As indicated earlier, the International Classification divides all seizures into two types—generalized, in which the clinical and EEG manifestations indicate bilateral and diffuse cerebral cortical involvement from the onset, and focal, in which the seizure is often the product of a demonstrable focal lesion or EEG abnormality in some part of the cerebral cortex. The manifestations of focal seizures reflect the locale of the lesion. In the past focal seizures were classified by whether consciousness was retained (partial) or impaired (complex), but are now subsumed under the term “dyscognitive” if awareness is altered. Focal seizures with sensory or motor features at the onset most often arise from foci in the sensorimotor cortex. Those with impairment of consciousness, which occurs in many forms, most often have their focus in the limbic and autonomic areas or in the temporal lobe, but a frontal localization is also known. Table 15-2 lists the common sites of the lesions and the corresponding types of seizures.
Relatively few focal seizures can be localized precisely from clinical data alone. However, when combined with scalp and intracranial EEG recording and MRI, the localization is reasonably accurate.
Focal motor seizures are attributable to a discharging lesion of the frontal lobe. A common type, originating in the supplementary motor area, takes the form of a turning movement of the head and eyes to the side opposite the irritative focus, often associated with a tonic extension of limbs, also on the side contralateral to the affected hemisphere. This may constitute the entire seizure, or it may be followed by generalized clonic movements. The extension of the limbs may occur just before or simultaneously with loss of consciousness but a lesion in one frontal lobe may give rise to a major generalized convulsion without an initial turning of the head and eyes. It has been postulated that, if there is loss of consciousness, it is the result of a rapid spread of the discharge from the frontal lobe to integrating centers in the thalamic or high midbrain reticular formation.
One form of focal frontal convulsion is the Jacksonian motor seizure, which begins with tonic and then clonic contraction of the fingers of one hand, the face on one side, or the muscles of one foot. Sometimes a series of clonic movements of increasing frequency build up to a tonic contraction. The characteristic feature is that the movements spread (march) from the part first affected to other contiguous muscles on the same side of the body. In its typical form, the seizure spreads from the hand, up the arm, to the face, and down the leg; or if the first movement is in the foot, the seizure marches up the leg, down the arm, and to the face, usually in a matter of 20 to 30 s. Rarely, the first muscular contraction is in the abdomen, thorax, or neck. In some cases, the one-sided seizure activity is followed by turning of the head and eyes to the convulsing side, occasionally to the opposite side, and then by a generalized seizure with loss of consciousness. Consciousness is not lost if the sensorimotor symptoms remain confined to one side.
The frontal lobe, being so large, can give rise to numerous forms of seizure including the typical Jacksonian type described above, but also adversive (contralateral turning of the body or of a body part), speech arrest, frontal, absence types, and a number of unusual disorders related to discharges from the supplementary motor area including hyperkinetic and postural tonic varieties. In practice, it is often difficult to distinguish such seizures from parasomnic (sleep related) events (see Chap. 18).