Residual Visual Responses in Patients With Retinitis Pigmentosa Revealed by Functional Magnetic Resonance Imaging

Abstract

PurposeWe evaluated the potential of magnetic resonance imaging in identifying signs of cortical visual processing with greater sensitivity than standard ophthalmological measures in patients with retinitis pigmentosa (RP) at advanced stages.MethodsEight patients affected with RP with only bare light perception and weak or absent visual evoked potential (VEP) or electroretinogram (ERG) responses to flashes of light were tested. Visual impairment was evaluated by means of psychophysical testing, where patients were asked to discriminate the drifting direction of a contrast modulated grating. Patients underwent magnetic resonance imaging scanning, and the behavioral performance was correlated with both blood oxygenation level-dependent (BOLD) signal elicited by flashes of lights and cortical thickness measured in primary visual area.ResultsContrast sensitivity to drifting gratings of very low spatial and temporal frequency was greatly impaired, yet measurable in all patients. Weak luminance flashes elicited significant BOLD responses in the striate and extrastriate cortex, despite that the stimuli were not perceived during scanning. Importantly, patients with less severe impairment of contrast sensitivity showed stronger V1 BOLD responses. Striate cortical thickness did not correlate with visual sensitivity.ConclusionsBOLD responses provide a sensitive and reliable index of visual sparing more than VEPs or ERGs, which are often absent in RP patients. The minimal residual vision can be assessed by optimal visual stimulation in two alternative forced choice discrimination tasks and by BOLD responses. Imaging techniques provide useful information to monitor progressive vision loss.Translational RelevanceFunctional magnetic resonance imaging might be a practical tool for assessing visual sparing, as it is more feasible and sensitive than psychophysical or ophthalmological testing.

Visual task-related functional and structural magnetic resonance imaging for the objective quantitation of visual function in patients with advanced retinitis pigmentosa

“…In this study, we suggested that fMRI was a feasible method to objectively and quantitatively discriminate FP and NFP patients with advanced RP. An fMRI study has shown that severe impairment of RP had fewer V1 BOLD responses in contrast sensitivity visual tasks (Castaldi et al, 2019), another finding that was consistent with our study. However, due to the limited number of subjects, the study did not report a difference in different levels of patients with advanced RP, whereas our study showed that the response intensity of participants in the NFP group was significantly lower than that of participants in the FP group.…”

Structural evaluation in inherited retinal diseases

“…Magnetic resonance imaging (MRI) affords the ability to obtain high resolution structural images of the visual cortex and also sensitively record associated responses (functional MRI). 122 Importantly, several MRI studies have described a degree of plasticity/remodelling following visual field defects in RCD or MD, and also with congenital visual impairment, including in ACHM. [123][124][125] The remapping of the primary visual cortex (V1) consists of a shift of central retinal inputs to more peripheral locations in V1, and this phenomenon was found to be larger in patients with more constricted visual fields.…”

Robust visual cortex evoked potentials (VEP) in Gnat1 and Gnat2 knockout mice

“…If melanopsin was sufficient for visual function under bright enough conditions, one would expect to have identified this phenomenon in certain individuals affected by various retinal pathologies, some of which largely spare ipRGC populations despite outer retinal degeneration ( Semo et al, 2003 ; Cui et al, 2015 ; Tran et al, 2019 ; Procyk et al, 2022 ). Although there is some evidence for residual melanopsin-mediated activity in such individuals ( Castaldi et al, 2019 ), it is unclear whether any of this activity can even be perceived. It seems more likely that melanopsin, rather than directly extending the dynamic range of the retina to extremely bright intensities, instead primarily mediates a range of functions that serve to accentuate and fine-tune certain aspects of pattern-forming vision.…”