Optic Nerve Tractography Prediction of Visual Deficit in Pituitary Macroadenoma
The presentation of nonfunctioning pituitary adenomas is frequently delayed due to the absence of a hormonal syndrome, and tumors have often attained a large size by the time of diagnosis. Pronounced compression of the optic chiasm can be evident on imaging, and may be associated with deficits in visual acuity and visual fields. However, the severity of visual symptoms varies widely among patients, and some retain intact visual function despite dramatic optic chiasm deformation. MRI features including optic nerve atrophy and vertical tumor extension, and other modalities such as optical coherence tomography, have not been found to reliably predict visual deficits or recovery of visual function following pituitary tumor resection. Diffusion tensor imaging (DTI) utilizes the principle of anisotropic water diffusion along white matter fibers to reconstruct fiber tracts. Tractography has long been used to delineate large fiber bundles in glioma surgery, but the feasibility of tracking small fiber bundles including cranial nerves has also recently been demonstrated. The optic nerves, chiasm, optic tracts and optic radiations can be displayed to a high degree of detail using DTI. Since processes such as inflammation and demyelination affect water anisotropy, DTI parameters can detect pathologic changes in axonal properties. Patients with a new diagnosis of pituitary adenoma causing compression of the optic chiasm undergo neuro-opthalmologic evaluation including assessment of visual acuity and Humphrey visual field testing. Thin-slice imaging of the sellar region including T1-weighted anatomic imaging and diffusion-weighted imaging (DWI) scans are obtained on 3T MRI, and processed using 3D Slicer software. Neuro-opthalmologic and imaging evaluation are repeated post-operatively in patients who undergo surgical resection of the pituitary tumor. DTI parameters including fractional anisotropy, radial diffusivity and axial diffusivity are measured and correlated to visual findings. The ability to reliably correlate non-invasive imaging with visual function in the setting of optic chiasm compression may allow for accurate identification of patients who will develop visual deficits, and who therefore require more urgent surgical intervention. Such imaging may also be valuable in predicting recovery of visual function following decompressive surgery.