Epileptic Disorders
MENUNeuroimaging of focal cortical dysplasia: neuropathological correlations Volume 5, numéro 2, June 2003
Illustrations
Auteur(s) : Nadia Colombo 1, Alberto Citterio 1, Carlo Galli 2, Laura Tassi 3, Giorgio Lo Russo 3, G. Scialfa 1, Roberto Spreafico 4.
1. Neuroradiology Department; 2. Pathology Department and 3.
Epilepsy Surgery Centre ‘Claudio Munari’, Ospedale Cà Granda
Niguarda
4. Department of Experimental Neurophysiology and Epileptology,
Istituto Nazionale Neurologico C.Besta, Milano, Italy
Focal cortical dysplasia (FCD) is a peculiar developmental anomaly of the cortex, first identified by Taylor [1] in lobectomy specimens obtained from drug-resistant epilepsy patients. It was described as an area of neocortical laminar disruption in which abnormal dysmorphic neurones were littered in all the cortical mantle and more or less associated with the so-called ‘balloon cells’ (BC), representing cells of uncertain lineage.
Taylor separated this entity from other MCD such as agyria,
pachygyria, polymicrogyria, heterotopia and hemimegalencephaly,
frequently referred to in the literature using the general term
‘cortical dysplasia’. Based on subsequent neuropathological
observations, it has become clear, nevertheless, that the term FCD
encompasses a wide spectrum of abnormalities of differing
severity.
For a long time, confusing terminology and classification of the
different forms of FCD were used in the literature: while Taylor’s
FCD (TFCD) was unanimously recognised as the most severe form, with
well defined histopathological features, the less severe forms of
FCD were variably called ‘microdysgenesis’ or ‘mild cortical
dysplasia’, and heterogeneous microscopic abnormalities were
considered, in isolation or together, as being distinctive of them
[2-13].
Our group recently proposed a simplified, three-category
classification of FCDs, based on easily recognised
histopathological characteristics, that avoids complicated
terminology (table 1) [14, 15].
Table 1. Neuropathological classification and findings of FCDs.
A) Architectural (AD) | - heterotopic neurones in the WM |
- disarray of cortical lamination | |
B) Cytoarchitectural (CD) | - heterotopic neurones in the WM |
- disarray of cortical lamination | |
- giant neurones | |
C) Taylor’s (TFCD) | - heterotopic neurones in the WM |
- disarray of cortical lamination | |
- giant + dysmorphic neurones | |
- without or with balloon cells (BC) |
Since the preoperative recognition of lesions on MR imaging can modify the diagnostic work-up of patients who are candidates for surgery, the most appropriate MR technique should be employed and the study should be carefully focussed on the clinical presentation [26-28]. Sequences in three different anatomical planes should always be acquired to assure the best visualisation and characterisation of the pathology.
MR technique. Our examinations using a 1.5 T magnet (Philips ACS NT) included: transverse spin-echo (SE) DP-T2W images of the whole brain (2 000-2 500/20-90) [TR msec/TE ms], 1 avg, 128 × 256 matrix, 230 mm field of view, 4-5 mm slice thickness with 10% intersection gap; coronal turbo spin-echo (TSE) TW2 images (2 300/100) [TR msec/TE ms], 4 avgs, 256 × 256 matrix, 230 mm field of view, 3 mm-thick sections with 10% intersection gap, turbo factor of 15; coronal TSE fluid-attenuated inversion-recovery (FLAIR) T2W sequence (6 000/100/2 000) [TR msec/TE msec/inversion time msec], 3 avgs, 238 × 256 matrix, 230 mm field of view, 3 mm-thick sections with 10% intersection gap, turbo factor of 15; coronal TSE inversion recovery (IR) T1W images (3 000/20/400) [TR msec/TE ms/inversion time msec], avgs, 256 × 256 matrix, 230 mm field of view, 3 mm-thick sections with 20% intersection gap, turbo factor of 4. Coronal sequences were acquired over the area of the brain suspected of being the epileptogenic zone (EZ), that is, the area of seizure generation, based on electro-clinical data. In most patients, 3D volume Fast Field Echo (FFE) T1-W images are also obtained in the sagittal plane (30/4.6) [TR msec/TE msec], 30° flip angle, 1 avg, 512 × 512 matrix, 230 mm field of view, 1 mm-thick contiguous slices, and source images are subsequently reformatted in transverse and coronal sections. Additional sagittal TSE FLAIR T2W images are sometimes acquired to obtain further definition of hippocampal/parahippocampal pathology.
No contrast medium is usually injected.
To investigate temporal lobe epilepsy, transverse images are acquired along the major axis of the hippocampus, and coronal images perpendicular. For extra-temporal lobe epilepsies, sections are obtained parallel and perpendicular, respectively, to the anterior-posterior commissural (AC-PC) line.
Personal data
FDCs were found in approximately 23% of the histopathological
specimens from our total series of 360 patients undergoing
surgery during the preceding six years. To find distinctive MR
findings for the different subgroups of FCDs, we conducted a
retrospective study correlating imaging and histopathology in a
group of 49 patients, selected on the basis of their
histological diagnosis of FCD, among 224 patients who
underwent surgery between May 1996 and November 2000 and for
whom a clinical follow-up of at least one year was available.
According to the adopted neuropathological classification,
15 TFCD (13 with BC; 2 without BC) (30.6%),
6 cytoarchitectural dysplasia (CD) (12.2%) and
28 architectural dysplasia (AD) (57%) were identified in this
series.
Differences were found at imaging between TFCD and the other two
categories of cortical dysplasia (non-TFCD), even though some
overlapping of MR features was observed [29].
The most striking features of Taylor’s FCD included: focal
thickening of the cortex with poor definition of the grey-white
matter junction; decreased signal of the subcortical white matter
on T1-W sequences that increased on T2-W images and frequently
tapered as it extended to the ventricle (figures 1, 2). Cortical gyri were most
frequently normal or slightly wider. Calcifications, peripheral
oedema or mass effect were usually absent.
TFCD favoured an extra-temporal location with a predilection for
the frontal lobe. Our data are therefore consistent with the
literature for both characteristic MR appearance and site of
TFCD.
Architectural dysplasia was mainly characterised by focal
volume loss of the involved brain with shrinkage of the white
matter, which exhibited variable increased signal on T2-W images
and mild blurring with the overlying cortex. Normal cortical
thickness was usually observed. The temporal lobe was the most
common location for this subtype of cortical dysplasia (figure
3). This quite peculiar MR aspect of AD is not sufficiently
underlined in the literature.
In about 50% of specimens taken from the temporal lobe, AD coexisted with hippocampal sclerosis (HS), configuring ‘dual pathology’. At visual assessment, HS was revealed by atrophy, decreased signal of the hippocampus on T1-W images and increased signal on T2-W images and by loss of definition of internal anatomy (figure 3).
The various subgroups of cortical dysplasia were not only
characterised by different neuropathological and MR aspects, but
also by distinctive electroclinical patterns and post-operative
outcome. Briefly, the patients with AD had lower seizure frequency
than those with CD and TFCD, and the epileptogenic zone involved
mainly the temporal lobe. In patients with TFCD, the epileptogenic
zone was most frequently extratemporal, and interictal stereo-EEG
was distinctive [15, 30].
Patients with TFCD had the best outcome after surgery, with 75%
being seizure-free (Engel class 1a) [31] after at least one year of
clinical follow-up, compared with 50% of CD and 43% of AD
patients. □
Acknowledgements
The authors dedicate this study to the late Professor Claudio
Munari, an unforgettable teacher and friend.