Histopathologic Findings of Malformations of Cortical Development in an Epilepsy Surgery Cohort

Malformations of cortical development (MCDs) are an important cause of pharmacoresistent epilepsy. Incidence studies based on contemporary magnetic resonance imaging and surgical findings estimate that 12% to 38% of pediatric and adult patients with medically recalcitrant epilepsy harbor a cortical malformation.12 Because the seizure burden is often excessive, patients commonly experience social stigmatization and alienation. Fortunately, neurosurgical resection of malformed tissue has proven potentially curative and is therefore offered to select patients with localized or focal MCD.3-6

With increased availability of surgical tissue, our understanding of the pathologic substrate of MCD has burgeoned. Modern investigational techniques utilizing surgical specimens have demonstrated potential pathogenetic mechanisms disruptive of normal development and identified pathophysiologic states conducive to abnormal electrical discharge.7-10 Immunohistochemistry has assisted with the identification and delineation of several patterns of malformation that may affect appropriate surgical and clinical management.11,12 In addition, postsurgical seizure control may be related in part to the particular type of focal MCD resected.13-15

Because histologie MCD patterning is well recognized and presumed to predict surgical outcome, consistent and appropriate nomenclature of the neuropathologic findings is essential. Unfortunately, the literature contains a variety of terminologies without clear or consistent definitions. Since Taylor et al16 published their seminal study more than 30 years ago, terms such as cortical dysplasia, cortical dysgenesis, cortical dysplasia with balloon cells, Taylor-type focal cortical dysplasia, neiironal migration disorder, and microdysgenesis have been inconsistently applied to MCDs. The lack of a standard nomenclature hampers effective communication, creates diagnostic confusion, and potentially compromises one's ability to best direct patient care. Perhaps the difficulty with developing a consistent terminology is that few clinicopathologic series exist for reference, and among these, a variety of histologic and immunohistochemical techniques have been employed.6,11-13,15,17-20

We suggest that histologie description of surgically resected MCDs from patients with intractable epilepsy could be made more uniform and reproducible by the application of a standardized approach to tissue handling, processing, and histopathoiogic analysis. Utilizing a large surgical epilepsy cohort, the specific aims of this study were to develop a practical method of histologie analysis and classification that highlights the germane features of MCDs.

MATERIALS AND METHODS

Ascertainment of Cohort

All patient materials were obtained from the Mayo Clinic Rochester Surgical Pathology Database. The database was searched from January 1987 to January 2003 using the search terms cortical dysplasia, polymicrogyria, and pachygyria and referencing patients from previous studies involving MCDs at our institution. Approval from the Mayo Clinic Investigational Review Board was obtained prior to study onset.

Clinical Record Extraction

The cohort's medical records were reviewed and demographic data, clinical history relevant to the seizure disorder (eg, seizure type, onset, frequency, and anticonvulsant treatment), significant past medical history, and family history was abstracted. Inclusion in the study required previous neurosurgery for treatment of medically intractable epilepsy. Exclusion criteria included a clinical history not consistent with intractable epilepsy, a diagnosis of tuberous sclerosis, an initial pathology report not consistent with MCD, or the unavailability of a patient's original pathologic specimen.

lmmunohistochemical Technique and Review

Surgically resected tissue was fixed in 10% buffered formalin and processed in paraffin. sections were routinely stained with hematoxylin-eosin, Luxol fast blue/cresyl violet (Kluver-Barrera), and immunostained for neurofilament protein (NFP, DAKO Clone 2F11) and glial fibrillary acidic protein (GFAP, DAKO Z0334). Each specimen was reviewed by a neuropathologist (J.E.P.) blinded to the original histologic and clinical diagnosis and systematically examined for the following features:

1. Abnormal cortical architecture: a disorganized cortical mantle with disruption of the usual laminar appearance because of ectopically placed neurons.

2. Abnormal neurons: abnormally enlarged or disfigured neurons scattered within the cortical mantle containing increased NFP.

3. Balloon cells: enlarged cells with glassy eosinophilic cytoplasm and eccentric nuclei, often found in clusters, with inconsistent immunoreactivity to either/both GFAP and NFP.

4. Neoplastic elements.

Classification

The overall structure of the proposed classification was derived from Barkovich et al,-1 but categories only relevant to pathology found at epilepsy surgery are included. Two other important alterations to the original Barkovich nomenclature were made (as indicated in italics later):

1. The term dysplasia (and its attendant implication of a preneoplastic condition) is avoided and is substituted with dysmorphisin when referring to the individual cell or disorganization when referring to cortical laminar patterning.

2. Architectural disorganization is used in favor of microdysgenesis and cytoarchitectural dysniorphism replaces cortical dysplasia.

The result is a classification system with language that focuses on MCD histology commonly found within surgical epilepsy specimens22 but retains a conceptual framework that aids mechanistic understanding of these disorders. Our rationale is elaborated within the "Comment" section of this paper.

The proposed classification is as follows:

I. Malformations because of abnormal proliferation

A. Nonneoplastic

1. Cortical hamartomas of tuberous sclerosis

2. Cytoarchitectural dysmorphism with balloon cells

B. Neoplastic

1. Dysembryoplastic neuroepithelial tumor

2. Ganglioglioma

3. Gangliocytoma

II. Malformations because of abnormal cortical organization

A. Architectural disorganization

B. Cytoarchitectural dysmorphism without balloon cells

C. Polymicrogyria

It is recognized that both lamination and cytoskeletal abnormalities often coexist; we have held to the Barkovich convention that lesions are classified according to the earliest perturbed developmental step (ie, proliferation prior to migration prior to organization) with the recognition that there is temporal overlap in these steps.

RESULTS

Clinical and Surgical Data

Fifty-three patients satisfied all criteria for inclusion into this study (Table 1). All patients had medically intractable epilepsy (average 5.2 medications tried per patient) and underwent neurosurgery for seizure control. The age of the patient at seizure onset ranged from birth to 54 years (mean age at onset 7.5 years). Localization-related epilepsy was diagnosed in 52 patients and 1 patient presented with infantile spasms. Patient age at the time of surgery ranged from 8 months to 58 years (mean age, 21.8 years). Cortical resection occurred with the following distribution: temporal lobe in 16 patients (8 right sided), frontal lobe in 14 patients (6 left sided), parietal lobe in 8 patients (5 left sided), and occipital lobe in 7 patients (4 left sided). Multiple lobes were resected in 11 patients (8 left sided), including 2 patients with left hemispherectomies. Three patients had additional neurosurgery for persistent medically intractable epilepsy.

Histopathologic Findings

A malformation of cortical development was recognized in 49 patients and grouped into the following 4 categories:

IA2: Cytoarchitectural dysmorphism with balloon cells (Figures 1 through 3)-19 patients demonstrated large dysmorphic neurons and balloon cells. Disrupted cortical lamination was present to some extent in all specimens.

IIA: Architectural disorganization, characterized by abnormal cortical lamination and/or ectopic neurons in white matter (Figure 4)-8 patients possessed a disorganized cortical mantle only.

IIB: Cytoarchitectural dysmorphism without balloon cells (Figure 5)-12 patients demonstrated large dysmorphic neurons but no balloon cells. Disrupted cortical lamination was present to some extent in all specimens.

IIC: Polymicrogyria-9 patients had pathology characterized by multiple shallow sulci overlying abnormal cortex. Disrupted cortical lamination was present to some extent in all specimens.

Because patients with tuberous sclerosis were excluded from this study, cortical tubers were not seen. In addition, none of the patients studied had evidence for a concomitant neoplasm and no patients possessing both polymicrogyria and pachygyria were found in this series. Four patients after additional review had no histopathologic evidence for an MCD: gliosis only (2 patients), mesial temporal sclerosis only (1 patient), and no apparent pathology (1 patient). One patient sample was too obscured by artifact to be diagnostic.

Clinical Characteristics and MCD Classification

Table 2 details the clinical characteristics as related to histopathologic diagnosis. Although descriptive in nature, patients harboring cytoarchitectural dysmorphism with balloon cells tended to have seizure onset at a younger age and more localized lesions. Patients with polymicrogyria also had seizures at a young age, but their pathology was more diffuse, often involving multiple lobes. In this cohort, surgery for all diagnoses occurred primarily in the adult population.

COMMENT

The term MCD is quite general and is meant to encompass the gamut of developmental malformations that are known to occur in the cerebral cortex.21 These malformations may be focal (restricted in area) or diffuse, involving much if not all of the cortex and subcortical structures. Depending on the extent of malformation, significant neurologic deficits may result including mental retardation, language and visual impairment, seizures, and paresis. Although first noted more than 100 years ago, the relationship between MCD and epilepsy was not well established until Taylor et al116 published their seminal histologie study of epilepsy surgical specimens containing what has subsequently been referred to as focal cortical dysplasia. Given the implied proclivity to malignant degeneration and issues raised in the following discussion, we propose that the group of malformations presently called focal cortical dysplasia be renamed to focal malformations of cortical development (FMCD). Subsequent pathologic, radiographie, and clinical studies have implicated FMCD to be an important cause of medically recalcitrant epilepsy.11,12,15,17,19,23

Unfortunately, histologie characterization of FMCD has proven problematic as terminology is often idiosyncratic or vague. Popular descriptors such as focal cortical dysplasia and microdysgenesis do not fully reflect the variety of microscopic features encountered, whereas, confusion occurs when terms such as Taylor-type focal cortical dysplasia and cortical dysgenesis are inconsistently employed.6 Several attempts to standardize nomenclature have been suggested, but none of these have been fully accepted.1,14,15,18,21,22

Perhaps the most widely regarded contemporary MCD classification system is that of Barkovich and colleagues,24 proposed in 1996, revised in 2001(25) and again in 2005.21 This system organizes both neuroradiologic and known genetic features of MCD within a framework based on 3 broad and overlapping developmental steps: neuroglial proliferation, neuronal migration, and cortical organization. Conceptualization of perturbed development and its consequence is possible for both diffuse and FMCD.

Despite its conceptual strength, this system does not well address specific histologie features present in malformations commonly found in surgical epilepsy tissue. For instance, neuronal cytomegaly with associated cytoskeletal abnormalities (other than balloon cells) is not specifically recognized. In addition, the terms cortical dysplasia and microdysgenesis are ambiguous and poorly delineate particularities recognized within these lesions. Although the overarching framework is considered valuable and potentially useful for classifying FMCD, the lack of specificity limits the utility of this scheme with respect to epilepsy surgical histopathology.

Rather than further complicate matters with unique terminology, we chose to supplement the Barkovich classification using terms addressing FMCD already present in the literature.15 Specifically, the terms architectural disorganization and cytoarchitectitral dysmorphism are used to differentiate abnormal laminar cortex only versus the presence of abnormal cell morphology in addition to altered cortical lamination. Like Barkovich et al, we have also avoided the misleading term focal cortical dysplasia of Taylor. Additionally, because FMCD are generally not considered precancerous, we have suggested eliminating the term dysplasia in favor of more histopathologically appropriate terminology such as dysmorphisin to describe individual cells and disorganization to described altered cortical lamination.

As previously mentioned, Taylor-type focal cortical dysplasia has been the subject of much confusion.14 Currently, most identify this particular pattern when enlarged, bizarre-appearing neurons are observed scattered across cortical layers-a finding reminiscent to that seen in tuberous sclerosis and hemimegalencephaly. Further differentiation based on the presence or absence of balloon cells is also common.14,22 We suggest the term cytoarchitectural dysmorphisin to represent lesions with large, bizarre, and dysmorphic neurons in addition to altered cortical lamination. As per Barkovich et al, whether or not these malformations are to be considered the effect of inappropriate cellular proliferation or cortical organization is dependent on whether balloon cells are also present.

Cytoarchitectural dysmorphisin with balloon cells is probably the most interesting finding in FMCD. The balloon cells are known to express glial and neuronal proteins, as well as proteins found in more primitive cells26 (Figures 1 through 3). Although seen in chronic epilepsy patients without neurocutaneous stigmata, the balloon cell phenotype is similar to the characteristic cell found in cortical tubers. Similar to the experience of others, these cells showed inconsistent immunoreactivity to both GFAP and NFP. Because tuberous sclerosis is a potentially confounding condition, patients who met clinical criteria for this disorder were excluded from this study. Nevertheless, in our series, balloon cells were found more often than the other forms of MCD. Although hematoxylin-eosin and Luxol fast blue/cresyl violet readily identified balloon cells amongst a disorganized background, NFP and GFAP distinguished cells with primarily neuronal or glial expression. What clinical significance, if any, the degree of immunoreactivity or neuronal versus glial cell type predominance has is unknown. Given its ambiguous origin and primitive protein expression, this malformation is considered a consequence of abnormal cellular proliferation.

Architectural disorganization is meant to replace more ambiguous terms such as mild focal cortical dysplasia or microdysgenesis. It is primarily characterized by abnormal cortical lamination, columnar disorganization, and ectopic neurons in white matter. Neurons of similar size and shape to pyramidal neurons may also be seen scattered within cortical layers other than layer V. Again, this pattern of malformation can be subtle; however, we found Luxol fast blue/cresyl violet and NFP to be particularly helpful in differentiating normal from abnormal cortex (Figure 4). In keeping with the Barkovich classification, architectural disorganization is best considered a malformation resulting from abnormal cortical organization.

Cytoarchitectural dysmorphism without balloon cells has a background of disorganized cortex with dysmorphic neurons but lacks balloon cells. Although NFP highlighted the bizarre cells, hematoxylin-eosin and Luxol fast blue/cresyl violet were sufficient in delineating the salient features (Figure 5). Despite similar appearance, recent literature addressing neuroimaging, epilepsy surgical outcome, and etiology of focal MCD has supported separating this lesion without balloon cells from those with balloon cells.21 Therefore, as the Barkovich classification suggests, cytoarchitectural dysmorphism without balloon cells is conceptualized as a malformation resulting from abnormal cortical organization.

The last pattern of FMCD identified in this cohort was polymicrogyria. This malformation also is associated with abnormal cortical organization. In our series, when focal polymicrogyria was present, the patient often had a concomitant medical condition (porencephalic cyst, lineus nevus sebaceous syndrome). Conventional histochemistry did adequately reveal these malformations, although NFP may provide further detail, especially when trying to distinguish an abnormal 4-layer versus normal 6-layer cortex.

Our classification system of FMCD is primarily derived from a contemporary, well-received, and overarching classification of MCD. We have also attempted to use language that reduces ambiguity; the terms dysplasia and Taylor-type cortical dysplasia have been purposely avoided. Our proposed system retains the rationale of classifying histopathologic findings of FMCD within a developmental framework. Whether this strategy will prove to assist patient care is not yet known, but a clinical outcome study based on this work is currently in progress. Further research regarding timing and nature of insults that initiate these malformations is also needed. For instance, better understanding of the differing phenotype of lesions presumably caused by disruption of cortical organization is required. The nature and meaning of balloon cells is also an area of interest that needs further clarification. In addition, the ability of this standardized approach to detect morphologic and cytoarchitectural abnormalities not detected with other staining procedures is unknown because all specimens examined were selected on the basis of a previous pathologic diagnosis. Application of these methods to tissue obtained for epilepsy surgery may result in an increased detection of developmental abnormalities.

In conclusion, neurosurgery is recognized as a therapeutic option in select patients with medically intractable epilepsy. Consequently, cortical tissue available for pathologic review from this population has increased and has revealed malformations of cortical development to be an important cause of pharmacologie resistant epilepsy. To aid conceptual understanding of FMCD we have presented a classification system that can supplement or enhance the current overarching classification proposed by Barkovich et al. In addition, terminology meant to limit ambiguity has been utilized when describing FMCD commonly found in epilepsy surgical histopathology.

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