Physiological Correlates and Predictors of Functional Recovery After Chiasmal Decompression

Abstract

Background: The intrinsic abilities and limits of the nervous system to repair itself after damage may be assessed using a model of optic chiasmal compression, before and after a corrective surgical procedure.

Methods: Visual fields (VFs), multifocal visual evoked potentials (mfVEP), retinal nerve fiber layer (RNFL) thickness, and diffusion tensor imaging were used to evaluate a patient before and after removal of a meningioma compressing the chiasm. Normally sighted individuals served as controls. The advantage of each modality to document visual function and predict postoperative outcome (2-year follow-up) was evaluated.

Results: Postsurgery visual recovery was best explained by critical mass of normally conducting fibers and not associated with average conduction amplitudes. Recovered VF was observed in quadrants in which more than 50% of fibers were identified, characterized by intact mfVEP latencies, but severely reduced amplitudes. Recovery was evident despite additional reduction of RNFL thickness and abnormal optic tract diffusivity. The critical mass of normally conducting fibers was also the best prognostic indicator for functional outcome 2 years later.

Conclusions: Our results highlight the ability of the remaining normally conductive axons to predict visual recovery after decompression of the optic chiasm. The redundancy in anterior visual pathways may be explained, neuroanatomically, by overlapping receptive fields.

Figure 1

Testing results over a 2 year period following removal of a tuberculum sellae meningioma. mfVEP, multifocal visual evoked potential; SD-OCT, spectral- domain optical coherence tomography.

Figure 2

Diffusion tensor imaging and fiber tractography. Left: the optic tracts delineated in the patient pre-operatively and four months following surgery. Right: FA values along the optic tracts in control subjects (n=17, dark blue plots), patient prior to surgery (pale blue plots) and during the post-surgical period (light blue plots). Error bars represent standard deviation. Red dotted line represents the border of the meningioma. FA, fractional anisotropy.