Objective Hypotonic encephalopathy refers to a syndrome in which the extracellular fluid is hypotonic and some water moves into the cells, resulting in brain cell edema, which causes metabolic and functional disorders in the brain and a series of psychoneurological symptoms. Hypotonic encephalopathy is not uncommon in clinical practice, and is often a secondary condition to the primary disease, for which physicians generally lack vigilance and awareness. In particular, hypotonic encephalopathy after acute cervical medullary injury is often masked by the symptoms of spinal cord injury and has been neglected, and has not been reported clinically. Methods Retrospective analysis of clinical data of 29 patients with diagnosed hypo-osmolar encephalopathy after acute cervical spinal cord injury admitted to our hospital from October 2003 to October 2008. All patients had symptoms of high paraplegia, and imaging and physical examination were performed after admission to clarify the type of injury and the plane of injury. Level of spinal cord injury: 9 cases of high cervical spinal cord injury (C4 and above C4 spinal cord level) and 20 cases of low cervical spinal cord injury. According to the level of spinal cord injury: 22 cases with complete (Frankel A) and 7 cases with incomplete (below Frankel B) injuries. The shortest period from injury to hyponatremia was 3 d, and the longest period was 11 d, with a mean of 6.4 d±2.7 d. Plasma osmolality and blood electrolytes were measured in 29 patients immediately after admission, and all of them developed hyponatremia, hypochlorhydria, and hypocalcemia successively from the 3rd to the 11th d after injury, with a decrease in plasma osmolality, and the lowest blood sodium, blood chloride, and blood calcium levels: blood sodium averaged 122 (116-128) mmol/L, and blood chloride averaged 94.2 (94.2) mmol/L. All 29 cases showed different degrees of central nervous system symptoms and signs. Among them, 11 cases with plasma osmolality between 250 and 270 mosm/L showed symptoms such as fatigue, apathy and mental depression; 8 cases with plasma osmolality between 240+250 mosm/L showed symptoms such as headache, drowsiness and lethargy, and 8 cases with plasma osmolality between.