Non-radiographic axial spondyloarthritis localized to the occipito-atlanto-axial region — the articulations between the occiput, atlas (C1), and axis (C2) — where active inflammation is confirmed by MRI or clinical criteria but plain radiographs show no definitive structural damage.

Documentation tips

What should appear in the chart to support M45.A1.

Source · Editorial brief grounded in 6 cited references ↓

• Record the specific spinal region by name — 'occipito-atlanto-axial region' or 'craniocervical junction' — so the site-specific code M45.A1 is defensible over the unspecified parent M45.A0.
• Document that plain radiographs of the SI joints and the affected spinal region do NOT show definitive structural damage; this is the core criterion separating nr-axSpA from AS (M45.1).
• Include MRI findings for the occipito-atlanto-axial region and/or sacroiliac joints — bone marrow edema, synovitis, enthesitis — as positive imaging supports medical necessity when X-rays are negative.
• Note HLA-B27 status and CRP/ESR results; elevated inflammatory markers and positive HLA-B27 strengthen the nr-axSpA diagnosis and support payer review.
• Capture inflammatory back pain characteristics: insidious onset before age 45, improvement with activity but not rest, morning stiffness >30 minutes. These clinical criteria are payer-audited for biologics.
• If the patient has a prior code of M46.8 (other specified inflammatory spondylopathy) from before FY2022, flag the transition to M45.A1 in the problem list for continuity and data integrity.

Related CPT procedures

Procedure codes commonly billed with M45.A1. Linking the right diagnosis to the right procedure is what establishes medical necessity.

Source · CMS LCDs · AAOS specialty guidance · claims-pattern analysis

Common coding pitfalls

The recurring mistakes coders make with M45.A1 and adjacent codes.

Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓

• Using M45.1 (ankylosing spondylitis of occipito-atlanto-axial region) when radiographic damage is absent — M45.1 requires radiographic evidence; without it, M45.A1 is the correct code.
• Defaulting to the unspecified parent M45.A0 when the physician has documented a specific spinal region; M45.A1 is billable and more specific, reducing audit risk.
• Coding M46.8 for nr-axSpA in encounters dated on or after October 1, 2021 — the dedicated M45.A subcategory superseded that practice.
• Applying M45.A1 when the patient has juvenile onset — M08.1 (juvenile ankylosing spondylitis) is required by the Excludes1 note at the M45 category level.
• Combining M45.A1 with M02.3- (Reiter's disease arthropathy) or M35.2 (Behçet's disease) in the same encounter — both are blocked by the category-level Excludes notes.

Clinical context

Source · Editorial summary grounded in 6 cited references ↓

Sibling codes

Other billable codes under M45.A (laterality / anatomic variants).