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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 3  |  Issue : 1  |  Page : 31-43

Intracranial monitoring and resective epilepsy surgery: Preoperative predictors of nonprogression to therapeutic surgery and long-term outcomes


Department of Neurosurgery, Rush University, Chicago, IL 60612, USA

Date of Web Publication7-Jul-2017

Correspondence Address:
Steven M Falowski
Department of Neurosurgery, St. Luke's University Health Network, Bethlehem, PA
USA
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2455-5568.209856

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  Abstract 


Background: Resective surgery is efficacious in treating intractable epilepsy when an epileptogenic focus is accurately identified. Invasive monitoring is crucial when noninvasive studies are indeterminate or nonconcordant.
Objective: Some patients do not progress to definitive surgery, following invasive monitoring; we aim to elucidate predictive characteristics and determine long-term outcome.
Methods: Characteristics of patients in the Institutional Review Board-approved Rush University Surgical Epilepsy database who underwent invasive electroencephalography monitoring for were retrospectively analyzed to determine premonitoring factors correlating with nonprogression to definitive surgery.
Results: Among 127 patients analyzed, 112 underwent resective surgery and 15 did not. Seizure freedom (Engel Class I) was realized in 63% of the surgery group and 7% of the nonsurgical group. The most common reason for not undergoing resective surgery was indeterminate epileptogenic focus localization. Factors correlating with nonprogression to surgery included bilateral pathology (P = 0.023), and factors exhibiting a trend include location, with frontal and parietal having lowest operative rates, and female gender.
Conclusions: In all patient categories studied, subgroups that progressed to therapeutic surgery had better outcomes than those that did not. Lengthening invasive monitoring duration, employing newer diagnostic technologies for better seizure localization, and optimization of patient selection should be explored to improve overall outcomes.
The following core competencies are addressed in this article: Medical knowledge, Patient care,
Practice-based learning and improvement, Systems-based practice.

Keywords: Brain mapping, electroencephalography, epilepsy, epilepsy surgery, hippocampus, intractable, seizure, subdural electrodes, temporal lobe epilepsy, temporal lobectomy


How to cite this article:
DiLorenzo DJ, Falowski SM, Wallace DJ, Corley JA, Fogg LF, Smith MC, Rossi MA, Balabanov AJ, Byrne RW. Intracranial monitoring and resective epilepsy surgery: Preoperative predictors of nonprogression to therapeutic surgery and long-term outcomes. Int J Acad Med 2017;3:31-43

How to cite this URL:
DiLorenzo DJ, Falowski SM, Wallace DJ, Corley JA, Fogg LF, Smith MC, Rossi MA, Balabanov AJ, Byrne RW. Intracranial monitoring and resective epilepsy surgery: Preoperative predictors of nonprogression to therapeutic surgery and long-term outcomes. Int J Acad Med [serial online] 2017 [cited 2021 Jan 25];3:31-43. Available from: https://www.ijam-web.org/text.asp?2017/3/1/31/209856




  Introduction Top


Epilepsy is a potentially debilitating condition affecting nearly 0.5% of adults,[1] leading to an estimated 150,000 new patients in the United States each year.[2] Medically intractable epilepsy is imprecisely defined but often determined when seizures persist for 2 years despite adequate monotherapy of two antiepileptic drugs (AEDs) with or without one trial of polytherapy.[3],[4] Up to 40% of patients will have persistent epilepsy despite medical treatment.[5] Recent studies of patients with new-onset seizures have shown that only 64% have seizure freedom by the time they try their third AED,[5] and quality of life can be decreased with polytherapy.[6] This decreases dramatically with each additional AED or combination. In deciding to pursue surgical intervention, the physicians and patients must weigh the benefits and risks of surgical therapy versus the potential long-term effects of continued seizures with further nonsurgical therapy.[7],[8]

Surgical intervention is an important treatment strategy for patients with intractable epilepsy. Surgery has the potential to offer patients an improved quality of life, as well as the chance of obtaining seizure freedom.[9],[10] The identification and localization of the seizure focus is the key to successful outcomes. However, an epileptogenic zone is not always easily determined by presentation, electroencephalography (EEG), and imaging.[8]

The decision to pursue surgical therapy involves one or two branch points, depending on the degree of source localization achieved with noninvasive EEG monitoring. If noninvasive monitoring does not provide sufficient localization for surgery, invasive monitoring may be pursued.

Predictors of good surgical outcome include concordance of preoperative imaging and EEG localization as well as absence of a need for intracranial monitoring for preoperative localization.[11] For cases in which noninvasive monitoring is insufficient for localization, invasive (Phase 2) monitoring is pursued and is an essential component in localization and in determining whether to proceed to resective surgery and what surgical strategy to pursue.

Good surgical and clinical results have been observed from resective surgery, following invasive electrode recording.[12],[13],[14],[15],[16],[17],[18],[19] A small subset of patients are deemed either poor surgical candidates or nonoperative after invasive electrode monitoring and do not proceed with a definitive surgery. Nonprogression to surgery is most commonly due to failure to in localization of a seizure focus. In this study, we attempt to determine predictors of nonprogression to surgery as well as to determine and compare long-term outcomes of the subgroups which do and which do not progress to definitive surgical treatment after invasive electrode monitoring.


  Methods Top


Study group

This is a Rush University Institutional Review Board-approved retrospective analysis using a prospectively maintained institutional epilepsy surgery database. There were 127 patients who underwent invasive EEG monitoring with subdural electrodes placed by a single surgeon between 1997 and 2010. The sample group comprised 76 males and 51 females. Ages ranged from 3 to 63 years, with a mean age of 30 years. Follow-up ranged from 2 to 11 years, with an average follow-up of 4.6 years. One hundred and twelve patients progressed to undergo definitive therapeutic resective surgery following electrode implantation while 15 patients did not progress to definitive surgery.

Surgical procedures

All surgeries were performed at a single institution (Rush University) by a single surgeon (RWB). Evaluation of each patient included case discussion at a multidisciplinary conference, in which it was agreed that the patient was medically intractable and a surgical candidate. Patients underwent video-EEG monitoring as well as appropriate imaging. Imaging in most cases included a 1.5 Tesla magnetic resonance imaging (MRI), and if negative, a 3 Tesla MRI for selected cases. Additional imaging, when indicated, included positron emission tomography, single photon emission computed tomography (SPECT), and subtraction ictal SPECT with coregistration on MRI. All patients underwent an intracarotid amobarbital procedure (WADA test) and neuropsychological evaluation before surgery. Intracranial electrodes were inserted and patients underwent inpatient EEG monitoring. Following the monitoring period, surgical eligibility and planning determinations were made. Patient surgeries included 77 temporal lobe resections and 35 extratemporal resections. Vagal nerve stimulator placement was discussed in the 15 patients not undergoing definitive surgery. Corpus callosotomies were excluded secondary to being an accepted palliative procedure for epilepsy. There was inclusion of subpial transection when in conjunction with other surgical modalities.

Analysis

Potentially relevant demographic features and other factors that may influence progression to resective epilepsy surgery were recorded including age, sex, laterality of monitoring, location of monitoring, and number of electrode contacts. Patient seizure frequency at the last follow-up was recorded as a marker for long-term epilepsy treatment. Patients were evaluated based on the modified Engel Seizure Outcome Grading Scale (I: Seizure free; II: Rare or nocturnal seizures within 1 year of current follow-up and less than three seizures per year; III: Significant reduction in seizure frequency or severity >90%; IV: <90% reduction in seizure frequency).


  Results Top


[Table 1] lists the demographics of those patients evaluated. The sample group comprised 127 patients, of whom 112 patients did and 15 patients did not undergo definitive surgery, following electrode implantation and monitoring. Mean age was 30 years in each of these two groups. There was a larger age range in those undergoing surgery. Among the patients undergoing surgery after electrode implantation, 62.7% of the patients had a modified Engel Class I outcome at follow-up while only one patient (6.7%) had a modified Engel Class I outcome in the group not undergoing definitive surgery.
Table 1: Patient demographics

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[Table 2] presents characteristics of the 15 patients who did not undergo therapeutic surgery, following invasive monitoring with implanted electrodes. Of these patients, one patient (7%) received a vagal nerve stimulator became seizure free, one patient (7%) realized significant improvement, eight patients (53%) had no change in seizure frequency, and five patients (33%) experienced worsening of their seizure frequency. This was observed over an average follow-up of 4.7 years. In the one patient (7%) experiencing an Engel Class I outcome, the seizure frequency was two seizures per month before electrode implantation. Significant reductions in seizure frequency correlated with changes in AED treatment in the two patients with improvement.
Table 2: Characteristics of patients not undergoing surgery following monitoring with implanted electrodes

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Among the 15 patients not undergoing surgery following electrode implantation, the reasons that their treatment did not progress to surgery included (1) indeterminate localization of the seizure focus in eight patients (53%), (2) multiple epileptogenic foci in four patients (27%), (3) epileptogenic foci in eloquent cortex in two patients (13%), and (4) deep focus in one patient (7%).

Four patients underwent placement of a vagal nerve stimulator and two patients also underwent placement of a responsive neurostimulator (NeuroPace), following electrode implantation and monitoring. Two other patients had a vagal nerve stimulator placed before electrode implantation for monitoring. One of the patients undergoing vagal nerve stimulator placement following electrode implantation and monitoring demonstrated some improvement achieving an Engels Class III outcome.

Among the surgical resection group, ten did not have follow-up sufficient to determine Engel Class; hence, these percentages are calculated from the 102 patients, for which this follow-up classification is available.

Overall outcomes: Surgical versus nonsurgical management

[Table 3] displays a detailed comparison of the outcomes for the entirety of each of the surgical resection and the nonresective group. In the surgical group, 62.7% of patients (64/102) achieved seizure freedom (Engel Class I), 22.5% (23/102) realized significant reduction (Engel Class II and III), and 14.7% (15/102) had no improvement (Engel Class IV). In the group not undergoing definitive resection surgery (the “nonsurgical” group), the outcomes sharply contrasted those of the surgical group, comprising 1 (7%), 1 (7%), and 13 (87%) patients who realized Engel Class I, Class II or III, and Class IV outcomes, respectively. Applying Chi-square analysis to these six numbers with 2 degrees of freedom results in a P value of P< 0.005 (P = 0.000000007925148).
Table 3: Outcome following resective surgery vs. nonresective management for all patients

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Detailed analysis of candidate factors for prediction of progression to surgery

Progression to surgery and outcomes: Laterality

[Table 4.1] and [Table 4.2] shows the progression to surgery characteristics for each laterality subgroup, and [Table 4.3], [Table 4.4], and [Table 4.5] shows the clinical outcome data for the patients partitioned into subgroups by laterality.


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Right-sided versus left-sided versus bilateral monitoring

In [Table 4.1], patient numbers and proportions are presented for surgical and nonsurgical groups segmented into the three laterality subgroups comprising those with electrodes implanted the right side, left side, and bilaterally. In this patient sample, progression to surgery was slightly higher for patients who underwent left-sided invasive monitoring than right-sided invasive monitoring, and both of these unilateral monitoring groups have a higher rate of progression to surgery than those patients who underwent bilateral monitoring. As shown in [Table 4.1] with three categories for laterality, Chi-square analysis produces a P = 0.07, consistent with a trend.

Unilateral versus bilateral monitoring

In [Table 4.2], the analysis described above for [Table 4.1] is repeated with the left- and right-sided groups combined into unilateral groups and compared as unilateral versus bilateral laterality. In this analysis, 90.8% of unilateral monitoring patients progressed to surgery versus only 72.2% of patients undergoing bilateral invasive monitoring. Chi-square analysis for this grouping demonstrated statistical significance (P < 0.05, P = 0.023).

Left-sided monitoring

[Table 4.3] shows an outcome analysis of patients undergoing left-sided invasive monitoring. Among those undergoing subsequent therapeutic surgery, 44 patients in [Table 4.3]a comprising 71.0% in [Table 4.3]c achieved seizure freedom (Engel Class I), and only 5 (8.1%) patients realized no improvement. This contrasts dramatically with those six patients who did not undergo surgery, all of whom had no improvement. In this group of patients undergoing left-sided invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups was statistically significant (P < 0.05, P = 3.9 × 10−8).

Right-sided monitoring

[Table 4.4] shows an outcome analysis of patients undergoing right-sided invasive monitoring. Among those undergoing subsequent therapeutic surgery, 14 patients in [Table 4.4]a comprising 50.0% in [Table 4.4]c achieved seizure freedom (Engel Class I), and 7 (25.0%) patients realized no improvement. This is moderately better than the nonsurgical subgroup, in which 1 (25%) patient realized seizure freedom and 2 (50%) patients had no improvement. In this group of patients undergoing right-sided invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups did not reach statistical significance (P = 0.54).

Bilateral monitoring

[Table 4.5] shows an outcome analysis of patients undergoing bilateral invasive monitoring. Among those undergoing subsequent therapeutic surgery, six patients in [Table 4.5]a comprising 50.0% in [Table 4.5]c achieved seizure freedom (Engel Class I), and three patients (25.0%) realized no improvement. This is substantially better than the nonsurgical subgroup, in which no patients (0%) realized seizure freedom and all five patients (100%) had no improvement. In this group of patients undergoing bilateral invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups was statistically significant (P < 0.05, P = 0.0186).

Unilateral monitoring

[Table 4.6] shows an outcome analysis of patients undergoing unilateral (left or right sided) invasive monitoring. Among those undergoing subsequent therapeutic surgery, 58 patients in [Table 4.6]a comprising 64.4% in [Table 4.6]c achieved seizure freedom (Engel Class I), and 12 patients (13.3%) realized no improvement. This is dramatically better than the nonsurgical subgroup, in which one patient (10%) realized seizure freedom and eight patients (80%) had no improvement. In this group of patients undergoing unilateral invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups was statistically significant (P < 0.005, P = 3.4 × 10−6).

Unilateral versus bilateral monitoring

In [Table 4.7], an outcome analysis of patients who underwent invasive monitoring and progressed to resective surgery is shown with patients divided into groups, who underwent unilateral versus bilateral invasive monitoring. Among the ninety patients undergoing unilateral (right or left sided) monitoring, 58 patients in [Table 4.7]a comprising 64.4% in [Table 4.7]c achieved seizure freedom (Engel Class I), and 12 patients (13.3%) realized no improvement. This is similar to the 12 in patients who underwent bilateral (right and left sided) monitoring, 6 (50.0%) of whom realized seizure freedom and 3 (25.0%) of whom had no improvement. In this group of patients undergoing resection, Chi-square analysis of the differences in the outcomes of the temporal versus extratemporal subgroups did not reach statistical significance (P = 0.50).

Progression to surgery and outcomes: Anatomical location

[Table 5.1] shows the progression to surgery characteristics for each anatomical location monitored subgroup, and [Table 5.2], [Table 5.3], [Table 5.4], [Table 5.5], and [Table 5.6] shows the clinical outcome data for the patients partitioned into subgroups by location. These groupings are inclusive such that a patient with electrodes in two regions is included in both subgroups.


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In [Table 5.1], patient numbers and proportions are presented for surgical and nonsurgical groups segmented into the four location subgroups comprising those with electrodes implanted the frontal, temporal, parietal, and occipital lobes. As seen in [Table 5.1]b, progression to surgery rates were the highest for temporal and occipital locations (89.6% and 88.2%, respectively) and the lowest for frontal and parietal locations (74.5 and 70.0%, respectively). Chi-square analysis produces a P = 0.059, consistent with a trend.

Frontal lobe monitoring

[Table 5.2] shows an outcome analysis of patients undergoing frontal lobe invasive monitoring. Among those undergoing subsequent therapeutic surgery, 19 patients in [Table 5.2]a comprising 55.9% in [Table 5.2]c achieved seizure freedom (Engel Class I), and seven patients (8.1%) realized no improvement. This contrasts dramatically with the 14 patients who did not undergo surgery, 1 (7.1%) of whom realized seizure freedom and 12 (85.7%) of whom had no improvement. In this group of patients undergoing frontal lobe invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups was statistically significant (P < 0.001, P = 0.000143).

Temporal lobe monitoring

[Table 5.3] shows an outcome analysis of patients undergoing temporal lobe invasive monitoring. Among those undergoing subsequent therapeutic surgery, 52 patients in [Table 5.3]a comprising 65.0% in [Table 5.3]c achieved seizure freedom (Engel Class I), and nine patients (11.3%) realized no improvement. This contrasts dramatically with the ten patients who did not undergo surgery, none (0%) of whom realized seizure freedom and 9 (90.0%) of whom had no improvement. In this group of patients undergoing temporal lobe invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups was statistically significant (P < 0.001, P = 2.75 × 10−8).

Parietal lobe monitoring

[Table 5.4] shows an outcome analysis of patients undergoing parietal lobe invasive monitoring. Among those undergoing subsequent therapeutic surgery, four patients in [Table 5.4]a comprising 80.0% in [Table 5.4]c achieved seizure freedom (Engel Class I), and one patient (20.0%) realized no improvement. This contrasts dramatically with the three patients who did not undergo surgery, none (0%) of whom realized seizure freedom and all 3 (100.0%) of whom had no improvement. In this group of patients undergoing parietal lobe invasive monitoring, Chi-square analysis of the differences in the outcomes of the surgical versus nonsurgical subgroups was statistically significant (P < 0.05, P = 0.028).

Occipital lobe monitoring

[Table 5.5] shows an outcome analysis of patients undergoing occipital lobe invasive monitoring. Among those undergoing subsequent therapeutic surgery, eight patients in [Table 5.5]a comprising 61.5% in [Table 5.5]c achieved seizure freedom (Engel Class I), and two patients (15.4%) realized no improvement. This contrasts with the two patients who did not undergo surgery, none (0%) of whom realized seizure freedom and both (100.0%) of whom had no improvement. In this group of patients undergoing occipital lobe invasive monitoring, Chi-square analysis of the differences in the outcomes in the surgical versus nonsurgical subgroups was statistically significant (P < 0.05, P = 0.042).

Temporal versus extratemporal monitoring

In [Table 5.6], an outcome analysis of patients who underwent invasive monitoring and progressed to resective surgery is shown with patients divided into temporal versus extratemporal resection. Among the 71 patients undergoing temporal resection, 47 patients in [Table 5.6]a comprising 66.2% in [Table 5.6]c achieved seizure freedom (Engel Class I), and eight patients (11.3%) realized no improvement. This contrasts with the 31 patients who underwent extratemporal resection, 17 (54.8%) of whom realized seizure freedom and 7 (22.6%) of whom had no improvement. In this group of patients undergoing resection, Chi-square analysis of the differences in the outcomes in the temporal versus extratemporal subgroups did not reach statistical significance (P = 0.31).

Progression to surgery and outcomes: Gender

[Table 6.1] shows the progression to surgery characteristics for each gender subgroup, and [Table 6.2] and [Table 6.3] shows the clinical outcome data for the patients partitioned into subgroups by gender.


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In [Table 6.1], patient numbers and proportions are presented for surgical and nonsurgical groups segmented into the two gender subgroups comprising male and female patients. As seen in [Table 6.1]b, progression to surgery rates was slightly higher for male than female gender (92.1% vs. 84.4%, respectively). Chi-square analysis produces a P = 0.095, consistent with a trend.

Female patients

[Table 6.2] shows an outcome analysis of female patients. Among the 36 patients undergoing subsequent therapeutic surgery, 25 patients in [Table 6.2]a comprising 69.4% in [Table 6.2]c achieved seizure freedom (Engel Class I), and five patients (13.9%) realized no improvement. This contrasts dramatically with the nine patients who did not undergo surgery, 1 (11.1%) of whom realized seizure freedom and 8 (88.9%) of whom had no improvement. In this group of female patients, Chi-square analysis of the differences in the outcomes in the surgical versus nonsurgical subgroups was statistically significant (P < 0.001, P = 5.11 × 10−5).

Male patients

[Table 6.3] shows an outcome analysis of male patients. Among the 66 patients undergoing subsequent therapeutic surgery, 39 patients in [Table 6.3]a comprising 59.1% in [Table 6.3]c achieved seizure freedom (Engel Class I), and 10 patients (15.2%) realized no improvement. This contrasts dramatically with the six patients who did not undergo surgery, none (0%) of whom realized seizure freedom and 5 (83.3%) of whom had no improvement. In this group of male patients, Chi-square analysis of the differences in the outcomes in the surgical versus nonsurgical subgroups was statistically significant (P < 0.001, P = 3.3 × 10−4).

Summary of analyses

In [Table 7], these analyses are summarized. Differences in rates for progression to surgery were found to be statistically significant for laterality, specifically with patients undergoing unilateral invasive monitoring having a greater rate of progression to surgery than those undergoing bilateral monitoring. Both subgroups did better with surgical than nonsurgical treatment. Limitations to interpretation of the data include small sample size and retrospective analysis. For each subgroup, considering surgical and nonsurgical patients lumped together, those who underwent unilateral monitoring had better outcomes than those who underwent bilateral monitoring.
Table 7: Summary of progression to surgery and outcomes

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Differences in rates for progression to surgery were found to show a trend but not reached statistical significance among the four location subgroups [P = 0.05888; from [Table 7]. For each of the four location subgroups, patients treated surgically had statistically significantly better outcomes than patients treated nonsurgically [P = 0.00014 for frontal, P = 2.7E-08 for temporal, P = 0.02846 for parietal, and P = 0.04187 for occipital; from [Table 7].

Differences in rates for progression to surgery were also found to show a trend but not reached statistical significance among the two genders, with males being more likely to progress to surgery [P = 0.095; from [Table 7]. For each gender, patients treated surgically had statistically significantly better outcomes than patients treated nonsurgically [P = 5.1E-05 for female, P = 0.00034 for male; from [Table 7]. Overall, with surgical and nonsurgical subgroups considered together, patients of both gender groups had statistically similar overall outcomes [P = 0.2653; from [Table 7].


  Discussion Top


Invasive monitoring: Perioperative risks and nonlocalization risks

The risks of invasive monitoring have generally been characterized and reported in terms of the perioperative surgical risks. Although these perioperative complication rates for invasive EEG monitoring are acceptably low to justify invasive monitoring,[9],[20],[21] they are still a factor in determining whether a patient undergoes invasive workup when seizure source localization is unclear. These low complication rates continue to improve with surgeon and team experience and may be further reduced with refinement in surgical technique and perioperative care.[9]

Related literature on nonprogression to surgery

There is a dearth of literature on the risks or likelihood of having a nondiagnostic outcome from invasive monitoring and consequently failing to proceed to resective surgery. In our study, we aim to characterize the risks or likelihood of achieving insufficient diagnostic information to justify subsequent progression to definitive therapeutic surgery.

Furthermore, we seek to identify predictors of progression and nonprogression to therapeutic surgery and to characterize outcomes for patients according to these potential predictors. In our study, demographic parameters were assessed to identify correlates of progression versus nonprogression to surgery. Age was found not to be a correlate since in both groups, the average age was 30. The number of implanted monitoring electrodes was also found not to be statistically significantly different in the two groups.

Progression to surgery and superiority of surgical versus nonsurgical outcomes

Overall, the patients who had surgical therapy did significantly better than those who did not progress to have surgical therapy, with 62.7% versus 6.7% in the surgical and nonsurgical groups, respectively, achieving seizure freedom. Patients in the group that did not undergo definitive surgery continued to either worsen or remain unchanged in their seizure frequency and only a small subset of nonsurgical patients demonstrated any improvement. The course of the disease in these patients suggests poor outcomes with further medical management. This is consistent with the known literature on efficacy of surgical intervention for epilepsy.

In our series of 127 patients, 88% (112/127) underwent resective surgery and 12% (15/127) did not progress to surgery and were managed medically. This is consistent with the published literature, including rates of nonprogression to surgery of 10% (17/160) as reported by Behrens et al. n a series of 160 patients undergoing invasive monitoring [22] and as high as 24% (12/50) in a series of fifty patients reported by Bekelis et al.[23]

Predictors of nonprogression to surgery

Because of the significant difference in seizure-free outcome between surgical and nonsurgical therapy, prognostic information predictive of a diagnostic invasive study and progression to surgery would be valuable for patients, families, and physicians in deciding whether to pursue invasive monitoring. This stark differential in efficacy between surgical versus nonsurgical management leads one to wonder how the diagnostic workup could be further optimized and to consider the options of prolonging the invasive monitoring session, expanding the electrode coverage, and investigating promising new technologies for augmented source localization.[10]

Limited information has been published on predictors of diagnostic invasive monitoring. Pondal-Sordo et al., 2007 reported on the effectiveness of intracranial monitoring and found that the yield of the intracranial EEG monitoring was higher in groups of patients with lack of congruence between MRI and scalp EEG and lower in patients with congruent but subtle or uncertain scalp EEG and MRI findings.[24]

Laterality

With respect to laterality, patients who underwent unilateral invasive monitoring (regardless of side, left or right) were more likely to progress to surgery (90.8% vs. 72.2%) than those undergoing bilateral monitoring. In our sample of 122 patients in whom outcome was assessed, those who underwent left-sided monitoring and progressed to surgery were significantly more likely to achieve seizure freedom than those who did not progress to surgery. This contrasts with the set of patients who underwent right-sided invasive monitoring; in this smaller group of 32 patients, there was not a statistically significant difference in outcome between the surgical and nonsurgical groups. We suspect that this is likely to be artifactual due to small sample size. In addition, because of the risk of language deficits with left sided surgery, a lower threshold for presurgical invasive monitoring may have been applied to cases with left-sided pathology than right-sided pathology. A resulting potential selection bias may have resulted in the inclusion of more cases of well-localized left-sided pathology, leaving relatively less well-localized pathology in the right-sided patient subgroup. Among the patients who underwent bilateral invasive monitoring, those who progressed to surgery had a significantly higher rate of achieving seizure freedom than those who did not.

Since the outcomes of patients undergoing unilateral versus bilateral monitoring, with seizure freedom in 64% versus 50%, are not statistically significantly different as seen in [Table 4].6, the prospect of undergoing bilateral invasive monitoring should therefore not be a deterrent for pursuing surgical treatment. This is consistent with Placantonakis et al., 2010, in which 26 patients undergoing bilateral invasive monitoring were studied and 17 patients (65.4%) subsequently progressed to undergo therapeutic resective surgery.[25] In that study, among 25 assessed at follow-up, 12 (48%) were Engel Class I, 3 (12%) were Class II, 1 (4%) was Class III, and 9 (36%) were Class IV.[25] This is comparable to our subgroup undergoing bilateral monitoring, in which 50% of surgical patients and 35% overall (surgical and nonsurgical) achieved seizure freedom.

Anatomical location

Substantial variation in rates for progression to surgery was seen among patient subsets partitioned by anatomical location monitored. For each of the locations, the subgroup of patients who progressed to surgery did better than those who did not. The best seizure-free rates of 80% and 65% were achieved among patients who had parietal and temporal invasive monitoring, respectively. Temporal lobectomy outcomes are consistent with the published literature, including Engels Class I outcomes of 65% by Foldvary et al. in 2000,[26] 58%[27] and 73%[28] as found in two randomized clinical trials by Wiebe et al. in 2001[27] and Engel et al. in 2012,[28],[29] respectively, and 71% (15 of 21) in a recent study from our institution on 21 consecutive patient with normal MRI undergoing temporal lobectomy.[30]

Gender

In this small set of patients, the analysis of gender subsets revealed a trend for a higher rate of progression to surgery for males. This may be artifactual due to small sample size, may represent a neurophysiological difference resulting in seizures being more readily localized in males, or other as yet to be elucidated factor.


  Conclusions Top


In this paper, we have identified and analyzed potential predictors for progression to surgery in patients being considered for invasive monitoring. In our small and initial study, we found that patients undergoing bilateral monitoring and those undergoing monitoring of extratemporal locations and particularly frontal and parietal regions had statistically significantly lower rates of successful localization and progression to surgery. Further, patients of female gender had a trend toward lower rates of successful localization. Given the small and retrospective nature of this study, these findings do not prove causality, rather they are an initial attempt to characterize such predictors and suggest areas for future investigation.

All cohorts who progressed to surgery did better than those who did not; therefore, further study is warranted to better understand predictive characteristics and underlying mechanisms that limit successful localization with invasive monitoring. This is valuable for several applications: (1) In preoperative counseling of patients who may be at higher risk of a nondiagnostic study, (2) in developing an understanding of causality and insights into fruitful alternate potential monitoring techniques or technologies, and (3) considering chronic invasive monitoring [10],[31] in patients who may be less likely to achieve localization with subacute inpatient invasive monitoring. With such better prognostic information, better surgical outcomes may be realized and diagnostic and therapeutic strategies may be better tailored to the individual patient.

This study sheds insight into the use of some potential identifying predictors of outcome in this patient cohort. As the present work focuses on the outcomes of these patients undergoing surgery, this fills in the potential gaps in those patients not undergoing resective surgery and the outcomes they face. Evaluation of this patient population has demonstrated that there is a significant risk with long-term seizures in those not undergoing surgery. Future work should follow these patients in a prospective manner to formulate a predictive tool for those who may undergo surgery.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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