Quantitative Diffusion is used as a biomarker of the severity of hippocampal sclerosis

Chronic epilepsy affects about 1% of the world population. It is estimated that 1000 individuals per year with refractory epilepsy undergo evaluation for epilepsy surgery for every 50 million people in the developed world.¹ MRI is essential for epilepsy surgery candidates.² An identifiable epileptogenic structural lesion on MRI that is successfully removed by surgery improves the chances of seizure freedom, rendering full remission in ≈70% of patients.³

There are benefits to using tailored MRI protocols for particular subsets of epileptic syndromes, such as mesial temporal lobe epilepsy (MTLE).

Dedicated neuroradiology of the limbic system provides biomarkers to assess the severity and extent of tissue damage associated with the neurodegenerative and inflammatory processes of mesial temporal sclerosis,⁴ which often extend beyond the limits of the hippocampus, reaching the amygdala,⁵ the parahippocampal and piriform cortices,^6,7 the cingulum, and the central gray matter nuclei.⁸

MRI epileptogenic lesions can be studied with quantitative MR methods generally available in referral centers for epilepsy surgery candidates.⁹ One of these techniques is diffusion-weighted imaging, which has been applied to focal epileptic syndromes such as MTLE.¹⁰

The diffusivity of water measured by the apparent diffusion coefficient (ADC) increases in the sclerotic hippocampus and in the amygdala.¹⁰ It was found that the mean ADC value computed at several hippocampal subfields is significantly higher on the ictogenic side as compared with the contralateral normal-appearing hippocampus of the patient and with the ADC values of healthy volunteers.

Additionally, hippocampal and amygdala ADC values are strongly correlated with volume atrophy, with the decrease of N-acetylaspartate to choline-plus-creatine ratios, and with T2-signal increase, which are hallmarks of hippocampal sclerosis.

Accordingly, ADC measurements provide a reliable postoperative prognostic indicator in patient candidates for anterior temporal lobe resection.

In this group of patients,¹⁰ ADC values were useful predictors of postoperative seizure remission, in particular in cases where the asymmetry between the ictogenic hippocampus and the contralateral side of the patient was higher than 3 standard deviations relative to control mean. A regression model provided a sound estimate that for each 0.1 mm²/s × 10⁵ increase in the ADC of the hippocampus, the probability of a favorable surgical outcome was multiplied by a factor of 88%, and that every 1-unit increase in the asymmetry index yielded a probability of a better outcome multiplied by a factor of 84%. On the contrary, lower hippocampal ADC values on the side of surgery are associated with a poor surgical outcome determined by Engel class.

These neuroimaging advances provide the clinical community with robust diagnostic tools to assess patients with MTLE. Further research with newly developed quantitative MRI technologies will strengthen the noninvasive diagnostic measures to determine which individuals would most benefit from early surgical interventions to treat epilepsies and those that will benefit from conservative medical and pharmacologic interventions in this disabilitating chronic condition.

References

1. Lhatoo SD, Solomon, JK McEvoy AW, et al. A prospective study of the requirement for and the provision of epilepsy surgery in the United Kingdom. Epilepsia 2003;44:673–76
2. Duncan JS. Imaging in the surgical treatment of epilepsy. Nat Rev Neurol 2010;6:537–50
3. Spencer S, Huh L. Outcomes of epilepsy surgery in adults and children. Lancet Neurol 2008;7:525–37
4. Gonçalves Pereira PM. Severity and extent of tissue damage in human and experimental temporal lobe epilepsy. PhD Thesis. Oporto University, Portugal; 2006
   
5. Kälviäinen R, Salmenperä T, Partanen K, et al. MRI volumetry and T2 relaxometry of the amygdala in newly diagnosed and chronic temporal lobe epilepsy. Epilepsy Res 1997;28:39–50
6. Jutila L, Ylinen A, Partanen K, et al. MR volumetry of the entorhinal, perirhinal, and temporopolar cortices in drug-refractory temporal lobe epilepsy. AJNR Am J Neuroradiol 2001;22:1490–1501
7. Gonçalves Pereira PM, Insausti R, Artacho-Pérula E, et al. MR volumetric analysis of the piriform cortex and cortical amygdala in drug-refractory temporal lobe epilepsy. AJNR Am J Neuroradiol 2005;26:319–32
8. Bernasconi N,  Duchesne S, Janke A, et al. Whole-brain voxel-based statistical analysis of gray matter and white matter in temporal lobe epilepsy. Neuroimage 2004;23:717–23
9. Sakamoto AC, Benbadis SR, Godoy J, et al. Essentials for the establishment of an epilepsy surgery program (appendix B). In: Luders H, Comair Y, eds. Epilepsy Surgery. 2nd ed. Philadelphia: Lippincott Williams & Wilkins, 2001: 979–86
10. Gonçalves Pereira PM, Oliveira E, Rosado P. Apparent diffusion coefficient mapping of the hippocampus and the amygdala in pharmaco-resistant temporal lobe epilepsy. AJNR Am J Neuroradiol 2006;27:671–83

 

Read this article at AJNR.org . . .