Is the atlas size associated with the pathophysiology of symptomatic spinal canal stenosis at the C1 level?

Narrow cervical spinal canal is an important risk factor for the development of cervical myelopathy. Patients with this disease often present with congenital narrowness of the cervical spinal canal. While there are studies on patients with subaxial spinal canal stenosis (SAS), few examined the coexistence of congenital narrow spinal canal in patients with cervical myelopathy at the C1 level. We investigated the characteristics of patients with C1 stenosis (C1S) with special reference to the size of the atlas. Thirteen patients (8 men, 5 women, mean age 76 years) with C1S were retrospectively analyzed and their clinical characteristics and radiological findings were compared with 27 SAS patients and with 26 age-, sex-, and body habitus-matched asymptomatic individuals. Of the 13 C1S patients, 6 presented with a retro-odontoid pseudotumor, 5 with atlantoaxial subluxation, and 2 with ossification or calcification of the transverse ligament; they were significantly older and shorter, and their body weight was significantly lower than in SAS patients (p < 0.001). Their average C1 anteroposterior- and spinal canal diameter was 26.9 ± 2.4 mm and 12.8 ± 4.1 mm, respectively and significantly smaller than in patients with subaxial stenosis (p = 0.004). These measurements were also statistically smaller than in the controls, even after matching for age, gender, height, and body weight (p < 0.05). In patients with C1S, the atlas size was significantly smaller than in SAS patients and asymptomatic controls, indicating an association between a small atlas size and symptomatic spinal canal stenosis at the C1 level.