Tonic-clonic generalised seizures affect the whole brain and produce abnormal electrical activity at the frontal, temporal and occipital sites. A tonic-clonic seizure is typified by an initial strong contraction of the whole musculature, causing a rigid extensor spasm. This ‘tonic’ phase is followed by a series of synchronous jerks, the ‘clonic’ phase. The patient remains unconscious for a few more minutes (post-convulsive coma), before gradually recovering.
Absence seizures affect the whole brain and produce abnormal electrical activity at the frontal, temporal and occipital sites. Absence seizures occur in children and are much less severe than the tonic-clonic seizure type. They are typified by a 3 second ‘spike and wave’ discharge where the patient has a brief moment of disorientation. Patients usually experience little or no motor disturbance.
Partial seizures do not affect the whole brain and are often caused by localised cerebral lesions. In this type of seizure, the abnormal electrical discharge in the brain is localised in the temporal, frontal or occipital site. These discharges produce mild symptoms, with no loss of consciousness. The incidence partial seizures increases with age.
An EEG is a record of brain activity that is measured using an electroencephalograph and a series of electrodes. In order to perform consistent testing of electrical brain activity, the ‘International Electrode Placement System’ or ‘10–20 system’ was developed in 1947. The system is based on the relationship of the location of the electrode and the underlying area of cerebral cortex. It takes into account varying head sizes by measuring the lateral distance between the ears and the longitudinal distance from the bridge of the nose to the back of the head, with electrodes being placed at 10 and 20 per cent values of a measured distance. Each site on the scalp has a letter (to identify the lobe) and a number or another letter to identify the hemisphere location. Electrodes positioned over the frontal (F), temporal (T), parietal (P) and occipital (O) lobes are represented by the corresponding letter. Odd (1, 3, 5, 7) and even (2, 4, 6, 8) numbers refer to the left and right hemispheres, respectively. The closer the position is to the midline, the smaller the number. A 'z' refers to an electrode placed on the midline.