Kernohan’s Notch

What is Kernohan's Notch?

Kernohan’s notch (also called the Kernohan’s sign or “false localizing sign”) is a radiologic and clinical finding that occurs when a large intracranial mass or swelling pushes the brainstem against the opposite side of the skull (the contralateral tentorial edge). This pressure creates an indentation—or “notch”—in the cerebral peduncle, which may be seen on CT or MRI scans. Paradoxically, the neurological deficits that result (most often motor weakness) appear on the same side as the original lesion, confusing clinicians who expect the deficits to be on the opposite side.

The phenomenon was first described by neurosurgeon James Kernohan in 1928 and remains an important clue in neuro‑trauma and neuro‑oncology because it signals a life‑threatening shift of brain structures (herniation) that often requires emergent decompression.¹

Common Causes

Any lesion that creates enough mass effect to force the brain past the tentorial notch can produce a Kernohan’s notch. The most frequent culprits are:

• Large supratentorial intracerebral hemorrhage (e.g., intracerebral or subdural bleed)
• Space‑occupying brain tumors (glioblastoma, meningioma, metastatic lesions)
• Acute traumatic brain injury with significant cerebral edema
• Massive cerebral infarction (especially in the MCA territory)
• Hydrocephalus with rapid ventricular enlargement
• Subarachnoid hemorrhage causing severe cerebral swelling
• Enlarged epidural hematoma that crosses the midline
• Cerebral abscess or granuloma with surrounding edema
• Venous sinus thrombosis leading to venous congestion and edema
• Severe intracranial infections (e.g., meningitis with marked edema)

Associated Symptoms

Because a Kernohan’s notch reflects a shift of deep brain structures, patients often display a mixture of focal and diffuse neurological signs:

• Motor weakness (hemiparesis) on the same side as the original lesion (the “false‑localizing” sign)
• Altered level of consciousness ranging from drowsiness to coma
• Pupillary abnormalities (e.g., dilated, non‑reactive pupil on the side of the lesion)
• Headache—often sudden, severe, and described as “worst ever”
• Nausea, vomiting, or projectile vomiting (sign of raised intracranial pressure)
• Seizures, especially if the underlying cause is a bleed or tumor
• Speech difficulties (aphasia) if dominant‑hemisphere structures are involved
• Visual field deficits (e.g., homonymous hemianopia) when occipital pathways are compressed

When to See a Doctor

Any sudden neurological change warrants prompt medical attention, but the following situations are especially urgent:

• New‑onset weakness or paralysis that does not improve within minutes
• Sudden, severe headache that is different from usual headaches
• Loss of consciousness or difficulty staying awake
• Repeated vomiting without a clear gastrointestinal cause
• Changes in speech, vision, or coordination
• Any head injury followed by worsening symptoms after an initial “lucid interval”

Because a Kernohan’s notch signals potentially fatal brain herniation, call emergency services (911 in the U.S.) or go to the nearest emergency department immediately.

Diagnosis

Diagnosing Kernohan’s notch involves a combination of clinical assessment and imaging studies.

Clinical Evaluation

• Focused neurological exam to document the side and severity of motor deficits
• Assessment of cranial‑nerve function (pupils, eye movements, facial symmetry)
• Glasgow Coma Scale (GCS) to gauge level of consciousness

Imaging

• CT scan (non‑contrast) – First‑line in emergencies; quickly shows hemorrhage, mass effect, and signs of herniation.
• MRI (T1, T2, FLAIR, diffusion‑weighted) – Provides higher resolution of the cerebral peduncles and can directly demonstrate the “notch” in the contralateral peduncle.
• CT or MR angiography – May be used to rule out vascular lesions (e.g., aneurysm) that could be the primary cause.

Other Tests

• Laboratory work‑up to assess coagulation status, electrolytes, and infection markers (CBC, PT/INR, CRP).
• Lumbar puncture is generally avoided until mass effect is ruled out because it can worsen herniation.

Treatment Options

Treatment is directed at the underlying cause and at relieving intracranial pressure (ICP). Management usually involves a multidisciplinary team (neurosurgery, intensive‑care, radiology, and rehabilitation).

Medical Management

• Osmotherapy – Intravenous mannitol or hypertonic saline to draw fluid out of brain tissue.
• Steroids – Dexamethasone (especially for tumor‑related edema).
• Anticonvulsants – Levetiracetam or phenytoin if seizures are present.
• Blood pressure control – Avoid hypertension that could worsen hemorrhage; target MAP per AHA/ASA guidelines.
• ICP monitoring – Invasive intraventricular catheter in severe cases.

Surgical Intervention

• Craniotomy or decompressive hemicraniectomy – Removes the mass (tumor, hematoma) and allows the brain to expand outward.
• Evacuation of hematoma – Burr‑hole or craniotomy drainage for subdural or epidural bleeds.
• Tumor resection – Microsurgical removal or stereotactic radiosurgery depending on tumor type.
• Ventriculostomy – Placement of an external ventricular drain (EVD) to relieve hydrocephalus.

Rehabilitation & Follow‑up

• Physical and occupational therapy to restore motor function.
• Speech‑language therapy if aphasia is present.
• Regular neuro‑imaging to confirm resolution of the notch and monitor for recurrence.

Prevention Tips

While Kernohan’s notch itself cannot always be prevented, reducing the risk of its underlying causes can lower the chance of this serious complication.

• Control blood pressure – Aim for < 130/80 mmHg; follow the American Heart Association recommendations.
• Use protective gear – Helmet use during cycling, motorcycling, and contact sports.
• Avoid excessive alcohol and anticoagulant misuse – Decreases risk of spontaneous intracerebral hemorrhage.
• Manage chronic conditions – Diabetes, hyperlipidemia, and smoking cessation reduce stroke risk.
• Prompt treatment of infections – Early antibiotics for sinus or ear infections reduce risk of intracranial spread.
• Regular medical surveillance – Annual MRI for known brain tumor patients, routine follow‑up after head trauma.

Emergency Warning Signs

• Sudden, severe headache (often described as “thunderclap”)
• Rapidly worsening weakness or paralysis on one side of the body
• New loss of consciousness, unresponsive state, or GCS < 8
• Repeated vomiting or seizures without an obvious cause
• Pupillary dilation or unequal pupils
• Any sign of “brain herniation” such as stiff neck with a bulging fontanelle in infants, or a “fixed, dilated pupil”
Action: Call emergency services immediately (e.g., 911 in the U.S.) and do not attempt to drive yourself.