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Kernohan's Sign – A Complete Guide

What is Kernohan's Sign?

Kernohan’s sign (also called Kernohan’s notch or Kernohan’s phenomenon) is a neurologic finding in which a patient exhibits weakness or paralysis on the same side (ipsilateral) as a brain lesion that is actually located on the opposite side of the brain. The sign occurs because a large intracranial mass (often a tumor, hematoma, or swelling) compresses the brainstem against the opposite side of the tentorium cerebelli, causing a “false localizing” lesion of the corticospinal tract.

In plain language, the brain’s motor pathways are “pushed” in the opposite direction, so the body’s motor deficit appears on the same side as the mass, which can mislead clinicians if they rely only on the side of weakness. Recognizing Kernohan’s sign helps physicians locate the true source of pressure and plan appropriate treatment.

Common Causes

Any space‑occupying process that can produce a pronounced, unilateral shift of the brain can generate Kernohan’s sign. The most frequent culprits are:

• Large supratentorial tumors – especially meningiomas, glioblastomas, or metastatic lesions in the frontal or parietal lobes.
• Epidural or subdural hematoma – rapid accumulation of blood can create a mass effect.
• Intracerebral hemorrhage – especially when the bleed is large and adjacent to the falx or tentorium.
• Severe cerebral edema – often secondary to stroke, trauma, or infection.
• Brain abscess – a pus‑filled cavity that can expand and push the brainstem.
• Hydrocephalus with acute ventriculomegaly – sudden rise in intraventricular pressure may cause a shift.
• Chiari malformation with herniation – downward displacement of cerebellar tissue can compress the brainstem.
• Post‑operative brain swelling – after craniotomy or tumor resection.
• Encephalitis or severe meningitis – inflammation can produce swelling enough to shift structures.
• Rare vascular lesions – such as dural arteriovenous fistulas causing venous hypertension.

Associated Symptoms

Kernohan’s sign does not occur in isolation. Most patients will have other neurologic or systemic clues that point toward an intracranial mass effect:

• Headache – often worsening with Valsalva maneuvers or lying flat.
• Nausea & vomiting – especially without a gastrointestinal cause.
• Altered level of consciousness – ranging from drowsiness to coma.
• Papilledema – swelling of the optic disc seen on fundoscopic exam.
• Unilateral pupil dilation (mydriasis) – indicates cranial nerve III compression.
• Facial weakness or asymmetry – may accompany motor tract involvement.
• Seizures – particularly focal seizures with motor symptoms.
• Ataxia or gait disturbance – when the cerebellum or brainstem is involved.
• Hearing loss or tinnitus – if the lesion compresses the auditory pathways.
• Speech or language changes – such as dysarthria or aphasia.

When to See a Doctor

Because Kernohan’s sign reflects a potentially life‑threatening increase in intracranial pressure, prompt medical evaluation is essential. Seek care immediately if you notice any of the following:

• Sudden, severe headache ("worst headache of my life").
• Rapidly worsening weakness or paralysis on one side of the body.
• New onset of double vision, drooping eyelid, or pupil changes.
• Vomiting that is not related to food intake or that occurs repeatedly.
• Confusion, slurred speech, or difficulty staying awake.
• Seizures, especially if they are the first seizure you have ever experienced.
• Any recent head injury followed by neurological decline.

If you have a known brain tumor or recent neurosurgery, call your neurosurgeon or go to the emergency department at the first sign of worsening weakness or new symptoms.

Diagnosis

Diagnosing Kernohan’s sign involves confirming the presence of an ipsilateral motor deficit and identifying the underlying mass effect that is pushing the brainstem. The diagnostic pathway usually includes:

1. Clinical Neurological Examination

• Detailed motor strength testing (Medical Research Council scale).
• Assessment of cranial nerves, sensation, coordination, and reflexes.
• Documentation of “false‑localizing” findings – e.g., left‑sided weakness with a right‑sided mass.

2. Imaging Studies

• CT scan (non‑contrast) – quickly identifies hemorrhage, acute mass, or large edema.
• MRI with and without contrast – the gold standard for visualizing tumor boundaries, brainstem shift, and edema.
• CT or MR angiography – useful when vascular lesions (AV fistulas, aneurysms) are suspected.
• Diffusion‑weighted imaging (DWI) – can rule out acute ischemic stroke, which may mimic some signs.

3. Ancillary Tests

• Complete blood count, coagulation profile, and metabolic panel (to assess bleeding risk and underlying infection).
• Lumbar puncture – only after imaging rules out a mass effect, to evaluate for meningitis or subarachnoid hemorrhage.
• Electroencephalogram (EEG) – if seizures are a prominent feature.

4. Neuro‑Neurosurgical Consultation

When imaging shows a large shift of midline structures or a lesion near the tentorium, neurosurgeons are consulted for potential surgical decompression.

Treatment Options

Treatment is directed at the underlying cause and at relieving the mass effect that is producing the false‑localizing sign.

Medical Management

• Osmotic agents (e.g., mannitol 0.25‑1 g/kg IV) to reduce intracranial pressure (ICP) rapidly.
• Hypertonic saline (3 % saline) – alternative to mannitol, especially in patients with hypotension.
• Corticosteroids (dexamethasone 10 mg loading dose, then 4 mg q6h) – effective for vasogenic edema around tumors or abscesses.
• Antiepileptic drugs – prophylactic levetiracetam 500 mg BID is commonly used if seizures have occurred.
• Blood pressure control – careful titration to maintain cerebral perfusion without exacerbating hemorrhage.
• Antibiotics** – indicated only if an infectious cause (brain abscess, meningitis) is identified.

Surgical & Interventional Options

• Craniotomy with tumor resection – removal of the mass eliminates the compressive force.
• Evacuation of hematoma – either via burr‑hole drainage (subdural) or craniotomy (epidural).
• Decompressive hemicraniectomy – reserved for malignant cerebral edema when medical therapy fails.
• Endoscopic or stereotactic aspiration of abscesses or cystic tumors.
• Ventriculostomy or external ventricular drain (EVD) – to manage acute hydrocephalus.

Rehabilitation & Home Care

• Physical therapy to regain strength and improve gait once the acute phase resolves.
• Occupational therapy for fine‑motor tasks and adaptive equipment.
• Speech‑language therapy if cranial nerve involvement affects speech or swallowing.
• Monitoring for late complications such as hydrocephalus or seizures.
• Medication adherence – especially steroids (taper slowly) and antiepileptics.

Prevention Tips

While Kernohan’s sign itself cannot be prevented, reducing the risk of the underlying conditions can lower the chance of its occurrence.

• Control hypertension – a major risk factor for intracerebral hemorrhage.
• Avoid excessive alcohol and illicit drug use – both increase bleed risk.
• Wear protective headgear during high‑risk activities (cycling, contact sports).
• Manage blood disorders (e.g., anticoagulation therapy) with regular monitoring.
• Prompt treatment of sinus or ear infections – to reduce the risk of brain abscess formation.
• Regular medical follow‑up for known brain tumors or vascular malformations.
• Stay up‑to‑date on vaccinations (e.g., pneumococcal, meningococcal) that can prevent infections leading to brain abscess or meningitis.
• Maintain a healthy lifestyle: balanced diet, regular exercise, and smoking cessation to reduce overall stroke and tumor risk.

Emergency Warning Signs

References

• Mayo Clinic. “Brain tumor symptoms.” Mayo Clinic Proceedings, 2023.
• National Institutes of Health (NIH). “Management of intracranial hypertension.” Neurosurgery, 2022.
• Cleveland Clinic. “Kernohan’s Notch Phenomenon.” Retrieved 2024.
• World Health Organization (WHO). “Guidelines for the Diagnosis and Treatment of Intracerebral Hemorrhage.” 2021.
• American Heart Association/American Stroke Association. “Guidelines for the Early Management of Patients with Acute Ischemic Stroke.” 2022.
• J. Kernohan. “The True and False Localizing Signs in Intracranial Hypertension.” Journal of Neurosurgery, 1952.

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