Journal of Neuro-Ophthalmology, March 2014, Volume 34, Issue 1

Diffusion Tensor Imaging in Traumatic Optic Tract Syndrome

Diffusion Tensor Imaging in Traumatic Optic Tract Syndrome We read with interest, the report by Bodanapally et al (1) of using diffusion tensor imaging (DTI) in eval-uating patients with traumatic optic neuropathy (TON). The authors concluded that decreased axial diffusivity and mean diffusivity in the posterior segment of the optic nerve might serve as a biomarker of axonal damage in patients with TON. We describe a patient with right homonymous hemi-anopia who presented 11 years after a severe closed head trauma with a negative conventional magnetic resonance imaging (MRI) and magnetic resonance angiography in whom left optic tract atrophy was documented with DTI. A 59-year-old woman complained of transient visual loss. In 1999, she was involved in a motor vehicle accident with loss of consciousness and associated memory loss. Recently, she reported 2 episodes of transient "tunneling of vision," each lasting 5 minutes. She was not certain if they were monocular or binocular. Her evaluation for these epi-sodes was unremarkable, and they spontaneously resolved with no recurrence. On examination, the patient was found to have a right homonymous hemianopia, but she was unaware of this visual field loss. MRI of the brain with and without contrast was unremarkable. Eighteen months later, despite normal intraocular pressures, the patient's optic discs were believed to be sus-picious for glaucoma, although she was not started on the treatment. Her medical, surgical, social, and family histories were noncontributory, and she was on no medications. Neuro-ophthalmological testing demonstrated visual acuity 20/25 in the right eye and 20/20 in the left eye. The pupils were isocoric with a right relative afferent papillary defect. Slip-lamp examination showed mild nuclear sclerotic cataracts, and intraocular pressures were 20 mm Hg, right eye, and 19 mm Hg, left eye. Funduscopy showed both optic discs to have enlarged cup-to-disc ratio, and the right disc demonstrated band optic atrophy (Fig. 1). Automated perimetry revealed an incongruous right homonymous hemi-anopia (Fig. 2). The pattern of retinal nerve fiber layer loss on optical coherence tomography of the right eye confirmed FIG. 1. The right optic disc shows band atrophy and horizontal cupping. The left disc has temporal pallor with vertical cupping. FIG. 2. Automated visual fields (24-2) demonstrate an incongruous right homonymous hemianopia. Letters to the Editor: J Neuro-Ophthalmol 2014; 34: 95-104 95 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. band optic atrophy, and there was temporal fiber atrophy in the left eye. A repeat MRI of the brain showed atrophy of the left optic tract (Fig. 3). This was confirmed using DTI (25 gradient directions, 1.8 mm isotropic spatial resolu-tion, b value 1,000 s/mm2) with fiber tractography of the visual pathways (2-4). MRI DTI fiber tractography (Diffusion Toolkit and TrackVis, Martinos Center for Biomedical Imaging, Massachusetts General Hospital (5); second-order Runge Kutta propagation algorithm, 35 angle threshold, min fractional anisotropy threshold automated by software, same tracking parameters for both right and left sides) of the optic tracts and optic radiations showed marked reduction in reconstructed fibers along the left optic tract (Fig. 4). The atrophic left optic tract had significantly decreased fractional anisotropy and in-creased mean diffusivity, consistent with chronic degen-eration (Fig. 5). The left optic radiation had mildly decreased fractional anisotropy and increased mean diffu-sivity that were not statistically significant. FIG. 3. Continguous, noncontrast T1 coronal magnetic resonance imaging of the anterior visual pathways. Arrows denote optic nerves (A), optic chiasm (B), and optic tracts (C and D). The left optic tract is atrophic. FIG. 4. Diffusion tensor imaging tractography of optic tracts (red arrows) and optic radiations (green arrows) shown in oblique (A), coronal (B), and axial (C) projections demonstrates atrophy of the left optic tract. The majority of left optic radiation fiber tracts generated from the fiber tracking algorithm have nonanatomical fiber trajectories proximal to the lateral geniculate nucleus (yellow arrows), presumably from dominant crossing fibers secondary to left optic tract atrophy. Fiber tracts were constructed from regions of interest placed at ends of either optic tract (chiasm and lateral geniculate nucleus [LGN]) or optic radiation (LGN and occipital lobe); some of the shorter tracts are specific for optic tract or optic radiation. Nonanatomical tracts were removed, which accounts for the fewer fiber tract on the left side due to crossing fibers dominating at the left LGN secondary to right optic tract atrophy. On the normal right side, the majority of fiber tracts are continuous through the LGN from the optic tract to optic radiation because the fiber tracts are mathematical constructions that follow principal diffusion eigenvector flow lines and do not demonstrate that axons synapse in the LGN. 96 Letters to the Editor: J Neuro-Ophthalmol 2014; 34: 95-104 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. We believe that the DTI results in this case are consistent with a posttraumatic optic tract lesion. Diffusion of water in tissue is affected by the presence of semiperme-able membranes and oriented structures in the intracellular, extracellular, and vascular compartments that result in preferential movement of water parallel to them. Anisotropy describes the directional dependence of diffusion, and white matter (e.g., optic tract) has relatively high anisotropy because longitudinal structures such as myelin, axonal membranes, and subcellular organelles contained in highly organized fiber bundles along white matter tracts contribute as barriers to water movement perpendicular to these structures. Fractional anisotropy is an index of anisotropy measuring the degree of diffusion differences in different directions. Decreased fractional anisotropy along white matter tracts can be from either reduced axial or increased radial diffusivity and should be correlated with mean diffusivity in determining changes in the underlying microstructural architecture. The combination of decreased fractional anisotropy and increased mean diffusivity in this case is likely due to chronic degeneration with membrane disruption, cell lysis, and gliosis. Atrophy of the optic tract often is difficult to detect with conventional neuroimaging in patients with an unexplained homonymous hemianopia. In our patient, previous MRI had been interpreted as normal, but more detailed analysis including DTI confirmed left optic tract atrophy. Clinicians should be aware of DTI as a diagnostic aid in evaluating patients with optic tract syndrome. Nagham Al-Zubidi, MD Department of Ophthalmology, The Methodist Hospital, Houston, Texas, Departments of Ophthalmology, Neurology, and Neurosurgery, Weill Cornell Medical College, Houston, Texas Waseem Ansari, BS The University of Texas Medical School at Houston, Houston, Texas Steve H. Fung, MD Department of Radiology, The Methodist Hospital, Houston, Texas, Department of Radiology, Weill Cornell Medical College, Houston, Texas Andrew G. Lee, MD Department of Radiology, The Methodist Hospital, Houston, Texas, Department of Radiology, Weill Cornell Medical College, Houston, Texas, FIG. 5. Diffusion tensor imaging gray scale and color directional fractional anisotropy maps through the levels of the optic tracts (red arrows and regions of interest [ROI] overlay) and optic radiations (green arrows and ROI overlay). The gray scale map shows decreased fractional anisotropy along the left optic tract. The color map shows fiber orientation in 3-dimensional space (red color for the x axis along right-left orientation, green color for the y axis along anterior-posterior orientation, and blue color for the z axis along cranial-caudal orientation). Letters to the Editor: J Neuro-Ophthalmol 2014; 34: 95-104 97 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. The University of Texas Medical School at Houston, Houston, Texas, Department of Ophthalmology, The University of Iowa Hospitals and Clinics, Iowa City, Iowa, Department of Ophthalmology, The University of Texas Medical Branch, Galveston, Texas, [e-mail] Supported in part by an unrestricted grant from Research to Prevent Blindness (RPB). The authors report no conflicts of interest. REFERENCES 1. Bodanapally UK, Kathirkamanathan S, Geraymovych E, Mirvis SE, Choi AY, McMillan AB, Zhuo J, Shin RK. Diagnosis of traumatic optic neuropathy: application of diffusion tensor magnetic resonance imaging. J Neuroophthalmol 2013;33:128-133. 2. Istoc A, Habas C, Iba-Zizen MT, Nguyen TH, Abanou A, Yoshida M, Bellinger L, Le Gargasson JF, Cabanis EA. Value of the functional neural tractography in the reconstruction of the visual pathways in DTMRI [in French]. J Fr Ophtalmol 2010;33:670-679. 3. Kier EL, Staib LH, Davis LM, Bronen RA. MR imaging of the temporal stem: anatomic dissection tractography of the uncinate fasciculus, inferior occiptofrontal fasciculas, and Meyer's Loop of the optic radiation. AJNR Am J Neuroradiol 2004;25:677-691. 4. Hofer S, Karaus A, Frahm J. Reconstruction and dissection of the entire human visual pathway using diffusion tensor MRI. Front Neuroanat 2010;4:15. 5. Wang R, Benner T, Sorensen AG, Wedeen VJ. Diffusion Toolkit: a software package for diffusion imaging data processing and tractography. Proc Intl Soc Mag Reson Med 2007;15:3720. Restricted Diffusion in Isolated Superior Ophthalmic Vein Thrombosis Superior ophthalmic vein (SOV) thrombosis is a rare disease entity characterized by rapidly progressive orbital symptoms, including periorbital edema, chemosis, propto-sis, and ophthalmoplegia. It may be isolated or accompanied by cavernous sinus thrombosis. Parmar et al (1) reported the value of diffusion-weighted imaging (DWI) in establishing this diagnosis, and we describe our experience with this disorder. FIG. 1. Orbital magnetic resonance imaging. Postcontrast T1 axial (A) and coronal (B) scans show diffuse enhancement of preseptal and retrobulbar regions of the left orbit, and absence of normal enhancement within the left superior ophthalmic vein (arrows). Diffusion-weighted imaging (C) reveals hyperintense signal in the left superior ophthalmic vein (arrows) and apparent diffusion coefficient map (D) shows a corresponding hypointense signal (arrow) confirming restricted diffusion. 98 Letters to the Editor: J Neuro-Ophthalmol 2014; 34: 95-104 Letters to the Editor Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.