A 60-year-old woman with long-standing rheumatoid arthritis presents with sudden neck pain and new occipital headache that worsens on neck flexion, with upper motor neurone signs. MRI shows anterior atlantoaxial subluxation with an atlanto-dens interval (ADI) of 12 mm. What is the threshold for ADI in adults above which cord compression risk is significant, and what is the management?

• A ADI >8 mm is normal in rheumatoid arthritis; no intervention required unless paralysis occurs
• B ADI >3–4 mm (adults) signifies instability; surgical atlantoaxial fusion is indicated if neurological compromise exists ✓
• C ADI >2 mm mandates immediate halo-vest immobilisation as definitive treatment
• D ADI measurement is irrelevant; the posterior atlanto-dens interval (PADI) alone guides surgery

Explanation

In adults, an ADI >3 mm is abnormal (versus ≤5 mm in children). In rheumatoid atlantoaxial subluxation, ADI >10–12 mm indicates severe instability; however, the posterior atlanto-dens interval (PADI <14 mm) is actually a better predictor of cord compromise. This patient with ADI of 12 mm, upper motor neurone signs, and occipital pain from C2 root compression requires posterior atlantoaxial or occipitocervical fusion. Non-surgical treatment with a cervical collar may be used preoperatively or in mild cases without neurological deficit.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.