Quantitative Assessment of Secondary White Matter Injury in the Visual Pathway by Pituitary Adenomas: A Multimodal Study at 7 Tesla MRI

Objective: Pituitary adenomas are known to comprise the optic apparatus, and the evaluation of vision loss for these patients continues to be a clinical and diagnostic challenge. Diffusion-weighted 7 Tesla MRI (dMRI) and structural imaging were used to evaluate secondary damage of the visual pathway by pituitary adenomas. dMRI is routinely acquired in the clinic and allows for mapping of white matter microstructure in vivo. Imaging findings were correlated with neuro-ophthalmological results to assess the utility of ultra-high-field MRI for objective evaluation of damage to the anterior and posterior visual pathways.

Methods: Probabilistic tractography using 7 Tesla dMRI (1.05-mm isotropic) was used to reconstruct the optic tracts and radiations in 18 patients with pituitary adenomas and in 16 healthy volunteers. MP2RAGE (0.7-mm isotropic) and T2-TSE (0.4 × 0.4 × 2 mm) sequences were also acquired to obtain high-resolution structural imaging. Diffusion indices were calculated along the tracts and radiations and correlated with tumor volumes and results from neuro-ophthalmological examinations. Neuroophthalmology included pattern standard deviation (PSD), mean deviation, and retinal nerve fiber layer (RNFL) thickness. Optic nerve sheath diameters (ONSD) and primary visual cortical (V1) thicknesses were also assessed.

Results: Fractional anisotropy (FA) was reduced by 21.9% in the optic tracts (p < 0.001) and 17.7% in the optic radiations (p < 0.001) in patients with adenomas. Patients showed an 8.5% increase in mean diffusivity (MD) of optic radiations compared with healthy controls (p < 0.001). PSD scores positively correlated with tumor volume (r = 0.89, p = 0.004), and MD of the optic tracts (r = 0.87, p = 0.006) and radiations (r = 0.69, p = 0.05). PSD was anti-correlated with FA of the tracts (r = −0.73, p = 0.05). Mean deviation positively correlated with FA of the tracts (r = 0.81, p = 0.016), and negatively correlated with MD of the tracts (r = −0.88, p = 0.008). V1 thickness was reduced by 6.4% (p = 0.007) and ONSD was increased by 18.2% (p = 0.01) in adenoma patients.

Conclusion: We demonstrated significant damage in otherwise normal-appearing white matter of the anterior and posterior visual pathways in pituitary adenoma patients. These microstructural findings, in addition to the anatomical changes to the optic nerve sheath and V1 may reflect transsynaptic degeneration in the visual pathways resulting from chiasmal compression. Imaging-based quantification of secondary neuronal damage strongly correlated with neuro-ophthalmological findings. Diffusion-weighted imaging of the visual pathways offers a promising method for determining secondary effects of chiasmal compression and identifying tumor-related degeneration in otherwise healthy-appearing structures. These findings may be useful in (1) objective grading of existing damage to the visual pathway to determine indication for surgical decompression and (2) predicting the considerable variability in visual recovery postsurgery, serving as a preoperative marker of vision recoverability for patients with pituitary adenomas.