Background: Atlas occipital assimilation (AOA) is a rare developmental anomaly of the craniocervical junction, with a poorly understood natural history. Therefore, in 15 cases, we reassess the clinical profile of three types of AOA. Methods: For 15 patients, we applied the 3 classifications: type 1 (4 patients; unilateral joint union (hemi-assimilation), type 2 (7 patients; bilateral joint union with atlas cleft (three-way), and type 3 (4 patients; bilateral joint union and complete union of the posterior arch (circumferential). Variables studied included age, sex, neurological status, pathological magnetic resonance findings, and clivo-axial angles (CAAs). Results: For these 15 patients, we discovered no significant gender differences. However, type 1 AOA affected younger people, type 2 involved more middle-aged/elderly people, and type 3 had most elderly population. Interestingly, all 7 type 2 patients were asymptomatic, and symptoms for the remaining patients included cough, headache, upper limb numbness (i.e., due to Chiari type 1 malformation CM1 in type 1 patients), and myelopathy (i.e., due to spinal canal stenosis at C1-2 in type 3 patients). In addition, 75% of type 3 AOA patients had C2-3 fusions. The average CAA was 127.5° for type 1 and 139° for types 2 and 3. Decompressive surgery was performed for type 1 and decompression with fusions for type 3 patients. Conclusion: Half of the AOA cases are asymptomatic. However, symptomatic cases require attention to CM1 in young people and atlanto-axial dislocation in elderly people.
Uchikado et al. (2025) studied this question.