Journal of Neuroradiology (2014) 41, 131—135

Available online at www.sciencedirect.com

ORIGINAL ARTICLE

Impaired white-matter integrity in photosensitive epilepsy: A DTI study using tract-based spatial statistics Hanjian Du a,1, Bing Xie b,2, Peigang Lu a,3, Hua Feng c,4, Jian Wang b,5, Shaoji Yuan a,∗ a

Department of Neurosurgery, Jinan Military General Hospital, Jinan 250000, China Department of Radiology, Southwest Hospital, Third Military Medical University, Chongqing 400038, China c Department of Neurosurgery, Southwest Hospital, Third Military Medical University, Chongqing 400038, China b

KEYWORDS Photosensitive epilepsy; White-matter; DTI; Tract-based spatial statistics

Summary Background and purpose: The present study was designed to map alterations in brain whitematter in photosensitive epilepsy (PSE) by applying tract-based spatial statistics (TBSS) analysis. Methods: Diffusion tensor-imaging (DTI) data from MRI brain scans were collected from eight PSE patients and 16 gender- and age-matched non-epileptic controls using a SIEMENS Trio 3.0Tesla scanner. For the white-matter analysis, DTI scans were processed using FSL software (http://www.fmrib.ox.ac.uk/fsl/index.html). Fractional anisotropy (FA) values in the PSE and control groups were compared using TBSS analysis corrected for multiple comparisons using threshold-free cluster enhancement. Results: Compared with the control subjects, the corpus callosum of PSE patients had significantly lower FA values. Conclusion: Our DTI study indicates that white-matter in the corpus callosum was abnormal in PSE patients, and that DTI methods can serve as useful non-invasive tools to evaluate whitematter changes in PSE patients. © 2013 Elsevier Masson SAS. All rights reserved.

∗

Corresponding author. Tel.: +86 13064027615. E-mail addresses: [email protected] (H. Du), [email protected] (B. Xie), [email protected] (P. Lu), [email protected] (H. Feng), [email protected] (J. Wang), [email protected] (S. Yuan). 1 Tel.: +86 18764166131. 2 Tel.: +86 23 65486355. 3 Tel.: +86 15972184088. 4 Tel.:+86 65463026. 5 Tel.: +86 23 68754419. 0150-9861/$ – see front matter © 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.neurad.2013.06.002

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H. Du et al.

Introduction

or brain trauma, and none showed any abnormalities on routine magnetic resonance imaging (MRI) examination. All had generalized spike-and-wave (GSW) or polyspike discharges with normal backgrounds on video—EEG monitoring during IPS [20]. The IPS procedure consisted of four sessions and was carried out using standard photostimulators in a dimly lit room. The distance from the stimulator to the subject’s nasion was approximately 25 cm [20,21]. Over the first 30 s, the flash frequency was increased to up to 60 per second, then reduced to 4 per second over the next 30 seconds. Subsequently, flash frequencies of 5, 10, 12, 15, 20 and 25 per second were used for periods of 20 seconds each, followed by 30-second periods of stimulation with irregular frequencies up to 60 per second. Also, during IPS the effects of three-eye conditions (eyes open, eyes closing and eyes closed) were tested once in each subject [3,22]. The stimuli were terminated as quickly as possible after a generalized discharge was elicited. All of our PSE patients’ diagnoses were based on their seizure history, signs and symptoms, and IPS stimulation results on video—EEG, according to the Commission on Classification and Terminology of the International League Against Epilepsy (ILAE) [23]. At the time of our study, all patients were controlled by sodium valproate as antiepileptic treatment. The clinical features of the patients are presented in Table 1. The controls consisted of 16 age- and gender-matched healthy, non-epileptic individuals (eight men and eight women; mean age: 23.6 years, range: 19—28 years) who were recruited from the surrounding region through advertisements. The controls underwent comprehensive brain examination to ensure they had normal brain structure and no neurological lesions. Subjects with brain trauma, brain tumor(s), psychiatric disorders, systemic disease or other MRI contraindications such as phobia or claustrophobia were excluded from the study. The Ethics Committee of the Third Military Medical University and Southwest Hospital approved the study protocols, and written informed consent was obtained from all participants.

Photosensitivity, or an evoked photoparoxysmal response (PPR), shows as generalized spikes or spike-and-wave discharges on electroencephalography (EEG) under intermittent photic stimulation (IPS) [1], and is a common feature of idiopathic generalized epilepsy [2,3]. The incidence of PPR in non-epileptic people is 0.5—9.9% [4], while only around 5% of epilepsy patients have photosensitive epilepsy (PSE) [5], with around 60% of such seizures induced by watching TV, patterns of lines, ceiling fans, cinema films and computer monitors, among other triggers [6,7]. Many experiments have previously been done involving the superficial layers of the brain and found that diffuse cortical hyperexcitability contributes to the development and propagation of the PPR [8—10]. However, few studies have investigated the role that white-matter may play in PSE. Diffusion tensor imaging (DTI) can quantitatively evaluate white-matter using anisotropic diffusion (Brownian movement) of water to detect nerve fiber bundle direction and integrity [11—13]. In the present study, our main goal was to map alterations in white-matter in PSE by applying tract-based spatial statistics (TBSS) analysis to DTI data to avoid the disadvantages of using fractional anisotropy (FA) images in voxel-wise statistical analyses [14], and also to perhaps improve sensitivity and objectivity by using nonlinear registration and projection onto the mean FA skeleton [15]. TBSS is known to provide important insights regarding white-matter and has already been widely used in research involving nervous system diseases such as glaucoma [16], early blindness [17] and Alzheimer’s disease [18].

Materials and methods Subjects The PSE group consisted of eight patients (four men and four women; mean age: 23.1 years, range: 19—28 years) recruited from our epilepsy clinic in Chongqing Southwest Hospital. The patients’ clinical information, including their National Hospital Seizure Severity Scale (NHS3) scores [19], was collected through interviews with the patients and relatives who had witnessed their epileptic seizures. None of the patients had a history of drug intoxication, encephalopathy

Data acquisition DTI data were collected using a Siemens Trio 3.0-Tesla scanner with a head matrix coil at the Department of Radiology of Southwest Hospital. During scanning, each subject

Table 1

Clinical features of patients with photosensitive epilepsy (PSE).

Patient

Gender

Age

Onset

Seizure type

PPR

Duration (years)

Family history

AED

NH3

Right-handed

1 2 3 4 5 6 7 8

M F M M F F F M

19 28 26 19 19 23 26 25

12 27 18 15 18 20 23 15

GTCS GTCS GTCS GTCS GTCS GTCS GTCS GTCS

Yes Yes Yes Yes Yes Yes Yes Yes

7 1 8 4 1 3 3 10

No No No No No No No No

VPA VPA VPA VPA VPA VPA VPA VPA

10 12 12 13 17 18 21 14

Yes Yes Yes Yes Yes Yes Yes Yes

Impaired white-matter integrity in photosensitive epilepsy lay supine within the scanner with eyes closed. Scanner noise was attenuated with earplugs, and the subject’s head movement was restricted by foam pads (provided by the scanner manufacturer) placed about the head. The importance of head immobility was emphasized to each subject. A single-shot echo-planar sequence was used with the following parameters: slice number = 45; matrix = 128 × 128; slice thickness = 3 mm; TR = 6100 ms; and TE = 93 ms. After an acquisition without diffusion, images were then acquired with diffusion gradients (b = 1000 s/mm2 ) applied in 64 directions. In addition, T2-weighted axial images were acquired as part of the examination. All images were submitted for visual analysis by experienced neuroradiologists and all were considered normal.

Preprocessing of raw DTI data Preprocessing of the raw DTI data was performed using FSL software (http://www.fmrib.ox.ac.uk/fsl/index.html). Motion and eddy current corrections were carried out by means of affine registration to the reference volume, and the corrected data were brain-extracted using FSL’s Brain Extraction Tool (BET) [24]. FA images were then created by fitting a tensor model to the raw diffusion data using the FMRIB Diffusion Toolbox (FDT).

TBSS analysis Voxel-wise statistical analysis of the FA data was performed using TBSS [15], a tool included in FSL software [14]. All of our subjects’ FA data were subsequently aligned into a stereotactic coordinate system with the MNI152 template [15,25], using the FMRIB non-linear registration tool (FNIRT) with B-spline representation of the registration warp field [26]. Next, all FA maps were averaged to produce a group mean FA image. The resultant mean FA map was then thinned to create a mean FA skeleton, representing the centers of all white-matter tracts in both study groups with a threshold FA value of 0.20 [15]. Statistical analysis was performed using the ‘randomize’ command in FSL. The number of permutations was set to 5000, and correction for multiple comparisons was achieved using threshold-free cluster enhancement (TFCE) [27] at a cluster level of P < 0.05.

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Results There was no significant difference in mean age between the PSE and control groups (P > 0.05). The FA in each group was compared using TBSS analysis, and correction for multiple comparisons was performed using TFCE. Compared with the non-epileptic controls, the PSE patients showed significantly decreased FA values (P < 0.05, corrected; Fig. 1) in the corpus callosum. Also, there was no correlation between the duration of epilepsy and corpus callosum FA values.

Discussion This study aimed to investigate nerve fiber integrity between PSE patients and a matching control group using DTI scanning and TBSS analysis. TBSS is more robust and sensitive than other voxel-wise analysis methods [15], such as voxel-based morphometry (VBM), which can be influenced by alignment and spatial smoothing [28], and uses assumptions that are difficult to verify [29]. In addition, TBSS automatically investigates the whole brain without manual intervention. During the conventional MRI examination of our patients, no macroscopic alterations in the brain were detected and all appearances were normal. However, with the TBSS analysis method, it was found that FA values, which reflect the microstructural integrity of the corpus callosum areas, were reduced, suggesting that the white-matter may have deteriorated. This finding directly confirms that DTI is able to detect small abnormalities that are otherwise not evident with conventional MRI techniques [30]. The presence of altered white-matter was determined by comparing the FA maps of the eight PSE patients with those of the 16 age- and gender-matched controls. This demonstrated that, using the TBSS method, the decreased FA values were mostly concentrated in the corpus callosum areas in PSE patients. These findings suggest that corpus callosum white-matter may be impaired and that this may be involved in the pathogenesis of epilepsy. The corpus callosum, a brain structure found only in placental mammals, is the most important interhemispheric commissural connection between the left and right cerebral hemispheres. It contains 200 million intra- and interhemispheric myelinated axonal projections and is considered

Figure 1 Comparison of fractional anisotropy (FA) between patients with photosensitive epilepsy (PSE) and their matching controls. Coronal (A), axial (B) and sagittal (C) views of the FA skeleton (green = FA > 0.2) are overlaid by the mean FA map. The yellow and red areas indicate the parts of the corpus callosum with significantly decreased FA values (P < 0.05, corrected) along the skeletonized fiber tracts in PSE patients versus controls, and may also represent regions of abnormal axonal integrity.

134 the largest white-matter structure in the brain [31]. It plays an important role in communication between the bilateral hemispheres and connects the cortical regions of the hemispheres while integrating information between the hemispheres [32]. The anterior corona radiata of the corpus callosum are projection fibers that make the reciprocal connections between the thalamus and frontal cortex [33]. Previous studies in PSE patients had shown that the frontal cortex is involved in the wave discharges seen on visual inspection of EEG traces [34]. The study by Aydin-Ozemir et al. [35], using the magnetic resonance spectroscopy method, demonstrated significantly decreased concentrations of N-acetylaspartate in the frontal lobe of PSE patients, whereas our present results indicate that a specific cortical—subcortical network may be present in PSE. In addition, partial epileptic activity of the cerebral hemispheres can spread via the corpus callosum to the contralateral hemisphere, thereby causing a generalized seizure. The surgical approach known as ‘corpus callosotomy’ has been adopted to treat refractory epilepsy by blocking the interhemispheric electrical storm [36], thus leading to independent spike-and-wave discharges in the left and right hemispheres without contralateral synchrony [37]. Based on our present findings, it also appears that the corpus callosum plays a prominent role in ictal propagation, and seizure-induced neuronal damage may lead to secondary axonal injury of callosal fibers. Numerous studies of gray matter [36] have found that the occipital lobe is involved in PSE. However, our present study found no differences in occipital white-matter between our patient and control groups, most likely as a result of the small sample size in our experiment. Larger samples will be needed to characterize the alteration of such structures in PSE in future studies.

Conclusion Corpus callosum white-matter was abnormal in PSE patients, as demonstrated by our DTI study, which suggests that pathological changes occur in brain white-matter in association with PSE. Also, the DTI method served as a useful noninvasive tool for the evaluation of white-matter in PSE patients.

Disclosure of interest The authors declare that they have no conflicts of interest concerning this article.

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