Objective: To determine whether different mechanisms could lead to the emergence of visual hallucinations (VH) (defined as false perceptions with no external stimulus) versus illusions (VI) (defined as a misperception of a real stimulus) in Parkinson’s disease (PD), we assessed retinal, clinical and structural brain patterns depending on the presence of VH or VI in PD.

Background: Visual hallucinations (VH), defined as false perceptions in the absence of an external stimulus, can range from minor to complex in PD. Whilst Visual illusions (VI), defined as misperception of a real stimulus, are generally regarded on a spectrum as being part of the minor VH, it remains unclear whether VI and VH are different entities with distinct mechanisms. In particular, the specific relationship between oculo-visual abnormalities observed in PD and the emergence of VH or VI remains poorly understood. Retinal thinning is generally observed in PD and could be greater in patients with VH, but no study has specifically studied retinal thinning or oculo-visual abnormalities in PD patients with isolated VI.

Method: In this case-control study, we compared retinal thickness using optical coherence tomography (OCT), between PD patients with: VI (PD-I; n=28), VH (PD-H; n=28), and without VI or VH (PD-C; n=28), matched for age and sex, and assessed demographic data, disease severity, treatment, anatomical and functional visual complaints, cognitive and visuo-perceptive functions and MRI brain volumetry for each group of PD patients.

Results: Perifoveal retina was thinner in PD-H compared to PD-C (p=0.005) and PD-I (p=0.009) but did not differ between PD-I and PD-C (p=0.85). Multivariate analysis showed that 1/ retinal perifoveal thinning and total brain gray matter atrophy were independently associated with the presence of VH compared to PD-I;  2/ retinal perifoveal thickness, PD duration, sleep quality impairment and total brain gray matter volume were independent factors associated with the presence of VH compared to PD-C; 3/ anterior ocular abnormalities were the only factor independently associated with the presence of illusions compared to PD-C.

Conclusion: These findings reinforce the hypothesis that there may be different mechanisms contributing to VH and VI in PD, suggesting that these two entities may also have a different prognosis rather than simply lying along a continuous spectrum.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/visual-hallucinations-and-illusions-in-parkinsons-disease-the-role-of-ocular-pathology/