MRI features of Hirayama disease at different cervical flexion angles

Objective: To analyze MRI features of different cervical flexion positions in Hirayama disease (HD) and discuss the effects on these features by different cervical flexion angles. Methods: The cervical MR images of neutral and different flexion positions (20 degree, 25 degree, 30 degree, 35 degree, 40 degree) of 20 patients, who were clinically diagnosed as HD, were studied. At flexion positions, the appearance of anterior shifting of the posterior wall of the cervical dural canal and widening of epidural space was recorded. The maximum sagittal diameters (d) of widened cervical epidural space and the cervical canal sagittal diameters (D) on the same level were measured to calculate d/D value for quantitative evaluation of the two signs. Comparisons of appearance of the signs among different flexion positions were made using Fisher's exact test. Repeated measures analysis of variance (rmANOVA) was used to compare mean d/D values among groups with different positions, and paired comparisons were also performed. Results: The appearance of anterior shifting of the posterior wall of the cervical dural canal were different between 20 degree group (70%, 14/20) and other 4 larger angles groups (100%) (χ2=5.76, P=0.020). The d/D values were 0.51±0.06, 0.54± 0.08,0.57±0.09,0.61±0.09,0.59±0.07 respectively at abovementioned 5 flexion positions, which were different among groups (F=3.450, P=0.013). The value was greater at 35 degree than that at 20 degree and 25 degree (P< 0.05), and it was also greater at 40 degree than that at 20 degree (P<0.05). Conclusion: Cervical flexion angle has an effect on anterior shifting of the posterior wall of the cervical dural canal and widening of epidural space. (authors)