A unilaterally dilated pupil is the first sign secondary to a mass in the middle fossa. Traditionally, it was taught that this is the result of compression of the third nerve against the incisura. However, magnetic resonance imaging (MRI) studies have shown that third-nerve paresis is more commonly caused by distortion of the midbrain rather than direct third nerve compression. There is a close correlation between the degree of lateral displacement and the alteration of consciousness. A contralateral hemiplegia usually is present. Respiration and consciousness usually are unimpaired. Sometimes the ipsilateral pupil is small, rather than large, and rarely the contralateral pupil dilates before the ipsilateral one.

Progressive pressure increase leads to increasing stupor, a more complete third-nerve palsy, and sometimes an ipsilateral hemiplegia with bilateral Babinski responses. The ipsilateral hemiparesis is secondary to tentorial pressure against the opposite cerebral peduncle (Kernohan notch). Respiration may be normal or of the central neurogenic hyperventilation pattern. There is often decerebrate posturing (arms extended at the side with inward turning, spontaneously or when a noxious stimulus is applied). Decorticate posturing (arms flexed at the elbow “pointing” to the cortex) is not usually seen with the uncal syndrome.