Magnetic resonance imaging of Kernohan's notch.

Compression of the cerebral peduncle against the tentorial incisura contralateral to a supratentorial mass, the so-called Kernohan's notch, can be a cause of false localizing motor signs. The authors present a case of Kernohan's notch secondary to a traumatic extradural hematoma. The patient developed an oculomotor palsy and a dense motor deficit ipsilateral to the extra-axial hematoma. Magnetic resonance imaging in the postoperative period clearly showed the midbrain lesion. The motor deficit and 3rd nerve palsy subsequently resolved.