Fraction of anisotropy and apparent diffusion coefficient as diagnostic tools in trigeminal neuralgia

Abstract

Strong evidence suggests that classical trigeminal neuralgia (TN) is caused by a focal insult on the trigeminal root, usually due to neurovascular compression (NVC), justifying microvascular decompression (MVD) as the first neurosurgical option [29].High-resolution magnetic resonance imaging (MRI) sequences (three-dimensional [3D] T2-weighted, 3D time-offlight magnetic resonance angiography, and 3D T1-weightedgadolinium, in association) revealed effective for depicting the fine trigeminal neurovascular anatomy, of paramount importance for planning MVD [18]. Also, structural abnormalities-such as nerve deviation, groove formation, or atrophy-can be seen in a large number of cases with TN [12]. Volume and cross-sectional area measurements obtained by MRI were significantly smaller on the affected nerves than on the unaffected nerves [19]. Pathological examination of specimens collected during surgery showed axonal loss and demyelination [11], but these morphological changes can be difficult to interpret.As diffusion tensor imaging (DTI) is able to assess tissue integrity, this method has been applied to the study of abnormalities in white matter tracts, independent of their cause. DTI quantifies the amount of non-random water diffusion within tissues and provides unique in vivo information about the pathological processes that affect water diffusion as a result of microstructural damage [2]. Fraction of anisotropy (FA) and apparent diffusion coefficient (ADC) represent the two main DTI indices that are most widely used. FA describes how much molecular displacements vary in space and is related to the presence of oriented structures [28]. ADC, expressed as square millimeter per second, characterizes the overall mean-squared displacement of molecules and the overall presence of obstacles to diffusion [2, 28].These evidences motivated several studies to examine structural abnormalities in the trigeminal nerve (TGN) of patients with TN using FA [5-7, 9, 10, 13-15, 17, 22-25, 30] and ADC [7,13,17,[23][24][25]. Most of the studies examining FA by comparing the affected and unaffected sides of patients and controls revealed a significant decrease in FA values [5-7, 9, 10, 14, 15, 17, 22-25]. Chen et al., Leal et al., and Lummel et al. reported a significant elevation in ADC values in the affected sides of patients [7,17,23]. Other three studies did not find differences in ADC values between affected and unaffected sides of patients and controls [13,24,25]. Concerning only DTI studies about TN caused by NVC, six articles found a significant decrease in FA values [14,17,[22][23][24][25] and two a significant increase in ADC values [17,23] in the affected nerves.The measurement of FA and ADC provides robust values for quantifying the degree of microstructural abnormalities of TGN. Degeneration or damage of white matter tracts is expected to result in reduced FA, owing to the loss of directionality of diffusion and increase in ADC, that is due to diffusivity being averaged in all spatial directions as a result of the los...

Diffusion tensor imaging in trigeminal neuralgia: beyond the normal morphology

“…[6]. Also, some structural abnormalities were identified in large number of patients including deviation of the nerve and nerve atrophy [3].…”

“…It's one of the most common causes of facial pain and is commonly induced by mechanical irritation and there's a strong evidence suggesting an insult at the trigeminal nerve root which is usually caused by vascular loop from the superior cerebellar artery (neurovascular compression). Recognized risk factors for developing trigeminal neuralgia include hypertension, atherosclerosis, individual sensitivity and familial history [2,3].…”

“…MRI of patients with trigeminal neuralgia using high-resolution sequences such as 3D T2-weighted images and 3D T1-weighted gadolinium-enhanced images can depict the fine neurovascular anatomy of the trigeminal nerve and important in planning microvascular decompression [7]. However, the morphological and pathological changes of the TGN can be difficult to identify by conventional MRI sequences [3].…”

“…Recent studies demonstrated decreased fractional anisotropy (FA), increased radial diffusivity (RD) and apparent diffusion coefficient (ADC) in the affected side compared to the normal side; in spite of normal morphological appearance, thus depicting the microstructural abnormality by non-invasive means [2,3].…”