Kernohan’s Notch Syndrome

Kernohan’s Notch Syndrome

Raised intracranial pressure as a result of an expanding supratentorial lesion (e.g., tumor, subdural hematoma) may cause herniation of brain tissue through the tentorium into the subtentorial space, putting pressure on the midbrain. If the midbrain is shifted against the contralateral margin (free edge) of the tentorium, the cerebral peduncle on that side may be compressed, resulting in a hemiparesis which is ipsilateral to the supratentorial lesion (and hence may be considered "false-localizing").
There may also be an oculomotor nerve palsy ipsilateral to the lesion, which may be partial (unilateral pupil dilatation).

References

Cohen AR, Wilson J. Magnetic resonance imaging of Kernohan’s notch. Neurosurgery 1990; 27: 205-207
Kernohan JW, Woltman HW. Incisura of the crus due to contralateral brain tumor. Arch Neurol Psychiatry 1929; 21: 274-287
Kole MK, Hysell SE. MRI correlate of Kernohan’s notch. Neurology
2000; 55: 1751

Cross References

"False-localizing signs"; Hemiparesis; Hutchinson’s pupil