Spike and wave discharges (SWD) represent a specific pattern of electrical activity in the brain associated with certain types of epilepsy. This pattern is a hallmark of abnormal, synchronized firing among large groups of neurons, not a normal feature of brain function. SWD are detected using an electroencephalogram (EEG), a non-invasive test that measures electrical activity on the scalp. Identifying the precise characteristics of SWD is a foundational step for neurologists in diagnosing and classifying generalized epilepsy syndromes.
The Electrical Signature of Spike and Wave Discharges
The term “spike and wave discharge” describes the unique, rhythmic shape of the electrical signal captured by an EEG. This repeating pattern involves two distinct components: a sharp deflection followed by a slow, rounded one. The initial “spike” is a high-amplitude, brief burst of electrical activity, typically lasting between 20 and 70 milliseconds. This sharp wave represents the near-simultaneous, excessive excitation of cortical neurons.
The spike is immediately followed by the “wave,” a slow, rounded deflection lasting significantly longer, around 70 to 200 milliseconds. This slow wave reflects a period of neuronal hyperpolarization, or inhibition, as the cells recover from the initial burst. The entire complex repeats rhythmically, creating a visible, oscillating pattern on the EEG tracing.
The frequency of this repeating cycle, measured in Hertz (Hz), is a defining characteristic. For instance, the 3 Hz spike-and-wave discharge means the complex repeats three times every second. This frequency, regularity, and symmetry help distinguish between different epilepsy conditions. The discharges are typically “generalized,” appearing simultaneously across both hemispheres of the brain, often with maximum amplitude over the frontal regions.
Neuronal Basis: The Thalamocortical Mechanism
The physiological origin of spike and wave discharges is rooted in a specific, abnormal oscillation within the thalamocortical network, a circuit connecting the thalamus deep within the brain to the overlying cerebral cortex. This mechanism involves a synchronized feedback loop between the thalamic relay neurons and the inhibitory neurons of the thalamic reticular nucleus (TRN). The TRN acts as a gatekeeper, and its neurons are primarily inhibitory, releasing the neurotransmitter gamma-aminobutyric acid (GABA).
The cycle begins with a burst of firing in the thalamic reticular neurons, which powerfully inhibits the thalamic relay cells. This strong inhibition causes the relay cells to hyperpolarize, which in turn activates a specific ion channel called the T-type calcium channel. Once activated, these T-type channels allow a rapid influx of calcium ions, triggering a rebound burst of activity in the thalamic relay cell.
This rebound burst from the thalamus is then projected widely to the cortex, causing the excessive, synchronized excitation that generates the visible “spike” on the EEG. The cortical neurons then feed back to the TRN, sustaining the cycle and leading to the subsequent hyperpolarization phase, which corresponds to the slow “wave.” This rhythmic exchange between excitation in the cortex and inhibition in the thalamus creates the continuous, oscillating SWD pattern.
Typical vs. Atypical Spike and Wave Patterns
The precise appearance of the spike and wave discharge on the EEG is important for diagnosis, leading to a distinction between typical and atypical patterns. Typical spike and wave discharges are characterized by a high degree of regularity, symmetry, and a specific frequency. These discharges classically occur at a frequency of 3 Hz, meaning the spike and wave complex repeats exactly three times per second, and they have an abrupt start and stop on the EEG tracing.
Atypical, or slow, spike and wave patterns suggest a more widespread or complex underlying brain dysfunction. These patterns occur at a slower frequency, typically between 1.5 and 2.5 Hz. The morphology of the atypical discharge is irregular, meaning the shape and amplitude of the repeating complexes are inconsistent and often fragmented.
Atypical discharges are frequently asymmetrical, appearing more prominent in one area of the brain than another, and they tend to begin and end less abruptly than the typical 3 Hz pattern. This irregularity and slower frequency reflect a less synchronized and more diffuse involvement of the thalamocortical circuit, often indicating a more severe epilepsy syndrome.
Associated Epilepsy Syndromes
The presence of spike and wave discharges links a patient’s symptoms to specific epilepsy syndromes. Typical 3 Hz spike and wave discharges are the defining electrographic feature of Childhood Absence Epilepsy (CAE). CAE is characterized by brief absence seizures, involving a vacant stare and cessation of activity lasting only a few seconds. The sudden onset and termination of the 3 Hz pattern on the EEG mirrors the abrupt behavioral change seen in these seizures.
In contrast, the slower, irregular, and asymmetrical atypical spike and wave patterns are associated with more severe developmental and epileptic encephalopathies. The most recognized is Lennox-Gastaut Syndrome (LGS), a complex childhood condition involving multiple seizure types, including atypical absence, tonic, and atonic seizures. Atypical absence seizures in LGS are often longer and may include changes in muscle tone, accompanied by the characteristic slow spike-wave pattern of less than 2.5 Hz. Other syndromes, such as Juvenile Myoclonic Epilepsy (JME), feature faster discharges (4 to 6 Hz) that may include multiple spikes before the slow wave, known as polyspike-wave complexes.