Hippocampal sclerosis: the missing link of cysticercosis epileptogenesis?

Management of calcified cysticerci in the brain parenchyma: treating the dead parasite

INTRODUCTION

Calcifications are the end stage of many parenchymal brain cysticerci and may occur either spontaneously or as the result of treatment with cysticidal drugs. These lesions, traditionally considered inert and asymptomatic, have been associated with several complications that seem to be mostly related to brain damage and inflammation ensuing as the result of the exposure of the host's immune system to parasitic antigens trapped within calcifications.

AREAS COVERED

This review, based on the search of different electronic databases up to May 2024, focuses on the reported correlates and complications of calcified cysticerci (chronic headaches, seizures/epilepsy, hippocampal atrophy/sclerosis, gliomas), and the different interventions developed for their prevention and treatment. Common analgesics, non-steroidal anti-inflammatory drugs, corticosteroids, and antiseizure medications have been used with success but, with the exception of the latter, these drugs offer temporary relief of symptoms and support for their use is based on level 3 evidence.

EXPERT OPINION

Several strategies may reduce the severity of clinical consequences of calcified cysticerci. Probably, the most relevant intervention would be the prevention of their occurrence or reduction in their size. In this view, the use of bisphosphonates appears as a potential option that needs to be tested in humans.

Large (≥3-Millimeter) Parenchymal Brain Calcified Cysticerci Are More Often Associated with Hippocampal Atrophy than Smaller Ones in Seizure-Free Individuals with a Single Lesion

Calcified cysticerci are often associated with hippocampal atrophy (HA). While most studies suggest that repetitive seizures cause HA in these patients, others have demonstrated that HA may also occur in persons without epilepsy. Little is known about mechanisms triggering HA in seizure-free individuals with calcified cysticerci. Here, we aimed to assess whether the size of the calcification is associated with HA. Using a population-based design, we selected apparently seizure-free individuals with a single calcified cysticercus in whom interictal paroxysmal activity and other causes of HA have been discarded. A total of 55 individuals (mean age, 58.3 ± 13 years, 62% women) fulfilled inclusion criteria. Unadjusted and multivariate models were fitted to assess the association between the size of the calcification dichotomized into <3 mm and ≥3 mm (exposure) and the presence of HA (outcome). Sixteen participants (29%) had HA, which was asymmetric in eight (50%) cases. Hippocampal atrophy was noted in 11/20 (55%) participants with large calcifications and in 5/35 (14%) with small calcifications (P = 0.001). A multivariate logistic regression model showed a significant association between the presence of large calcifications and HA, after adjustment for relevant confounders (odds ratio: 7.78; 95% CI: 1.72-35.1). Participants with calcifications ≥3 mm in diameter were 7.8 times more likely to have HA than those with smaller ones. Study results open avenues of research for the use of agents to prevent HA progression.

Evaluating the Association of Calcified Neurocysticercosis and Mesial Temporal Lobe Epilepsy With Hippocampal Sclerosis in a Large Cohort of Patients With Epilepsy

BACKGROUND

Neurocysticercosis (NCC) is a parasitic infection of the central nervous system that has been associated with mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS). However, this association has not been completely established.

OBJECTIVE

To evaluate the prevalence of calcified NCC (cNCC), its characteristics and a possible association between cNCC and MTLE-HS in a cohort of 731 patients with epilepsy.

METHODS

We review clinical, EEG and neuroimaging findings of 731 patients with epilepsy. From these, 659 had CT-scans and 441 patients had complete neuroimaging with CT-scans and MRI. In these patients, we review the prevalence and characteristic of epilepsy in cNCC and in MTLE-HS patients.

RESULTS

Forty-two (6.4%) of the 659 patients studied with CT-scans had cNCC. cNCC lesions were more frequent in women than in men (n = 33-78.6% vs. n = 09-21.4%, respectively; OR = 3.64;(95%CI = 1.71-7.69); p < 0.001). cNCC was more often in patients who developed epilepsy later in life, in older patients, in patients who had a longer history of epilepsy, and in those with a lower educational level. MTLE-HS was observed in 93 (21.1%) of 441 patients that had complete neuroimaging, and 25 (26.9%) of these 93 patients also had cNCC. Calcified NCC was observed in only 17 (4.9%) of the remaining 348 patients that had other types of epilepsy rather than MTLE-HS. Thus, in our cohort, cNCC was more frequently associated with MTLE-HS than with other forms of epilepsy, O.R. = 11.90;(95%CI = 6.10-23.26); p < 0.0001).

CONCLUSIONS

As expected, in some patients the epilepsy was directly related to cNCC lesional zone, although this was observed in a surprisingly lower number of patients. Also, cNCC lesions were observed in other forms of epilepsy, a finding that could occur only by chance, with epilepsy probably being not related to cNCC at all. In this cohort, cNCC was very commonly associated with MTLE-HS, an observation in agreement with the hypothesis that NCC can contribute to or directly cause MTLE-HS in many patients. Given the broad world prevalence of NCC and the relatively few studies in this field, our findings add more data suggesting a possible and intriguing frequent interplay between NCC and MTLE-HS, two of the most common causes of focal epilepsy worldwide.

Incidence of Adult-Onset Epilepsy and the Contributory Role of Neurocysticercosis in a Five-Year, Population-Based, Prospective Study in Rural Ecuador

This prospective cohort study aimed to assess incidence and etiology of adult-onset epilepsy in previously seizure-free Atahualpa residents aged ≥ 20 years. Persons with adult-onset epilepsy occurring over 5 years were identified from annual door-to-door surveys and other overlapping sources. Those who emigrated or declined consent were excluded at the administrative censoring date of the last survey when these subjects were interviewed. Persons who died and those who developed incident epilepsy were censored at the time of these outcomes. Poisson regression models adjusted for demographics, education, alcohol intake, and the length of observation time, were used to estimate annual adult-onset epilepsy incidence rate ratio and cumulative incidence. Systematic neuroimaging screening was offered to participants to get insights on the etiology of epilepsy. Individuals enrolled in this cohort (N = 1,480) contributed to 6,811.6 years of follow-up. Seventeen developed incident adult-onset epilepsy, for an annual incident rate of 249.2 per 100,000 persons-year (95% CI: 130.7-367.7). Cumulative incidence was 1,245.9 per 100,000 persons (95% CI: 653.7-1,838.3) after a mean of 4.6 (SE: 0.06) years of follow-up. Six persons with incident epilepsy had neurocysticercosis (35%). Individuals with neurocysticercosis were six times more likely to develop adult-onset epilepsy than those without this disease (IRR: 6.01; 95% CI: 2.16-16.7), after adjusting for relevant covariates. The attributable fraction of incident adult-onset epilepsy due to neurocysticercosis was 30.9% (95% CI: 25.6-46.2%). This rural Ecuadorian population has a high incidence of adult-onset epilepsy, with neurocysticercosis being an important contributory cause.

Hippocampal Atrophy/Sclerosis Is Associated with Old, Calcified Parenchymal Brain Neurocysticercosis, But Not with More Recent, Viable Infections

Magnetic resonance images from 197 patients with calcified neurocysticercosis (NCC), 38 with viable NCC and 197 NCC-free healthy rural villagers were evaluated to compare the frequency of hippocampal atrophy/sclerosis (HAS) across these populations. Scheltens' medial temporal atrophy scale was used for hippocampal rating. The median age of the 432 study participants was 46 years (interquartile range, 29-62 years), and 58% were women. Hippocampal atrophy/sclerosis was disclosed in 26.9% patients with calcified NCC, compared with 7.9% in patients with viable NCC and 8.1% in healthy rural villagers. After adjusting for age, gender, and history of epilepsy, hippocampal atrophy/sclerosis was more frequent in patients with calcified NCC than in those with viable cysts (RR, 3.60; 95% CI, 1.18- 0.99; P = 0.025) and healthy rural villagers (RR, 3.43; 95% CI, 1.94-6.06; P < 0.001), suggesting that hippocampal damage develops late in the course of this parasitic disease.

Taenia solium Cysticercosis and Its Impact in Neurological Disease

Taenia solium neurocysticercosis (NCC) is endemic in most of the world and contributes significantly to the burden of epilepsy and other neurological morbidity. Also present in developed countries because of immigration and travel, NCC is one of few diseases targeted for eradication. This paper reviews all aspects of its life cycle (taeniasis, porcine cysticercosis, human cysticercosis), with a focus on recent advances in its diagnosis, management, and control. Diagnosis of taeniasis is limited by poor availability of immunological or molecular assays. Diagnosis of NCC rests on neuroimaging findings, supported by serological assays. The treatment of NCC should be approached in the context of the particular type of infection (intra- or extraparenchymal; number, location, and stage of lesions) and has evolved toward combined symptomatic and antiparasitic management, with particular attention to modulating inflammation. Research on NCC and particularly the use of recently available genome data and animal models of infection should help to elucidate mechanisms of brain inflammation, damage, and epileptogenesis.

EEG Patterns in Patients With Calcified Neurocysticercosis With or Without Hippocampal Atrophy

PURPOSE

To assess whether hippocampal atrophy develops in conjunction with clinical or subclinical epileptiform or encephalopathic activity in subjects with neurocysticercosis (NCC).

METHODS

Using a population-based and nested case-control study design, scalp EEGs and brain MRIs were performed in Atahualpa residents aged ≥40 years, who have imaging-confirmed NCC (case patients), as well as in age- and sex-matched NCC-free control subjects.

RESULTS

Sixty-two case patients and 62 control subjects were included. Encephalopathic EEG patterns were more common in five NCC subjects with epilepsy than in those without a history of seizures. Epileptiform EEG activity was noted in one patient with NCC but in none of the control subjects. This subject's focal epileptiform discharges correlated with the location of calcified cysticerci in the brain parenchyma, and the hippocampus ipsilateral to the epileptiform discharges was more atrophic than the contralateral hippocampus. The degree of hippocampal atrophy in patients with NCC without a history of seizures was significantly greater than in control subjects (P < 0.01) and tended to be even greater in patients with NCC with a history of seizures.

CONCLUSIONS

Hippocampal atrophy may not be exclusively related to seizure activity in patients with NCC. Other mechanisms, such as recurrent bouts of inflammation around calcified cysticerci, might explain the association between NCC and hippocampal atrophy.

Understanding the association of neurocysticercosis and mesial temporal lobe epilepsy and its impact on the surgical treatment of patients with drug-resistant epilepsy

Mesial temporal lobe epilepsy associated with hippocampal sclerosis (MTLE-HS) is one of the most common types of focal epilepsies. This is an epileptic syndrome commonly associated with treatment-resistant seizures, being also the most prevalent form of drug-resistant epilepsy which is treated surgically in most epilepsy surgery centers. Neurocysticercosis (NCC) is one of the most common parasitic infections of the central nervous system, and one of the most common etiological agents of focal epilepsy, affecting millions of patients worldwide. Recently, researchers reported a curious association between MTLE-HS with NCC, but this association remains poorly understood. Some argue that calcified NCC lesions in MTLE-HS patients is only a coincidental finding, since both disorders are prevalent worldwide. However, others suppose there might exist a pathogenic relationship between both disorders and some even suspect that NCC, by acting as an initial precipitating injury (IPI), might cause hippocampal damage and, eventually, MTLE-HS. In this review, we discuss the various reports that examine this association, and suggest possible explanations for why calcified NCC lesions are also observed in patients with MTLE-HS. We also propose mechanisms by which NCC could lead to MTLE-HS. Finally, we discuss the implications of NCC for the treatment of pharmacologically-resistant focal epilepsies in patients with calcified NCC or in patients with MTLE-HS and calcified NCC lesions. We believe that investigations in the relationship between NCC and MTLE-HS might offer further insights into how NCC may trigger epilepsy, and into how MTLE-HS originates. Moreover, observations in patients with drug-resistant epilepsy with both NCC and hippocampal sclerosis may not only aid in the understanding and treatment of patients with MTLE-HS, but also of patients with other forms of dual pathologies aside from NCC. This article is part of a Special Issue titled Neurocysticercosis and Epilepsy.

The Association Between Neurocysticercosis and Hippocampal Atrophy is Related to Age

Neurocysticercosis (NCC) has been associated with hippocampal atrophy, but the prevalence and pathogenic mechanisms implicated in this relationship are unknown. Using a population-based, case-control study design, residents in a rural village (Atahualpa) aged ≥ 40 years with calcified NCC were identified as cases and paired to NCC-free individuals (control subjects) matched by age, sex, and level of education. Cases and control subjects underwent magnetic resonance imaging for hippocampal rating according to the Scheltens' scale for medial temporal atrophy and were interviewed to identify those with a clinical seizure disorder. The prevalence of hippocampal atrophy was compared between cases and control subjects by the use of the McNemar's test for correlated proportions. Seventy-five individuals with calcified NCC and their matched control subjects were included in the analysis. Hippocampal atrophy was noted in 26 (34.7%) cases and nine (12%) control subjects (odds ratio: 4.4; 95% confidence interval: 1.6-14.9, P < 0.0021). Stratification of pairs according to tertiles of age revealed an age-related trend in this association, which became significant only in those aged ≥ 68 years (P = 0.027). Only five cases and one control had recurrent seizures (P = 0.221); three of these five cases had hippocampal atrophy, and the single control subject had normal hippocampi. This study confirms an association between NCC and hippocampal atrophy, and shows that this association is stronger in older age groups. This suggests that NCC-related hippocampal atrophy takes a long time to develop.

Pathogenesis of Taenia solium taeniasis and cysticercosis

Taenia solium infections (taeniasis/cysticercosis) are a major scourge to most developing countries. Neurocysticercosis, the infection of the human nervous system by the cystic larvae of this parasite, has a protean array of clinical manifestations varying from entirely asymptomatic infections to aggressive, lethal courses. The diversity of clinical manifestations reflects a series of contributing factors which include the number, size and location of the invading parasites, and particularly the inflammatory response of the host. This manuscript reviews the different presentations of T. solium infections in the human host with a focus on the mechanisms or processes responsible for their clinical expression.

Update on Cysticercosis Epileptogenesis: the Role of the Hippocampus

Neurocysticercosis (NCC) is the most common helminthic infection of the nervous system and a frequent cause of reactive seizures and epilepsy worldwide. In many cases, multiple episodes of focal seizures related to an identifiable parenchymal brain cyst (and likely attributable to local damage) continue for years after the cyst resolves. However, cases where seizure semiology, interictal EEG abnormalities, and parasites location do not correlate raise concerns about the causal relationship between NCC and either reactive seizures or epilepsy, as well as the epileptogenic potential of parasites. Neurosurgical series of patients with intractable epilepsy and cross-sectional population-based studies have shown a robust association between NCC and hippocampal sclerosis (HS), which might contribute to the above-referred inconsistencies. Current information does not allow to define whether in patients with NCC, HS could result from recurrent seizure activity from a local or distant focus or from chronic recurrent inflammation. In either case, HS may become the pathological substrate of subsequent mesial temporal lobe epilepsy (MTLE). Longitudinal clinical- and population-based cohort studies are needed to evaluate the causal relationship between NCC and HS and to characterize this association with the occurrence of MTLE. If a cause-and-effect relationship between NCC and HS is demonstrated, NCC patients could be assessed to examine neuronal mechanisms of hippocampal epileptogenesis in comparison with animal models, to identify biomarkers of hippocampal epileptogenesis, and to develop novel interventions to prevent epilepsy in NCC and perhaps in other forms of acquired epilepsy.

Neuroimaging observations linking neurocysticercosis and mesial temporal lobe epilepsy with hippocampal sclerosis

OBJECTIVE

To test if chronic calcificed neurocysticercosis (cNCC) and hippocampal sclerosis occur more often than by chance ipsilateral to the same brain hemisphere or brain region in mesial temporal lobe epilepsy associated with hippocampal sclerosis (MTLE-HS) plus neurocysticercosis. This proof-of-concept would provide important evidence of a direct pathogenic relationship between neurocysticercosis and MTLE-HS.

METHODS

A cohort of 290 consecutive MTLE-HS surgical patients was studied. A test of proportions was used to analyze if the proportion of patients with a single cNCC lesion matching the same brain hemisphere or region of hippocampal sclerosis was significantly greater than 50%, as expected by the chance.

RESULTS

Neuroimaging findings of cNCC were observed in 112 (38.6%) of 290 MTLE-HS patients and a single cNCC lesion occurred in 58 (51.8%) of them. There were no differences in main basal clinical characteristics of MTLE-HS patients with single or multiple cNCC lesions. In patients with single cNCC lesions, the lesion matched the side in which hippocampal sclerosis was observed in 43 (74.1%) patients, a proportion significantly greater than that expected to occur by chance (p=0.008). Neurocysticercosis in temporal lobe was ipsilateral to hippocampal sclerosis in 85.0% of patients and accounted mostly for this result.

CONCLUSIONS

This work is a proof-of-concept that the association of neurocysticercosis and MTLE-HS cannot be explained exclusively by patients sharing common biological or socio-economic predisposing variables. Instead, our results suggest the involvement of more direct pathogenic mechanisms like regional inflammation, repetitive seizures or both. Neurocysticercosis within temporal lobes was particularly related with ipsilateral hippocampal sclerosis in MTLE-HS, a finding adding new contributions for understanding MTLE-HS plus cNCC or perhaps to other forms of dual pathology in MTLE-HS.