Coma associated with flaccidity produced by fluid-percussion concussion in the cat. I: Is it due to depression of activity within the brainstem reticular formation?

This study is the first attempt to characterize neurological and behavioural consequences of fluid-percussion concussive head injury in the cat. Both animals initially anaesthetized by N2O as well as unanaesthetized, chronically prepared animals were subjected to injury. Injury with a fluid-pressure wave of 1.9-2.5 atm (duration 21-24 ms) produced a brief generalized areflexia. Following this initial response, injury greater than 2.1 atm frequently produced a period associated with hypotonia of postural muscles and suppression of postural motor responses (flaccidity). A close association between flaccidity and other indices of coma such as absence of eye-opening responses was noted. These consequences of injury can occur without fatal apnoae, circulatory collapse or overt intraparenchymal haemorrhages. This result suggests that mechanical stress predominantly restricted to the brain stem in fluid percussion may be sufficient, at least in the cat, to produce coma associated with flaccidity which has been previously documented for acceleration concussion. There was no evidence that fluid percussion produced EEG depression similar to the effects of lesions in the mesencephalic reticular activating system (RAS). Thus, depression of general levels of brain activity including those within the RAS seems not be necessary for production of this form of reversible coma.