A fixed midposition pupil may be seen with hypothermia or glutethimide. Small, fixed pupils may be seen with opiates, organophosphates, pilocarpine,phenothiazine, and following respiratory arrest from barbiturates. Brain herniation may result in fixed pupils even though the herniation may be a primary metabolic process. Testing of reflexes should includethe corneal reflex, gag reflex, sternutatory reflex, oculocephalics, andvestibular reflexes. Motor function testing should assess spontaneous movements,such as myoclonic jerks or asterixis or seizure activity, or if response tostimuli is appropriate, purposeful, or posturing.
Some general rules apply in the comatose patient.
In general,focalneurological signs indicate a structural lesion, however focal signs may beseen inTodd's paralysis following a generalized seizure disorder or ifthere is a preexisting focal deficit such as an old stroke. Nonfocalneurological signs usually indicate toxic or metabolic coma, however nonfocalsign also occur in subarachnoid hemorrhage, bilateral subdural hematoma, orvasculitis. Toxic and metabolic comas usually have an incomplete and symmetric affect onthe nervous system, affecting many levels of the neuraxis simultaneouslywhile retaining integrity at other levels. Respiratory patterns may notlocalize the level of the neurological lesion. Damage to the midbrain and higher brain stem structures mayresult in central neurogenic hyperventilation, which is a hyperventilatorypattern in excess of 20 respirations per minute without the crescendoamplitude seen in Cheyne-Stokes respiration.