Simple sensory seizures represent challenging clinical phenomena. Neurologists readily accept that a patient’s sensory symptoms prior to the appearance of motor symptoms or alteration of consciousness represent part of the patient’s clinical seizure semiology and provide valuable localizing information. A challenge appears when a patient has only simple partial sensory seizures. Simple partial seizures are a relatively common seizure presentation among the childhood epilepsies, but motor seizures are predominant.⁴⁷ The paucity of psychic or sensory symptoms may reflect the child’s inability to describe internal experiences in a way that is easily interpretable to adult observers. Since simple sensory seizures have no overt ictal behavior, the subjective description of the seizure symptomatology is the only clue that can lead to a proper clinical evaluation and treatment. Some patients react to sensory seizures in a stereotypic fashion. For example, a painful simple partial sensory seizure of the hand may result in hand rubbing or vocalization about the pain. Occasionally, physicians encounter patients who have simple partial seizures for many years prior to the appearance of more clinically apparent seizures. When patients with pure sensory seizures also have psychiatric complaints, the chances for misdiagnosis or delay of diagnosis are high.¹²¹

Simple sensory seizures were originally underrecognized as epilepsy.³⁵ While auras were known in ancient times as an event preceding seizures,¹²² it was not until the early 19th century that partial seizures were clearly related to localized brain disease. Fritsch and Hitzig described localization of function in 1870, which led to modern concepts of cortical specification.³⁸

Jackson also promoted the hypothesis that abnormal electrical activity in a localized region of cortex could cause symptoms such as those seen in epilepsy.^51,52 In the early 1900s, the work by Brodmann¹⁸ and other neuroanatomists showed that the cerebral cortex had regional histologic variations often corresponding to different cerebral functions. Penfield, Jasper, and others made major contributions to mapping the localization of cerebral function and defining motor and sensory areas of the brain, as well as areas without obvious effect on sensory, motor, or speech activity.^83,85

Sensory seizures were defined by Penfield and Jasper separately from simple seizures with autonomic symptoms or psychic symptoms:⁸⁵

“Sensory seizures. Discharge in an area of sensory representation produces a sensation that may serve as a warning (aura) of a motor or psychical seizure. Actually, the sensation constitutes, in itself, a seizure. Under this heading we have taken up the sensory attacks that involve the trunk, head, and extremities (somatosensory), and the special senses (visual, auditory, vertiginous, gustatory, and olfactory).

Penfield and Jasper’s classification of simple partial sensory seizures has withstood the test of time since the international classification²⁶ of simple partial seizures with somatosensory or special sensory symptoms is nearly identical to the original description except for lack of description of gustatory seizures (Table 1). It is unlikely that another sensory modality will be discovered and change the classification further.

Penfield and Jasper recognized the difficulty of separating a simple sensation from a more complicated hallucination, for example, separating simple visual hallucinations from more complicated formed illusions. Occasionally one finds more complicated hallucinations with intraoperative stimulation, but more reliably localizing are the simple hallucinations of rather elementary sensory perceptions. A conscious patient may attach many complex descriptions to simple
hallucinations, and since we observers have nothing to observe other than the patient’s subjective description, sometimes there may be a fine line between a classification of simple partial sensory seizure and a simple partial psychic seizure with prominent sensory complaints.

Ideally, understanding the epidemiology of simple partial seizures would require evaluation of a large population in which patients with epilepsy are identified accurately and their seizures and epilepsies are classified based on a thorough clinical and laboratory investigation. Such studies are usually based on patients referred to specialized epilepsy centers. Focal epilepsy occurs in approximately 60% of patients with epilepsy, and 10% to 21% of these patients have simple partial seizures alone.⁴⁰ Detection of simple partial seizures can be difficult. Some patients with simple partial sensory seizures may be undiagnosed or unaware that their symptom represents epilepsy unless the seizure progresses into a more disabling seizure type. In patients with obvious epilepsy and more severe types of seizures, only a careful history will elicit remote sensory complaints that may no longer occur, as in several cases reported by Williamson et al.¹²³

Isolated simple partial sensory seizures are uncommon in patients referred to major epilepsy centers. Among the focal epilepsies, temporal lobe and frontal lobe foci predominate in surgical series.¹¹⁹ Parietal and occipital cases are rare because those epilepsies are either rare, rarely intractable, or rarely brought to surgery. For example, occipital epilepsy was identified in only 12 of 502 nontumoral epilepsy cases by Bidzinski et al.¹² Williamson et al. reviewed reports of occipital lobe epilepsy in the literature and found that <2% of focal epilepsy is occipital.¹²⁴ They found only sporadic reports when they reviewed parietal lobe epilepsy.¹²³

These surgical data are typically grouped by lobe, not seizure type. It is likely that the epidemiology of simple partial sensory seizures differs from the lobar distribution of surgical cases. A review of the initial signs of seizure onset in data adapted from Penfield and Kristiansen showed that in 222 patients, 55 had somatosensory auras, 11 had visual auras, three had auditory auras, one had an olfactory aura, and two had gustatory auras.⁶² Penfield and Perot noted visual hallucinations in 41 of 1,132 (3.6%) patients with focal epilepsy; 21 had only visual hallucinations.⁸⁷ Mauguiere and Courjon found somatosensory epilepsy in 127 of 8,938 (1.4%) patients with epilepsy when evaluated after age 3 years.⁷² Young and Blume reported painful seizures in 24 of 858 (3%) epileptic patients¹²⁹ and Hausser-Hauw and Bancaud noted that 30 of 718 (4%) intractable epileptics had gustatory hallucinations.⁴³ West and Doty reviewed olfactory auras in epilepsy and concluded that prevalence is unknown but published estimates ranged from <1% to >30% depending on the epilepsy syndrome and pathology.¹²²

The advent of magnetic resonance imaging (MRI) technology has further complicated matters. Patients with lesions identified in known sensory cortical regions may not volunteer information on sensory symptoms until thoroughly interrogated about an aura. At times, only with inpatient video-electroencephalography (V-EEG) and MRI can sensory symptoms be suspected and assessed. This is especially true in patients with sensory symptomatology at seizure onset that progresses to loss of consciousness and amnesia for the aura. These patients often have more intense seizures and EEG changes are usually bilateral.¹⁰³ Additionally, sensory symptoms may result from spread of a seizure from an unexpected location; for example, many patients with supplementary motor seizures complain of a somatosensory aura even when lesions are not near the sensory cortex.¹⁰⁶ Gustatory hallucinations may also represent seizure spread.⁴³ Visual hallucinations have been reported with frontal lesions,¹⁰² as have jacksonian sensory seizures from a prefrontal lesion.¹¹⁸ Thus, at this time, there is still need for adequate epidemiologic studies of localization-related epilepsies and partial sensory seizures using modern technology and appropriate clinical information.

Understanding neocortical anatomy and regional interconnections has led to a better understanding of seizure semiology and evolution. The neocortex is a repository for all primary sensory input for somatosensory, visual, auditory, and gustatory senses, while olfaction mostly utilizes portions of the limbic system and is the only special sense without thalamic relays.²¹ Within the cerebral cortex, sensory functions are not so discreetly organized that there are sharp boundaries; for example, the sensory responses obtained by Penfield and Rasmussen⁸⁸ and Uematsu et al.^116,117 showed that sensory responses could be recorded from the precentral gyrus (see reference 85, p. 58, Fig III-12). Motor responses show similar dispersion. Penfield reported that removal of the precentral gyrus did not prevent motor responses from the postcentral gyrus, suggesting that activation of the postcentral gyrus alone could result in movements during seizures.

The sensory homunculus (see reference 85, p. 70, Fig III-15) must be considered as an artist’s schematic diagram of data obtained by Penfield and Jasper on many patients; it is not entirely accurate for each patient. However, the homunculus does show significant organization of the cerebral cortex in an anatomically meaningful way (see reference 85, p. 71, Fig. III-17).

Some body parts (e.g., the tongue and hand) have large cortical sensory areas. Picard and Olivier have extensively reviewed the anatomy of the sensory tongue cortex in humans. The language-dominant hemisphere had a larger sensory cortical tongue representation and the tip of the tongue had more extensive representation than the middle or back portions of the tongue.⁸⁹ Most cortical representation was contralateral to somatic localization, but ipsilateral and bilateral responses to stimulation were reported, suggesting possible stimulation of the secondary sensory area, located at the base of the perirolandic cortex.

Penfield and Rasmussen⁸⁸ found a human second sensory area in the region of the termination of the motor strip in the frontoparietal operculum (see reference 85, p. 79, Fig III-21). When stimulated, the second sensory area on the superior bank of the frontoparietal sylvian fissure produces bilateral, contralateral, or ipsilateral sensations, often of the perioral regions (see reference 85, p. 79, Fig III-21). Ipsilateral inputs are less numerous as shown by Adrian in cats.¹ Lüders et al. recorded evoked potentials from the second sensory area that were of lower amplitude than primary sensory potentials.⁶⁷

Other primary sensory cortices show highly organized anatomy; for example, the visual cortex deep within the calcarine fissure is organized so that the occipital pole is innervated polysynaptically by the macula and therefore is activated by the central visual field, whereas more rostral portions of the calcarine cortex contain representations of more peripheral visual fields.

The auditory cortex in the Heschl gyrus of the temporal lobe is tonotopically organized as shown by evoked potential mapping of longer-latency responses with magnetoencephal- ography.^{82,115,119,120} Vestibular cortex is also in the superior temporal gyrus rostral to the auditory cortex, but stimulation does not produce anatomically specific symptoms, just mild vertigo.⁸⁵

Taste is represented in the parietal operculum and possibly near the insula,⁸⁵ though Hausser-Hauw and Bancaud’s detailed depth electrode studies point primarily to the parietal operculum.⁴³ Olfaction initially involves pathways from the olfactory epithelium through the cribriform plate to the olfactory bulbs, olfactory tracts, and lateral stria. These areas project to the anterior perforated space, prepiriform cortex, lateral olfactory gyrus, periamygdaloid cortex, entorhinal cortex, amygdala, septal nuclei, and hypothalamus.¹²² Thus, most olfactory centers are located in the mesial temporal region and adjacent structures.

While a description of epileptogenesis is beyond the scope of this chapter, clearly any structural lesion, whether seen by MRI or not, involving a sensory cortex or adjacent area may provide the necessary and sufficient changes for chronic localization-related epilepsy to develop. The concept of the epileptogenic zone, as interpreted by Penfield and Jasper⁸⁵ and Ajmone-Marsan⁴ and reviewed by Lüders and Awad,⁶⁵ implies that the locations in the cortex where symptoms are produced, where seizures are generated, where interictal spikes occur, and of any structural lesions all may contribute to epileptogenesis or to the symptom expression during seizures. It is certainly possible for sensory cortex to become activated late in a seizure and thus produce symptoms after the actual EEG onset in a silent cortical area. To experience somatosensory, visual, or auditory symptoms, white matter pathways, association cortices, and brain structures regulating attention and consciousness all play a role.

Epileptogenesis producing sensory seizures and focal epilepsy may occur without brain lesion or injury from peripheral nervous system injury. Spiller et al. reported three cases with soft tissue injuries to the hand where sensory seizures developed within months in the injured limb and all cases eventually had secondarily generalized tonic–clonic seizures. The authors postulated that the injury resulted in cortical reorganization that was epileptogenic.¹⁰⁹

Simple sensory seizures show a wide variety of phenomenology and represent some of the most interesting seizures reported. Patients can have prolonged symptoms⁷⁰ and can relate elaborate descriptions of their seizure. Since sensory cortices are located in adjacent lobes, it is not surprising that multimodality sensory seizures are frequently reported in addition to sensory seizures from one modality.⁷²

Seizures of the primary sensory cortex typically produce contralateral positive or negative symptomatology. Unilateral symptoms thought to originate in or near the contralateral postcentral gyrus include tingling, numbness, sense of movement, desire to move, somatic pain, heat or cold, electric shock sensations, agnosia for a body part, and phantom sensations.^{60,72,85,99,123,129} The hand and fingers are most often involved initially.⁷² Symptoms may be stationary or have a sensory march. Somatosensory seizures may be interpreted as motor activity by the patient, even when there is no visible movement seen. This can often lead to misinterpretation of seizure localization by a physician taking the history. Cephalic pain, occasionally migrainous in nature,¹⁹ even if unilateral, may not have localizing value, while abdominal pain usually originates from the temporal lobe.¹²⁹ Genital pain probably originates at the mesial parietal termination of the sensory strip and is not necessarily associated with orgasmic seizures.^98,110,128 Attacks of limb agnosia (sudden loss of sensation for a body part) and phantom limb sensations (sense that the limb is in a position that is not the true position) probably originate in the posterior parietal region.^41,72,99 Ajmone-Marsan gives Foerster credit for pointing out that a postcentral-area seizure can be distinguished from the precentral seizure since motor symptoms stop earlier when the seizure originates in the postcentral gyrus and that the convulsing muscles are often more limited in distribution than when the primary motor cortex is seizing. Additionally, when a motor seizure occurs after activation of the sensory cortex, a tremor may be seen before true clonic spasms, if clonic spasms occur.⁴ Russell and Whitty observed that somatosensory auras often spread quickly to involve at least the entire limb and that prolonged sensory seizures, comparable to the epilepsia partialis continua of motor seizures, were not observed in their 85 cases. Motor seizures rarely progress to a simple somatosensory seizure, while the reverse is common. Sensory seizures often end abruptly, unlike the gradual cessation typical of focal motor attacks.⁹⁹

Seizures from the second sensory area may produce ipsilateral or bilateral symptoms that are identical in character to symptoms occurring in the primary sensory cortex (e.g., numbness and tingling). Affected body parts can be diffuse or axial but often symptoms localize to the fingertips, feet, lips, or tongue.^{2,6,11,13,61,85,93,123} Second sensory-area seizures can arise from the frontoparietal operculum or the inferior parietal lobule.¹²⁶ It is also possible to have ipsilateral sensations with seizures originating near the supplementary motor area since there may be a corresponding supplementary sensory region adjacent to the primary sensory foot area.⁸⁵ Arseni and Maretsis’ case 3 may have had ipsilateral seizures from this location.⁶

Somatosensory auras have been reported in temporal lobe epilepsy, usually with bilateral or unilateral tingling. Pain and numbness were also noted in Erickson et al.’s report and most symptoms were in the limbs but could involve the head or trunk.³⁶