1. Sharps

A sharp wave is a brief, high-amplitude EEG pattern that lasts between 70-200 milliseconds. It represents a rapid, synchronized firing of neurons in a specific area of the brain.

  • Duration: Lasts between 70-200 ms.
  • Appearance: Typically appears as a sharply contoured deflection from the baseline, resembling a brief, sharp spike.
  • Clinical significance: While the duration of a sharp is somewhat arbitrary and there’s no strict clinical threshold, it’s considered an epileptiform discharge when it disrupts the normal rhythm of the EEG background.

Sharps are often seen during interictal (between seizures) periods in people with epilepsy. While they signify abnormal brain activity, the presence of a sharp alone does not indicate the severity or potential for a seizure. It’s more about the abnormality in neuronal synchronization and timing.

2. Spikes

A spike is similar to a sharp, but it is even shorter in duration, typically between 20-70 milliseconds. Like sharps, spikes are also a manifestation of abnormal neuronal firing.

  • Duration: Lasts from 20-70 ms.
  • Appearance: Spikes are characterized by their fast, sharp deflection that’s very brief and occurs abruptly.
  • Clinical significance: The clinical importance of spikes is similar to sharps; their presence is an indication of epileptiform activity, but a single spike does not necessarily predict a seizure.

Spikes are often seen in interictal periods (i.e., between seizures), but like sharps, they don’t necessarily correlate with severity. The spike simply indicates a momentary disruption in the normal brainwave pattern.

3. Slow Waves

Slow waves follow spikes or sharps in the EEG and represent the refractory period of the neurons that were involved in the epileptiform discharge. After the large, synchronized excitatory postsynaptic potentials (EPSPs) that lead to the spike or sharp, the affected neurons take some time to recover, which is represented by slow waves.

  • Amplitude: Slow waves are often higher in amplitude compared to the spikes or sharps that precede them.
  • Timing: They immediately follow a sharp or spike and reflect the brain’s return to its normal state after a discharge.
  • Appearance: Slow waves typically have a broad, low-frequency appearance and can appear in multiple forms, such as delta waves (0.5-4 Hz) or theta waves (4-8 Hz).

Slow waves mark the brain's recovery phase after a brief episode of abnormal neuronal firing (the spike or sharp). They indicate a transient state of reduced neuronal activity as the brain attempts to reset itself. In clinical settings, the presence and size of slow waves help clinicians understand the scope and effect of the discharge on the brain.

4. Field of Discharge

The field of a discharge refers to the spread of abnormal brain activity that is detected in the EEG. After a sharp or spike occurs, its effects should be visible in surrounding electrodes. The discharge doesn’t just affect the location where the abnormal firing occurs, but the effects should radiate outwards.

  • The presence of ripples or field activity in surrounding electrodes indicates that the discharge is occurring in a localized region but is affecting a wider area of the brain.
  • Montages: In bipolar montages (which measure the voltage difference between two closely spaced electrodes), the discharge often shows clear patterns of the electrical “ripples” spreading outward from the site of the discharge.

A discharge with a clear field suggests it is a true interictal discharge, which means it occurs between seizures and could be a warning of potential seizures. If there is no observable field — that is, the discharge appears isolated and doesn’t spread across neighboring electrodes — it may be an isolated event. In this case, it may not be categorized as an interictal discharge, but further monitoring is necessary to confirm whether more discharges follow.

5. Clinical Implications of Sharps, Spikes, and Slow Waves

The presence of spikes and sharps, along with slow waves, in an EEG can be indicative of interictal epileptic activity, meaning the person has epilepsy but isn't currently having a seizure. These discharges suggest an abnormal neural synchronization but don't always correlate directly with seizure onset.

  • Seizure Prediction: While spikes and sharps are present in interictal periods, their occurrence can sometimes indicate a higher likelihood of seizures. However, it's not a direct one-to-one correlation — not all individuals with sharps or spikes will have seizures, and many people with epilepsy have these discharges without immediately following seizures.
  • Monitoring: Monitoring the field of these discharges, the amplitude of the slow waves, and the timing of the events can help doctors determine the severity and potential treatment strategies. A clear field and high-amplitude slow waves may suggest more widespread abnormal neural activity and could influence the treatment approach.

In conclusion, spikes, sharps, and slow waves are critical components of epileptiform discharges observed in EEG. Their duration, appearance, and field play an important role in diagnosing and monitoring epilepsy, and understanding these patterns is essential for predicting and managing seizures.