This is a case–control study conducted from January 2021 to November 2021, at the neurophysiology department of Al-Imamian Al-Kadhemian Medical City, Baghdad. The Iraqi Committee of Medical Specialization approved the study (Decision No. 291; Date 21/1/2021). All subjects who took part in this study gave their consent.
The study included 20 patients of either sex aged 40 to 70 years who have CSM, diagnosed by senior neurosurgeon and documented by MRI. An abductor pollicis brevis (APB) muscle power of grade ≥ 4 according to the Medical Research Council (MRC) scale was ensured for all patients prior to their inclusion in the study.
Patients with a history of cervical surgery, peripheral neuropathy, diabetes, spinal cord lesions, pregnancy, central nervous system disorders, restless leg syndrome, motor neuron disease, fibromyalgia, and epilepsy, brain surgery, the presence of a pacemaker, or any metallic foreign body in the body were all excluded from the study.
The control group consists of another 20 sex- and age-matched healthy volunteers.
The Nurick grading scale and the modified scale of the Japanese Orthopedic Association (mJOA) system were used to assess functional neurological state. Nurick’s grading  is as follow: grade 0 = signs and symptoms of root involvement without spinal cord disease; grade 1 = signs of spinal cord disease without difficulty in walking; grade 2 = slight difficulty in walking that does not prevent full-time employment; grade 3 = difficulty in walking that prevents full-time employment or daily takes without requiring assistance with walking; grade 4 = ability to walk only with assistance; and grade 5 = chair bound or bedridden. The lower grades indicating better walking ability.
For the mJOA , it is a questionnaire that assesses six functions: motor dysfunction in the upper and lower extremities (0–4), sensory function in the upper and lower extremities (0–2), sensory function in the trunk (0–2), and bladder function (0–3). The sum of these subscales varies from 0 to 17, with a minimum score of 0 being the lowest and a maximum score of 17 being the highest. CSM cores 15–17 for mild, 12–14 for moderate, and 0–11 for severe cases .
Cervical spine imaging
A comprehensive radiological evaluation of the cervical spine was performed, which included X-rays to identify spinal cord compressing pathology in the cervical area and MRI with a Siemens (MAGNETOM Aera 1.5T MRI) MRI machine using T1 and T2 sagittal, T2 axial, corono-myelo, sagitto-myelo sections.
The CuSP and H-reflex measurements were performed using a four-channel electromyography (EMG) equipment (Medtronic, Denmark). Room temperature was monitored between 25 and 28 °C during test procedures and skin temperature between 32 and 34 °C was ensured using a skin thermometer.
For the CuSP, ring electrodes were applied to stimulate the index finger, and EMG activity was recorded using surface electrodes from the APB with filter setting of 2 Hz–10 kHz.
Participants were asked to make an opposition of the right thumb to obtain a steady maximal contraction, and an EMG audiosignal was used to monitor muscle contraction. A single painful stimulus (80-mA intensity) with a 0.5-ms duration was delivered to the index finger during maximal voluntary contraction until a complete silent period of consistent latency and duration was attained. The experiment was repeated at least ten times until we obtained five ideal recordings demonstrating total silence of the motor unit potential with the longest duration and shortest delay.
We chose the mean value of the five best CuSPs as the final value for CuSP parameters in each subject to reduce any variation in CuSP parameters. The interval between stimulation and the start of the silent period was characterized as CuSP latency. The time between the start and end of the silent period was specified as CuSP duration. Visual assessment at the start of an abrupt reduction or upon recovery of EMG activity determined the onset and endpoint latencies of each CuSP .
To measure H-reflex, participants were sitting comfortably with their forearms on a table in front of them. Surface recording electrodes were used to capture the H-reflex of the flexor carpi radialis. The active electrode was put over the flexor carpi radialis muscle belly, and the reference electrode was placed over the brachioradialis, away from other forearm median-innervated muscles. A bipolar felt pad electrode was used to stimulate the median nerve just proximal to the elbow (cathode proximal, pulse width 0.5 ms). The ground electrode lies between the stimulator and the recording electrode .
Responses to median nerve stimulation were identified as H-reflexes if they had latencies between 12 and 25 ms. Five H-responses were analyzed to ensure that they were repeatable. After correcting for height (or limb length), latencies were determined from the onset of stimulus artifact until the start of the reflex response. The amplitudes were measured from the baseline to the highest negative peak and H/M amplitude ratio was calculated.
An EMG equipment (Micromed, Italy) was used for CoSP. Each participant sat on a comfortable chair. Three brief maximal voluntary isometric contractions (5 s) starting off the test. To avoid exhaustion, maximum efforts were spaced by approximately 60 s of rest. The subjects next performed isometric contractions, which resulted in the firing of one or two single motor units that could be identified.
The appropriate region of the hemisphere for APB muscle was stimulated with a transcranial magnetic stimulator (Magstim 2002, Magstim Co., Whitland, UK) using circular coil electrode with a 90-mm diameter (type 9784, UK). The stimulation site on the head was determined utilizing suprathreshold stimuli around the C3 region, as per the international 10–20 system .
During a weak isometric contraction of the contralateral APB muscle, five to ten random (between 4 and 6 s) single pulse transcranial magnetic stimulation at an intensity of roughly 140% of resting motor threshold were applied. The best location was then identified and used throughout the experiment .
To reduce variability, the individual was requested to maximally contract the APB muscle against resistance, then a single suprathreshold transcranial magnetic stimulation pulse was given to the motor cortex, followed by at least five repeatable responses. We looked for motor evoked potentials (MEPs) with the shortest latency and maximum amplitude. The duration of CoSP is measured from the start of muscle activity suppression to the return of muscle activity. 30-Hz low filter and 30-kHz high filter settings were employed.
For all statistical studies, statistical package of social sciences (SPSS) software version 25.0 was utilized (SPSS, Chicago). Continuous data were expressed as mean ± SD and analyzed with a Student’s t-test (between two groups) or an analysis of variance with least significant difference as a post hoc pairwise comparison (between more than two groups).
Categorical variables were expressed as numbers and percentages. The receiver operating characteristic curve was used to find out the discriminative value of some parameters to differentiate between patients and controls. The possible association of neurophysiological measures with age, disease duration, arm length, and JOA score was investigated using Pearson's correlation test. When the p-value was less than 0.05, the difference was considered statistically significant.