How does the neck flexion affect the cervical MRI features of Hirayama disease?

Although flexion cervical MRI has been recommended for the diagnosis of Hirayama disease (HD), no study focused on the MR features at different neck flexion angles. Moreover, no uniform flexion angle has been confirmed in clinical practice. The purpose of this study is to quantitatively investigate the MRI typical signs of HD patients in different neck flexion degree and gives a suggestion to the MR scanning. Cervical MRI in neutral and different flexion positions (cervical flexion angle 20°, 25°, 30°, 35°, and 40°) were performed in 45 HD patients. Three MRI features including anterior shifting of the posterior wall of the cervical dural canal (ASD), widening of cervical epidural space, and epidural flow voids (EFV) at each flexed position were summarized. To evaluate ASD quantitatively, the widest cervical epidural space with the maximum sagittal diameters (d) and cervical canal sagittal diameter (D) at the same level were measured. The d/D values at different angles were calculated and compared. ASD was demonstrated in 34 out of 45 cases (75.6%) at 20° and in all cases (100%) at other 4 angles (χ (2) = 25.728, P < 0.05). Significant difference was demonstrated for the appearance rate of EFV (mean 72.8%) among different angles (χ (2) = 11.373, P = 0.021). The peak mean d/D value was found at 35°. Neck flexion angles have effects on ASD, widening of cervical epidural space and EFV. 25° is recommended as the least effective diagnostic flexion angle for MRI diagnosis of HD, and 35° may be the best one.