Focal spikes and sharp waves usually indicate a seizure disorder of with partial onset, but focal discharges can also be indicative of a structural lesion in the absence of seizure activity. Frontocentral discharges may be seen in patients with simple partial seizures. Temporal or frontal spikes may be seen in patients with complex partial seizures. Normal focal spike-wave complexes include 14- and 6-Hz positive spikes, subclinical rhythmic electrographic discharge of adults (SREDA), and wicket spikes.
Focal spikes are interpreted if the spike is consistent, has an identifiable field, and cannot be explained by artifact. A single spike during the course of a recording should not be
interpreted as abnormal, although if the conformation is worrisome, note in the body of the report should be made. Also, great caution should be exercised when interpreting a spike that is seen from a single electrode; remember that a single electrode is usually represented on more than one channel in a montage.
Focal spikes associated with seizures
Focal spikes are associate with partial seizures and the benign epilepsies of childhood. Partial seizures are divided into simple and complex, based on symptomatology rather than EEG findings. The benign epilepsies of childhood can manifest as focal and generalized seizures.
Simple partial seizure
The EEG during a simple partial seizure usually shows prominent spiking over the involved cortex, although in some patients there may be localized slowing which may become generalized. A typical pattern might be left central spikes in a patient who
presents with focal seizures affecting the right arm. Occasionally, the sharp component of the discharge may be subtle or missing. The epileptiform activity may occur in deep layers of cortex and subcortical structures so that the spike potentials are not projected to the surface electrodes. Alternatively, there may not be sufficient synchrony to produce a spike detectable on the surface.
Partial seizures may spread throughout the hemispheres, resulting in a generalized
seizure. This is secondary generalization, as opposed to primary generalized seizures, like absence. Secondary generalized seizures may have a focal onset which can be detected
Table 5-3: Interpretation of focal spikes.
Type EEG features Clinical features
Rolandic spikes Spike and slow wave complex. Often triphasic and fast. Maximal at C3 and C4.
Benign rolandic epilepsy. May be seen in subjects without seizures.
Occipital spikes Negative or biphasic spikes over the occipital region. Unilateral or independent bilateral discharges.
May be benign occipital epilepsy, but occipital spikes are not always benign. May be seen in blindness.
Parietal sharp waves Sharp waves or spikes in the parietal region. Can be activated by forehead taps.
Often associate with versive head and eye movement or sensory seizures.
Temporal sharp waves
Sharp waves in the temporal region. Anterior temporal = F7, F8. Midtemporal = T3, T4. Posterior temporal = T5, T6.
Anterior temporal sharp waves often associated with partial complex seizures. Midtemporal sharp waves associated with seizures and with psychological complaints. Posterior temporal sharp waves associated with seizures in the majority of patients, often generalized tonic-clonic. Also associated with psychological complaints.
Periodic lateralized epileptiform discharges
Unilateral or bilateral independent sharp and slow wave complexes at 1-2/sec.
Any destructive process.
Often anoxia, herpes simplex encephalitis, stroke, tumor.
clinically, but this is not always the case. The generalization may occur so quickly that the focal onset can only be determined by EEG, and not by clinical appearance.
Complex partial seizure
The EEG during complex partial seizures usually shows focal spikes in the temporal or frontal region. Routine EEG may not detect the spikes if they originate in cortex that is not directly underlying the surface electrodes. Sphenoidal, nasopharyngeal, or depth electrodes may be needed to identify these discharges. Complex partial seizures may have secondary
generalization. If the rate of generalization is fast, the partial origin may not be evident, and the only clue would be separate complex partial seizures without generalization and EEG showing focal activity prior to the
generalization.
Benign focal epilepsies of childhood
Benign focal epilepsies of childhood are termed benign because they are age-related and seldom persist into adult life. There are two types: rolandic and occipital.
Rolandic epilepsy
Rolandic epilepsy is characterized by interictal discharges arising from the central regions, localized near electrodes C3 and C4. The interictal discharges are independent and augmented by sleep. Relatives of patients with rolandic epilepsy may have EEG abnormality as a genetic marker without clinical seizures.
The discharges of rolandic epilepsy are so characteristic in location and pattern that they are seldom confused with other pathologic activity. Independent central spikes are seen on an otherwise normal background. This must be differentiated from multifocal spikes, however.
Occipital epilepsy
Figure 5-5: Central spikes
Central spikes, of the types seen in rolandic epilepsy. The maximal negativity in at C3. Discharges are expected on the right side, as well, although the right hemisphere channels are not shown in this figure.
Occipital epilepsy is characterized by interictal sharp waves with predominance at O1 and O2. Rolandic and occipital epilepsy may occur in the same families, and relatives with no history of seizures may have either occipital or rolandic discharges on EEG. During the seizure, the EEG shows 2-3/sec spike-wave discharges with predominance in the occipital region. The interictal discharge may be blocked by photic stimulation or eye opening.
Focal sharp activity without seizures
Focal spikes or sharp waves are occasionally seen in patients with no clinical seizures. The EEG may have been ordered for some non-epileptic indication. Some of these are children who are genetic carriers of benign epilepsies, whereas in others there is no explanation. The interpretation of these records is controversial. Some neurophysiologists believe that all abnormal sharp activity is potentially epileptogenic and should be
interpreted as such. Unfortunately, this may result in unneeded use of antiepileptic drugs. Patients should be treated with antiepileptic drugs based on clinical presentation rather than on EEG findings. The old adage still is valid “Treat the patient, not the EEG.” Of course, the countering argument is that the patient may have seizures which are not always clinically identifiable.
About 3% of normal individuals exhibit epileptiform activity on EEG. [ck] The proportion is somewhat higher in children than adults. Approximately 25% of these discharges are focal. Some of these patients will go on to develop seizures, however, these patients should not be treated with anticonvulsants without clinical evidence of convulsive activity. Of patients with seizures, about 50% will show abnormalities on EEG, but this value differs dramatically, depending on the clinical setting. Patients with absence epilepsy are more likely to have abnormal EEG than patients with complex partial seizures,, for example.
Children with behavioral disturbances have been reported to have an increased incidence of focal sharp waves and spikes. The implication of these findings is controversial. Some investigators believe that the spikes may have contributed to the behavioral disturbance by interfering with normal social and intellectual development. Others believe that the spikes are incidental and should not be treated. The spikes are probably a reflection of brain dysfunction, which correlates with the behavioral disorder rather than being the cause of the dysfunction.
Subclinical rhythmic electrographic discharge of adults (SREDA) is sharply contoured rhythmic delta activity with prominent in the centroparietal region. This pattern is seen in older patients and has no definite clinical correlate. This is not an ictal discharge. Patients with this finding are said to be at increased risk for cerebrovascular disease, but the association is not convincing. [ck] This rhythm is not found in normal younger
individuals and is probably an abnormal pattern. The report should reflect the nonspecific clinical implications.
Some patients with congenital blindness may exhibit occipital spikes. These should not be interpreted as epileptiform.