The differential diagnosis of amyotrophic lateral sclerosis (ALS) and cervical spo ndylotic myelopathy (CSM), which both occur in middle-aged and elderly people, have similar clinical manifestations and can sometimes coexist, making the differential diagnosis difficult, but the management and prognosis of the two diseases are very different. CSM is expected to recover and heal with early surgery, while ALS is expected to worsen or even die with wrong surgery. Therefore, it is very important to distinguish the two diseases early and accurately. Early differentiation of the two diseases is based on the following criteria: (1) the presence or absence of neurogenic damage on the electromyogram of the three limbs or both limbs plus the tongue; (2) the presence or absence of abnormal somatosensory evoked potentials (SEP). However, these criteria are not specific, as the former may be false-negative or false-positive in ALS with limited early lesions or CSM with lumbar spine lesions, while the latter is controversial due to the large number of studies showing that SEP changes can also be present in ALS. In this study, based on the different pathogenesis of the two diseases, we firstly used Seernocleidomastoid Muscle EMG (SC M-EMG) and Dermatomal Somatosensory Evoked potential (DSSE P) to differentiate the two diseases. It is the first report in China and abroad, and also introduced the recently developed Precision Decomposition Electromyography (PDEMG) into clinical application for the first time. The normal values of sternocleidomastoid EMG and upper limb DSSEP were determined for the first time in China; ② The abnormality rates of sternocleidomastoid muscle in ALS and CSM were 97% and 0, respectively, with highly significant differences, including some of the early ALS or CSM with lumbar spine lesions that had false negative or false positive results by traditional EMG methods; ③ The abnormality rates of upper limb DSSEP in ALS and CSM were 8% and 100%, respectively, with highly significant differences. The abnormalities were significantly correlated with the degree of degenerative changes in the cervical spine and the degree of spinal cord compression on imaging, and helped to determine the coexistence of the two diseases; ¾ PDEMG showed a decrease in the number of motor units recruited and the average release rate in the motor control mechanism of ALS. ALS is a degenerative disease with a wide range of lesions, except for its initial predilection for cervical expansion, which can reach upward to the high cervical and medulla oblongata and downward to the thoracolumbar medulla, whereas CSM is a limited lesion, with cervical medulla damage rarely exceeding the C4 plane and not going below the T2 plane. Therefore, the extent of muscle lesions has been an important aspect in differentiating the two diseases. The sternocleidomastoid muscle (SCM) has spinal cord innervation segments at the C2 and C3 levels, mainly C2, so as the disease progresses, it is presumed that this muscle should be involved before the lingual or lower limb muscles. The abnormality rate of the SCM muscle in this group was higher than that of the trigeminal and lingual muscles, and the abnormality rate of this muscle was 100% in the 8 cases with postoperative exacerbation. 2 patients with ALS who had abnormalities of both upper limb muscles showed significant abnormalities of this muscle within 1 year of follow-up, while there were no abnormalities of the lingual and lower limb muscles, confirming the above inference. The abnormality rate of the tongue muscle in the ALS group was 48%. Therefore, SCM-EMG was superior to lingual EMG and eliminated the pain of lingual EMG, and provided an objective basis for prognosis. This is especially important when the two diseases coexist. CSM is caused by degenerative changes in the cervical spine resulting in spinal cord compression or localized spinal cord blood supply disorders. The DSSEP of the upper extremity, as an examination that encompasses the entire range of possible lesions from the C4-T1 dermatome, has been shown to accurately reflect the segmental lesion plane and correlate well with imaging and clinical findings, and to overcome the false-negative findings of conventional SEPS in CSM due to damage to the cervical medulla, which may lie below the highest level of entry of the afferent nerve fibers to the upper extremity. The DSSEP correlated significantly (P<0.01) with the imaging findings in CSM and with the degree of disease (P<0.01), making it a good objective indicator for surgical localization and prognosis, with very different results in ALS and CSM. In this study, we propose for the first time to use SCM muscle EMG and upper limb DSSEP to diagnose and differentiate between ALS and CSM, and to diagnose the coexistence of both diseases, in addition to the traditional electrophysiological examination methods. The results proved to be reliable and of sufficient clinical utility. The introduction of PDEMG into the study of ALS will provide further insight into the electrophysiological basis of the disease in the future.