Pathological disorder of ligamentous or muscular attachment sites at the occipito-atlanto-axial region of the spine — the junction of the skull base (occiput), atlas (C1), and axis (C2).
Verified May 8, 2026 · 4 sources ↓
- Status
- Billable
- Chapter
- 13
- Related CPT
- 11
- Region
- Spine
Documentation tips
What should appear in the chart to support M46.01.
Source · Editorial brief grounded in 4 cited references ↓
- Provider must explicitly name the occipito-atlanto-axial region (occiput, C1, C2, or upper cervical) — generic 'neck pain' or 'cervical enthesopathy' without anatomical localization will not support M46.01.
- Record imaging findings that confirm enthesopathic change at this level: ligamentous calcification, insertional edema on MRI, or osseous reaction at ligament attachment sites on CT.
- Document the specific structure involved when possible (e.g., apical ligament, alar ligament, ligamentum nuchae at occiput-C2), as this strengthens medical necessity for advanced imaging.
- Note the clinical basis for the inflammatory or degenerative enthesopathy diagnosis — onset, symptom pattern (suboccipital pain, restricted rotation), and any failed conservative management.
- If an underlying systemic inflammatory condition (e.g., ankylosing spondylitis, psoriatic arthritis) drives the enthesopathy, code that condition as a principal or additional diagnosis alongside M46.01.
Related CPT procedures
Procedure codes commonly billed with M46.01. 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 M46.01 and adjacent codes.
Source · Editorial brief grounded in CDC ICD-10-CM tabular guidance, AAOS coding references, and cited references ↓
- Defaulting to M46.00 (site unspecified) when the note references upper cervical or occipital structures — M46.01 should be used whenever the occiput-C1-C2 region is documented.
- Confusing M46.01 with M46.02 (cervical region, C3–C7) — the occipito-atlanto-axial region is anatomically and codably distinct; review imaging reports and clinical notes for the exact spinal level.
- Coding M46.01 for general suboccipital muscle tension or headache without documented enthesopathic pathology at ligament or tendon insertion sites — enthesopathy requires evidence of insertional pathology, not just regional pain.
- Assigning M46.01 when the underlying condition is a specific inflammatory spondyloarthropathy already captured by a more specific code (e.g., ankylosing spondylitis M45.x1 for the same region) — check whether M46.01 is the most precise code available.
- Omitting M46.01 from the imaging order claim when it is the primary diagnosis driving the head/neck MRI or CT — CMS A57215 explicitly lists it as a covered ICD-10 to support medical necessity for these scans.
Clinical context
Source · Editorial summary grounded in 4 cited references ↓
M46.01 captures spinal enthesopathy specifically localized to the occipito-atlanto-axial region, which includes the ligamentous and muscular insertion points at the occiput, C1, and C2. This is a highly specific anatomical designation within the M46.0 parent category (spinal enthesopathy). Use it when the treating provider has documented inflammation, degeneration, calcification, or irritation at the ligament or tendon attachment sites in this uppermost cervical segment — not merely cervical pain or generalized neck stiffness.
This code falls under 'Other inflammatory spondylopathies' (M46) in Chapter 13. It is classified separately from enthesopathy of the cervical region (M46.02) or cervicothoracic region (M46.03), so anatomical precision in the clinical note is mandatory. M46.00 (site unspecified) is the fallback only when documentation genuinely fails to localize the region — do not default to it when upper cervical anatomy is referenced.
M46.01 is listed by CMS as a covered diagnosis supporting medical necessity for MRI and CT scans of the head and neck (CMS Article A57215), making it directly relevant for imaging order justification and prior authorization workflows. Conditions such as ligamentum nuchae calcification, apical/alar ligament enthesopathy, and atlantoaxial ligamentous insertional pathology fall within this code's scope when documented at this region.
Sibling codes
Other billable codes under M46.0 (laterality / anatomic variants).
Frequently asked questions
Source · Generated from the editorial pipeline, verified against 4 cited references ↓
01What anatomical region does M46.01 cover?
02When should I use M46.00 instead of M46.01?
03Does M46.01 support medical necessity for head and neck MRI or CT?
04Can I use M46.01 alongside a spondyloarthropathy code like ankylosing spondylitis?
05Is M46.01 appropriate for ligamentum nuchae calcification at the upper cervical level?
06Does M46.01 require a 7th-character extension?
07What CPT codes are commonly billed with M46.01?
Sources & references
Editorial content was developed using the following public sources. Last verified May 8, 2026.
Mira AI Scribe
Mira AI Scribe captures the anatomical localization of enthesopathic findings at the occiput, C1, or C2 level — including ligament or tendon insertion pathology documented on MRI or CT, symptom laterality, and any systemic inflammatory context — from the encounter note. This prevents downcoding to M46.00 (unspecified site) and ensures the imaging order carries the specific diagnosis CMS requires for head/neck scan coverage under Article A57215.
See how Mira captures M46.01 documentation