The Electroencephalogram (EEG) is a noninvasive diagnostic tool that measures the electrical activity produced by the brain using small electrodes placed on the scalp. The resulting tracing is a continuous record of brain waves, categorized by their frequency, amplitude, and shape. While most patterns represent normal function, the presence of certain transient, high-amplitude waveforms, known as sharp waves, can indicate an underlying abnormality in clinical neurology.
Defining Sharp Waves on an EEG
A sharp wave is a specific type of abnormal electrical transient identified by its distinct morphology on the EEG tracing. It is defined primarily by its duration, which falls between 70 and 200 milliseconds, distinguishing it from a faster “spike” (less than 70 milliseconds).
These waves are sharply contoured, appearing as a pointed peak that disrupts the brain’s typical background rhythm. They exhibit high amplitude, making them stand out prominently compared to ongoing brain activity. Sharp waves are almost always recorded as a negative deflection, meaning the wave points upward on the conventional EEG display. They are often followed immediately by a slower, high-amplitude wave, creating a characteristic “sharp-and-slow-wave” complex.
Clinical Interpretation of Sharp Waves
Sharp waves are categorized as interictal epileptiform discharges (IEDs), meaning they are abnormal electrical events occurring between clinical seizures. They represent a brief, synchronous, and excessive discharge of electrical energy from a localized group of hyperexcitable neurons. This synchronized firing indicates an underlying propensity for abnormal brain activity and an increased risk of developing seizures.
The location of the sharp wave guides the clinician toward a focal or generalized classification of brain irritability. Focal sharp waves are confined to a specific brain region, such as a single hemisphere or lobe, and are the electrographic signature of focal epilepsies. Generalized sharp waves appear simultaneously across both hemispheres in a bilaterally synchronous and symmetrical pattern.
Generalized discharges are considered more predictive of seizure recurrence than focal ones. However, the presence of sharp waves alone is not sufficient to diagnose epilepsy, as a small percentage of healthy individuals (up to 5% of children) can exhibit these discharges without ever experiencing a seizure. The finding must always be correlated with the patient’s clinical history and symptoms to determine its true significance.
Associated Epileptic Syndromes and Conditions
The specific pattern and location of sharp waves are important for categorizing the patient’s condition and identifying an epileptic syndrome. For instance, centrotemporal spikes (Rolandic spikes) are characteristic of Benign Focal Epilepsy of Childhood with Centro-Temporal Spikes (BECTS). These focal discharges are typically enhanced during sleep and suggest an epilepsy syndrome that children usually outgrow.
Generalized sharp waves are a feature of several genetic generalized epilepsies, often appearing as spike-and-wave or polyspike-and-wave complexes. Slower, less regular spike-wave complexes are associated with symptomatic generalized epilepsies like Lennox-Gastaut syndrome. Sharp waves can also be seen in non-epileptic contexts, requiring differentiation from normal variants.
Non-Epileptic Sharp Waves
High-amplitude sharp waves known as triphasic waves are often a sign of a metabolic or toxic encephalopathy, such as uremia or liver failure, rather than a primary epileptic disorder. These waves appear diffusely. Another non-pathological example is the Positive Occipital Sharp Transients of Sleep (POSTS), a normal waveform seen during certain sleep stages.
Diagnostic Follow-Up and Management
Once sharp waves are identified on an EEG, the diagnostic process involves a comprehensive evaluation to determine the underlying cause and clinical relevance. This begins with a detailed review of the patient’s clinical history, focusing on whether any paroxysmal events were actual epileptic seizures. The next step often includes neuroimaging, such as Magnetic Resonance Imaging (MRI) or Computed Tomography (CT), to investigate potential structural causes.
Neuroimaging rules out underlying brain lesions, such as tumors, strokes, or malformations of cortical development, which can focus abnormal electrical activity. Blood tests may also be necessary to check for metabolic disturbances or toxic conditions that could be the source of generalized abnormalities like triphasic waves. The presence of IEDs after a patient’s first unprovoked seizure is significant, associated with approximately a two-fold increased likelihood of seizure recurrence.
Management decisions depend heavily on whether the patient has experienced clinical seizures in addition to the EEG finding. For individuals who exhibit sharp waves but remain asymptomatic without a history of seizures, treatment with Anti-Epileptic Drugs (AEDs) is generally not recommended. The potential risks and side effects of medication typically outweigh the benefits in asymptomatic cases. Treatment is usually initiated only if the patient has had seizures or if the sharp wave activity is correlated with specific cognitive dysfunction. Long-term monitoring, including follow-up EEGs, remains an important aspect of care.