Kernohan's notch is a cerebral peduncle indentation associated with some forms of transtentorial herniation (uncal herniation). It is a secondary condition caused by a primary injury on the opposite hemisphere of the brain. Kernohan's notch is an ipsilateral condition, in that a left-sided primary lesion (in which Kernohan's notch would be on the right side) evokes motor impairment in the left side of the body and a right-sided primary injury evokes motor impairment in the right side of the body. The seriousness of Kernohan's notch varies depending on the primary problem causing it, which may range from benign brain tumors to advanced subdural hematoma.
Mechanism Kernohan's notch phenomenon is a result of the compression of the cerebral peduncle, which is part of the mesencephalon, against the tentorium cerebelli due to transtentorial herniation. This produces ipsilateral hemiparesis or hemiplegia. The skull is an incompressible closed space with a limited volume (Monro-Kellie Doctrine). When there is increased cranial pressure in the brain, a shift in the brain forms towards the only opening of the skull, the foramen magnum. Thus, when an increase of pressure in a hemisphere of the brain exists, the cerebral peduncle on the opposite hemisphere is pushed up against the tentorium, which separates the posterior fossa from the middle fossa. This produces a visible "notch" in the cerebral peduncle. Because a Kernohan's notch is caused by an injury creating pressure on the opposite hemisphere of the brain, it is characterized as a false localizing sign. The Kernohan's notch phenomenon is unique in that it is not only a false localizing sign, but is also ipsilateral or same-sided. The left cerebral peduncle contains motor fibers that cross over to the right side of the body. Thus, if you have a right hemisphere trans-tentorial herniation, it causes a Kernohan's notch in the left cerebral peduncle which results in right-sided motor impairment. Therefore, you get, paradoxically, impairment of motor function on the same side of the body as the herniation which caused the Kernohan's notch on the contralateral side.
Causes The Kernohan's notch is a secondary phenomenon that results from a major primary injury. Non-tumoral, non-traumatic, intracranial haemorrhage rarely causes this phenomenon. A wide range of serious injuries can cause an increase in intracranial pressure to trigger the formation of a Kernohan's notch. In general, this phenomenon occurs in patients with advanced brain tumor or severe head injury. In the case of severe head injury, a clot can occur over the surface of the brain and can often cause shift of the middle part of the brain against the tentorium, which creates the Kernohan's notch. Chronic subdural hematomas have been known to be a familiar cause of Kernohan's notch. MRIs have shown evidence of Kernohan's notch from patients with traumatic head injury that are related to acute space-occupying lesions such as subdural hematoma, epidural hematoma, depressed skull fracture, or spontaneous intracerebral hematoma. Also, the anatomical size of tentorial notches vary considerably between individuals; however, very little evidence supports that a more narrow notch creates a predisposition towards Kernohan's notch.
Signs and symptoms Symptoms directly related to the Kernohan's notch is most commonly paralysis or weakness on one side of the body (ipsilateral paralysis / paresis), the so-called Kernohan's sign. Paralysis and weakness is known as hemiplegia and hemiparesis, respectively. This is due to destruction or pressure applied to the motor fibers located in the cerebral peduncle. A more rare sign of Kernohan's notch is ipsilateral oculomotor nerve palsy. However, most patients come into the clinic citing symptoms associated with the primary injury causing the Kernohan's notch. Since so many types of head injuries exist, virtually any symptom of brain trauma can be seen accompanying Kernohan's notch. These symptoms may range from total paralysis to simple headache, nausea, and vomiting.
Diagnosis Because of the ipsilateral characteristic of Kernohan's notch, diagnosis is unique. Many clinicians assume a right sided paralysis corresponds with an injury in the left hemisphere of the brain and misdiagnose Kernohan's notch. Despite this complication that ipsilateral paralysis causes, it makes it very easy to diagnose when coupled with imaging techniques. For example, when seeing an MRI of a blood clot on the left side of the brain coupled with left-sided paralysis, it immediately points to Kernohan's notch. In most head trauma cases, CT scans are the standard diagnostic method; however it is not ideal for imaging small lesions, so MRI is used to identify Kernohan's notch. It is important to distinguish Kernohan's notch from direct brain stem injuries. Case studies have shown that in patients with chronic subdural hematoma, a compressive deformity of the crus cerebri without an abnormal MRI signal may predict a better recovery in patients with Kernohan's notch.
Treatments There is no special treatment for Kernohan's notch since treatment is completely dependent on the injury causing it. The pressure against the tentorium should be relieved and the notch should go away. However, this does not mean that damage to the motor fibers disappears. Depending on the severity of the injury, there may or may not be persistent damage after pressure relief. Neurological deficits may resolve after surgery, but some degree of deficit, especially motor weakness, generally remains. Pressure relief to "un-notch" the cerebral peduncle may include the removal of brain tumors and blood clots or the suction of blood from a drilled hole in the skull.
Case studies
Kernohan's notch and misdiagnosis The ipsilateral and false localizing signs characteristic of Kernohan's notch sometimes cause confusion and result in misdiagnosis. Such a case is described in an account by Wolf at the University Hospital in Groningen, Netherlands:
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