Acute neurologic decompression illness in pigs: lesions of the spinal cord and brain

A detailed histopathologic description of central nervous system lesions from a porcine model of neurologic decompression illness is presented. Pigs were dived in a dry chamber to 200 feet of seawater for 24 min before the start of decompression. Of 120 pigs, 40 (33.3%) were functionally unaffected and 80 (66.6%) developed neurologic decompression illness; 16 died, 64 survived. Petechial hemorrhages were grossly visible in the spinal cord of 73% of the survivors, 63% of the fatalities, and 3% of the clinically unaffected pigs. The thoracic part of the cord was most commonly involved. Histologic cord lesions were found in 75 (63%) pigs: 83% of decompression illness survivors, 81% of the fatalities, and 23% of those clinically unaffected. Morphologically, hemorrhagic lesions were the most common (54%). Other common findings included spongiosis (48%), axonal swelling and loss (39%), and myelin degeneration (35%). White matter hemorrhages in the spinal cord were generally more numerous and extensive than those affecting the gray matter; however, gray matter hemorrhage was associated with increasing disease severity. Brain lesions were present in 23% of pigs and were most frequent in fatalities. Cerebellar and brain stem hemorrhages were the most common brain lesions; the molecular layer of the cerebellum appeared particularly susceptible. Pigs were chosen because of their cardiovascular and gas exchange similarities to humans. The clinical and histopathologic features of the pig model were compared with previous accounts in animals and humans; the model was judged analogous to severe human decompression illness. The finding of occult brain and cord lesions in clinically unaffected pigs is discussed. The model provides a useful tool for the study of dysbaric lesions of the central nervous system. Its noninvasive nature may facilitate the study of nervous system injury and repair processes.