axial involvement
- ✓Imaging studies showing inflammatory changes in upper cervical spine
- ✓Assessment of atlantoaxial stability and neurological function
- ✓Evaluation for spinal cord compression or myelopathy
- ✓Documentation of neck mobility restrictions
- ✓Monitoring for respiratory complications from cervical involvement
- ✓Treatment plan addressing high-risk location
Commonly Confused Codes
- •M45.2 — Ankylosing spondylitis of cervical region (broader cervical involvement)
- •M24.30 — Pathological dislocation of unspecified joint (mechanical instability)
- •G99.2 — Myelopathy in diseases classified elsewhere (neurological complication)
- •M46.01 — Spinal enthesopathy, occipito-atlanto-axial region (different pathology)
- •S13.111A — Dislocation of C0/C1 cervical vertebrae (traumatic cause)