Intracranial investigation of piriform cortex epilepsy during odor presentation

ABSTRACT The piriform cortex (PC) is part of the olfactory system, principally receiving input from the lateral olfactory tract and projecting to downstream components of the olfactory network, including the amygdala. Based on preclinical studies, PC is vulnerable to injury and can be easily kindled as an onset site for seizures. While the role of PC in human epilepsy has been studied indirectly and the subject of speculation, cases of demonstrated PC seizure onset from direct intracranial recording are rare. We present a pediatric patient with drug-resistant focal reflex epilepsy and right mesial temporal sclerosis with habitual seizures triggered by coconut aroma. The patient underwent stereoelectroencephalography with implantation of olfactory cortices including PC, through which we identified PC seizure onset, mapped high-frequency activity associated with presentation of olfactory stimuli and performance on cognitive tasks, and reproduced habitual seizures via cortical stimulation of PC. Coconut odor did not trigger seizures in our work with the patient. Surgical workup resulted in resection of the patient’s right amygdala, PC, and mesial temporal pole, following which she has been seizure free for 20 months without functional decline in cognition or smell. Histological findings from resected tissue showed astrogliosis and subpial gliosis.


Introduction
The human piriform cortex (PC) is a phylogenetically ancient three-layered allocortex (Vaughan & Jackson, 2014) and divided into frontal and temporal subregions (Lane et al., 2020).The PC is a key component of the olfactory system (Gottfried & Zald, 2005) receiving input from the olfactory bulb and anterior olfactory nucleus, with principal efferent connections to the lateral amygdala nucleus, entorhinal cortex, orbitofrontal cortex, and mediodorsal nucleus of the thalamus (Gottfried & Zald, 2005;Ray & Price, 1993;Zhou et al., 2019) and is involved in feedforward and feedback inhibitory processes (Suzuki & Bekkers, 2012).
Piriform cortex is vulnerable to excitotoxic injury and can be a seizure onset site in preclinical models.In humans, vulnerability of PC is seen in the context of status epilepticus (Vaughan & Jackson, 2014).Olfactory dysfunction in medial temporal lobe epilepsy is suggestive of injury to olfactory networks and highlights the effect of mesial temporal lobe (MTL) structures, especially the amygdala, and its impact and connectivity to the piriform area and olfactory cortices (Vaughan & Jackson, 2014), which may also account for some olfactory auras (Chen et al., 2003;Taşcı et al., 2021).Temporal lobectomy and amygdalectomy are used to treat drug-resistant MTL epilepsy (Hamasaki et al., 2014) and the likelihood of seizure freedom increases when PC is included (Borger et al., 2020;Galovic et al., 2019) and with greater amount of PC destroyed (Hwang et al., 2022; M. J. Kim et al., 2022).Targeting PC for therapeutic seizure intervention using deep brain stimulation is also underway (Kurada et al., 2020;Young et al., 2018).
Despite PC's vulnerability to seizures, few studies have investigated its role as a seizure onset zone in humans (Vaughan & Jackson, 2014) and interest in the role of the PC in human epilepsy has increased relatively recently.Most available research on PC has been animal focused but with emerging neuroimaging techniques there is a growing opportunity to investigate human PC.Most existing neuroimaging studies examining epilepsy involving human PC have used extracranial methods (Vaughan & Jackson, 2014) and intracranial studies are rare.In one such study, Vaughan and Jackson (2014) reported on a 37-year-old female patient undergoing intracranial EEG monitoring for drug-resistant epilepsy that included a depth electrode in frontal PC.Electrode recordings showed subclinical seizure activity in PC described as 1-to-2-minute runs of rhythmic sharp waves.Recordings during the patient's habitual clinical seizure captured attenuation and gamma frequencies in PC.Surgical resection included destruction of right temporal lobe structures, including frontal and temporal PC, but was incomplete due to proximity to the middle cerebral artery.Surgery resulted in reduced seizure burden without seizure freedom.
Our study adds to available research examining epilepsy involving human PC.We present novel data on a pediatric patient with drug-resistant, focal epilepsy and right mesial temporal sclerosis who underwent intracranial monitoring and mapping that included PC using stereoencephalography (SEEG) in our epilepsy center.

Case report
The patient was a 17-year-old, right-handed female at the time of presentation to our epilepsy center.She had a history of acute disseminating encephalomyelitis (ADEM) in the setting of mononucleosis at age 14 years old that presumably led to seizures and right hippocampal sclerosis.Seizures were treated unsuccessfully with oxcarbazepine.At age 16, she underwent laser ablation of right mesial temporal structures at an outside hospital (OSH) (Figure 1) without notable improvement in seizure control.Medical history was otherwise noncontributory.During repeated inpatient stays at OSH for epilepsy-related treatment, her mother used a strong-smelling coconut shampoo.Subsequently, patient's aura included coconut smell and a metallic taste in her mouth.
During a surgical workup at our center, coconut odor was reported to be a seizure trigger.A brain MRI at that time showed remnant amygdala from her initial laser ablation.A video EEG demonstrated interictal discharges in the right temporal region and one electrographic seizure with onset from the right temporal lobe.The patient underwent SEEG implantation targeting remnant amygdala, temporal pole, prefrontal cortex, anterior insula, anterior cingulate, orbitofrontal cortex, and frontal and temporal PC (Figure 1).Interictal SEEG showed frequent discharges with superimposed fast activity in PC, temporal greater than frontal, and remnant amygdala.Numerous subclinical electrographic seizures were recorded from the temporal PC with propagation into frontal PC and amygdala.A single habitual seizure was elicited during cortical stimulation of the most mesial contacts of the temporal PC (5-s train at 50 Hz, 0.3 msec pulse width, 3 mA intensity) showing propagation across various temporal and frontal lobe structures.Electrocortical stimulation of PC did not affect performance on cognitive mapping tasks.
The patient also underwent high-frequency activity (HFA) mapping via SEEG with presentation of a range of odors including orange, banana, vanilla, and coconut (Figure 2).Each odor produced an independent and highly reproducible burst ranging from 40 to 150 Hz activity, often with embedded spikes (see Figure 2 for specific frequency bands).Habituation to odors was observed, with activity becoming briefer and lower in amplitude over the course of mapping.Stimulation of the frontal piriform and remnant amygdala elicited the metallic taste described in the patient's typical aura.Based on the findings, she underwent an uncomplicated resection that included remnant right amygdala, temporal and frontal PC, and temporal pole.Histology showed widespread astrogliosis and subpial gliosis.There were no reported changes in cognition or olfaction following surgery.She has been seizure-free postoperatively (approximately 2 years) (Engel Class 1) and was weaned off oxcarbazepine.She has since graduated high school and is pursuing college.

Discussion
Here, we present a case with validated PC seizure onset along with passive mapping of the effects of an olfactory stimulus.Few intracranial studies exist examining seizures originating from human PC.The PC's primary role is processing olfactory information, and its contribution to cognitive function has yet to be elucidated.To our knowledge, ours is the first published study to document human focal epilepsy originating from PC, to map HFA associated with olfaction and cognition, and to use cortical stimulation to reproduce habitual seizures and riskstratify functional deficit from directly within PC.Although Vaughan and Jackson (2014) identified subclinical and habitual seizure activity involving PC in their adult patient with drugresistant epilepsy, they did not test piriform reactivity to odors nor produce a PC seizure.Findings from our study of increased HFA in response to odor presentation are consistent with results from Vaughan and Jackson's (2014) case study, and recent research showing theta, beta, and gamma HFA during odor presentation in groups of patients with drug-resistant epilepsy (Jiang et al., 2017).Although seizures have been triggered by odors as reported by our patient (Vaughan & Jackson, 2014), coconut presentation did not result in seizure during our work with her.
Like Vaughan and Jackson's (2014) case study, findings from our patient's surgical workup resulted in partial resection of her frontal and temporal PC.In contrast to their patient, however, our patient has been seizure free since surgery, which is likely because her resection included all targeted epileptogenic tissue.These findings are consistent with previous research indicating surgical destruction of PC in patients with drug-resistant MTL epilepsy is associated with seizure freedom (Borger et al., 2020;Galovic et al., 2019;Hwang et al., 2022; M. J. Kim et al., 2022).
Histological findings from our patient's resected tissue showed astrogliosis and subpial gliosis.These findings support existing research showing reactive gliosis, including hypertrophy and proliferation, associated with drug-resistant seizures (Khurgel & Ivy, 1996).Repeated seizure activity leads to pathological structural changes in glial cells that alter their function, including type and amount of neuroproteins produced (e.g., glial fibrillary acidic protein), thereby increasing immunoreactivity, seizure susceptibility, and resistance to antiseizure medication (Khurgel & Ivy, 1996; Y. J. Kim et al., 2015).
Our patient did not experience postsurgical functional decline in cognition or olfaction, completed high school without issue, and is pursuing college, although preserved olfaction may be due to the patient's intact left-sided PC.No cognitive decline was expected based on findings from intracranial cognitive mapping that showed no disruption in performance during task completion.
As has been reported in the previous research, our patient quickly habituated to odors, likely out of adaptive necessity to rapidly detect, and respond to environmental changes (Pellegrino et al., 2017).Regarding the cause of our patient's seizures, olfactory auras are frequently found in patients with a history of encephalitis (Lehrner et al., 1997) and she was diagnosed with ADEM at age 14 years old, shortly before the onset of seizures.This association is further supported by research indicating human PC is highly vulnerable to injury (Cheng et al., 2020;Sakurai et al., 2018;Vaughan & Jackson, 2014;Vismer et al., 2015;Young et al., 2019).

Conclusion
Our study used SEEG to monitor and map human PC epilepsy and showed PC HFA associated with odor presentation and induced a typical seizure using cortical stimulation to PC. Cognitive mapping suggested PC cortex could be resected without affecting patient function.The results guided surgical resection that included PC and resulted in seizure freedom without olfactory or cognitive changes.

Figure 1 .
Figure 1.MRI reconstruction of most mesial contacts of right frontal (D) and temporal (E) piriform electrodes marked in green.