Postoperative motor dominant C5 paralysis was known as one of several complications after laminoplasty. Several theories have been proposed for postoperative segmental paralysis after laminoplasty, but its etiology remains unclear.

To investigate the possible mechanism for postoperative motor dominant C5 paralysis from intraoperative electrophysiological studies using evoked spinal cord potentials (ESCPs).

A total of 66 patients who had undergone laminoplasty due to compressive cervical myelopathy were studied retrospectively. In all patients, the symptomatic intervertebral levels of cervical myelopathy were identified by several types of the ESCPs. Motor dominant C5 paralysis was determined as at least 1 level down compared with pre-operative shoulder abduction according to the manual muscle testing.

Five patients (7.6%) showed postoperative motor dominant C5 paralysis. C5 paralysis occurred from 1 to 3 days after surgery and compromised unilaterally in all 5 patients. The causes of cervical myelopathy were cervical spondylosis in 3 patients and ossification of the posterior longitudinal ligament in 2 patients. One patient with severe impairment (2 in manual muscle-testing [MMT] scale) did not show clinical recovery. The other 4 patients recovered to 4 or 5 on the MMT score from 3 to 6 months after the onset. Based on the findings of ESCPs, the C4-5 level was affected by cervical myelopathy in all 5 patients with postoperative motor dominant C5 paralysis (C4-5 level in 3 patients, both C4-5 and C5-6 levels in 2 patients). A high signal intensity area on T2-weighted magnetic resonance imaging (MRI) was observed in all patients who showed apparent motor dominant C5 paralysis in this study.

Cervical myelopathy at the C4-5 level is a potential risk for motor dominant C5 paralysis. Although it is merely a speculation, when C5 radiculopathy occurs after laminoplasty, C5 paralysis becomes clinically apparent because the deltoid muscle gets predominantly innervated by C5 root due to intramedullary spinal cord damage on the C6 segment in C4-5 myelopathy before surgery. It may represent the high signal intensity area on T2-weighted MRI at the C4-5 level.