Diffusion tensor imaging of time-dependent axonal and myelin degradation after corpus callosotomy in epilepsy patients

Abstract

Axonal degeneration of white matter fibers is a key consequence of neuronal or axonal injury. It is characterized by a series of time-related events with initial axonal membrane collapse followed by myelin degradation being its major hallmarks. Standard imaging cannot differentiate these phenomena, which would be useful for clinical investigations of degeneration, regeneration and plasticity. Animal models suggest that diffusion tensor magnetic resonance imaging (DTI) is capable of making such distinction. The applicability of this technique in humans would permit inferences on white matter microanatomy using a non-invasive technique. The surgical bisection of the anterior 2/3 of the corpus callosum for the palliative treatment of certain types of epilepsy serves as a unique opportunity to assess this method in humans. DTI was performed on three epilepsy patients before corpus callosotomy and at two time points (1 week and 2–4 months) after surgery. Tractography was used to define voxels of interest for analysis of mean diffusivity, fractional anisotropy and eigenvalues. Diffusion anisotropy was reduced in a spatially dependent manner in the genu and body of the corpus callosum at 1 week and remained low 2–4 months after the surgery. Decreased anisotropy at 1 week was due to a reduction in parallel diffusivity (consistent with axonal fragmentation), whereas at 2–4 months, it was due to an increase in perpendicular diffusivity (consistent with myelin degradation). DTI is capable of non-invasively detecting, staging and following the microstructural degradation of white matter following axonal injury.

Introduction

Wallerian degeneration (WD), described originally in 1850 (Waller, 1850) and extended by Ranvier (Ranvier, 1878) and Ramón y Cajal (Ramón-y-Cajal, 1928), is characterized by a series of events caused by neuronal injury that ultimately lead to the fibrosis and atrophy of the affected neuronal fibers. Such changes occur both upstream and downstream from the site of the lesion and therefore produce axonal changes in locations distant to the primary lesion. In the central nervous system (CNS), the acute phase of degeneration is composed primarily of fragmentation and dying-back of the axons (George and Griffin, 1994b, Kerschensteiner et al., 2005), whereas the chronic stage is characterized mainly by the slow and progressive degradation and phagocytosis of the myelin sheaths (George and Griffin, 1994a).

The demonstration, characterization and staging of the changes seen in axonal degeneration by means of a non-invasive approach would be of great importance in the clinical setting. In peripheral nerve degeneration, clinical magnetic resonance imaging (MRI) shows T2 signal hyperintensity of the nerve in sites distant from the precipitating injury as soon as 24 h from onset (Bendszus et al., 2004). However, due to the slow and progressive nature of axonal degeneration in the CNS, T2 signal changes distant from the lesion are not evident during the first 4 weeks (Kuhn et al., 1989, Khurana et al., 1999). At 4–14 weeks following injury, the white matter tracts undergoing degeneration become hypo-intense on T2-weighted images due to loss of myelin proteins (whereas myelin lipids remain intact), which produces a hydrophobic environment. As the myelin lipids are digested and gliosis ensues, the tissue becomes hydrophilic, causing increased signal intensity on T2-weighted images (Kuhn et al., 1989). Using magnetization transfer imaging, Lexa et al. (1993) demonstrated abnormalities in feline white matter within the first 2 weeks of degeneration, prior to the appearance of T2 changes.

In the last decade, there has been great interest in studying the microstructural environment of neural tissues by measuring the anisotropic diffusion of water molecules via MRI (Moseley et al., 1990). Normally, axonal membranes and myelin pose barriers to water displacement, such that water preferentially diffuses along the direction of the axons (Beaulieu, 2002). Given that the structural integrity of the axons governs the uneven displacement of water molecules (i.e., anisotropic diffusion), it is feasible to utilize DTI as a means to obtain information on the axonal state. As axons degenerate and break down with subsequent degradation of myelin, the barriers that normally hinder the diffusion of water across the axons disappear, allowing a more spatially uniform profile of water displacement (i.e., isotropic diffusion) (Beaulieu et al., 1996).

Previous diffusion MRI studies have demonstrated axonal degeneration in animal peripheral (Beaulieu et al., 1996, Stanisz et al., 2001) and central (Schwartz et al., 2003, Song et al., 2003, Schwartz et al., 2005) nervous systems. There is considerable evidence showing that myelin is a barrier to water diffusion and that its degradation (Beaulieu et al., 1996, Song et al., 2005) or absence (Gulani et al., 2001, Song et al., 2002) causes an increase in diffusivity perpendicular to the long axis of the fibers, a phenomenon that occurs rather late in the degenerative process. This abnormal diffusion pattern, consistent with chronic degeneration, has been demonstrated in humans (Pierpaoli et al., 2001, Glenn et al., 2003) and could be the underlying reason for the low diffusion anisotropy described in other series (Wieshmann et al., 1999, Werring et al., 2000, Thomalla et al., 2005, Thomas et al., 2005). The acute phase of the degeneration, invisible to conventional MR imaging and characterized by the fragmentation of the axons, reduces the diffusivity parallel to the principal axis of the fibers, as demonstrated using animal models (Ford et al., 1994, Beaulieu et al., 1996, Song et al., 2003). In a previous human study, parallel diffusivity was shown to be reduced 9 ± 4 days after stroke in the pyramidal tract, distally from the primary lesion (Thomalla et al., 2004). However, to our knowledge, a prospective study examining the time course of the full diffusion tensor after axonal injury has not been performed in humans.

Corpus callosotomy is a palliative surgical procedure performed in epilepsy patients with disabling seizures that do not respond to medication. During the surgery, the corpus callosum (typically the anterior two thirds) is transected in order to prevent the spread of epileptic activity from one hemisphere to the other and thus limit the generalized manifestation of seizures (Alonso-Vanegas and Castillo, 2002). The well-localized nature of the lesion, as well as the complete transection of the tract by corpus callosotomy, serves as a unique opportunity to study the evolution of axonal degeneration in vivo in a single white matter tract of considerable dimensions, as compared to its pre-surgical state.

The objectives of this study were (i) to assess the pattern and time course of water diffusion during degeneration of axons in a large white matter bundle, secondary to a well-localized and complete injury, namely, corpus callosotomy in epilepsy patients, and (ii) to relate the diffusion abnormalities with the known underlying stages of axonal degeneration in the human brain.