Categories of structural processes that impair the function of the arousal system:
Supratentorial mass lesions - may compress deep diencephalic structures → impair the function of both hemispheres;
infratentorial mass or destructive lesions - directly damage the RAS at its source (upper brainstem).
To produce stupor or coma - damage or ↓ of function of both cerebral hemispheres or bilateral damage to RAS (paramedian region of the upper brainstem, diencephalon)
Conversely, unilateral hemispheric lesions, or lesions at/below of the brainstem at the level of the midpons - do not cause coma. (Sx of sensory/motor deficits).
Locations and etiologies of lesions
Bilateral hemispheric damage - hypoxic-ischemic insult
Diencephalic injury - "Tip of the basilar" syndrome - occlusion of the perforating arteries (arise from the basilar apex or PCA) → bilateral thalamic infarction; thalamic hemorrhage, tumors, inflammatory lesions (e.g., Behcet's syndrome). ± ischemia and tumors of hypothalamus, pituitary tumors;
Upper brainstem injury