Abstract: Objective To explore the characteristics of cerebrospinal fluid and neuroelectrophysiology in patients with critical illness polyneuropathy (CIP). Methods Twenty-nine patients with CIP who developed the disease within 7 to 10 days treated in our hospital from September 2017 to March 2019 were selected as the subjects of the study.The causes,changes of cerebrospinal fluid and electromyographic electrophysiology were analyzed. Results Among the 29 cases,there were 16 males and 13 females,with an average age of (50.4±19.3) years.The primary causes included subarachnoid hemorrhage (8/29,27.6%),viral encephalitis (14/29,48.3%),severe pulmonary infection (4/29,13.8%),and advanced tumor (3/29 10.3%).Cerebrospinal fluid analysis showed protein cell separation (9/21,42.9%),suggesting that immune response might be involved in peripheral neuropathy.Electrophysiological examination showed that both motor and sensory nerves were damaged,nerve axons were obviously insulted.And there were significant differences in motor nerve response amplitude (51.7% vs 72.4%,P=00.022) and conduction velocity (20.7% vs 46.6%,P=00.003) between upper and lower limbs,indicating the injuries of the lower limbs were more severe.The spontaneous potential was captured by needle pole electromyography in 28 muscles of upper limb (28/44) and 49 muscles of lower limb (49/73) in the early stage,with no significant difference (P=0.700).The response amplitude and conduction velocity of upper limbs' motor nerve in 5 patients with CIP were normal,nevertheless,90% of the muscles innervated showed obvious spontaneous potential,suggesting that the change of membrane potential of skeletal muscle cells was earlier than the damage of nerve axons. Conclusion In patients with CIP,there is protein cell separation in cerebrospinal fluid,which may be mediated by immunity;Axonal damage in CIP is dominant,and the motor nerve injury is more obvious in lower limbs;The spontaneous potential onset earlier than axonal lesion is the specific change in the early stage of CIP.