Visual function and retinal nerve fibre layer degeneration in patients with Alzheimer disease: correlations with severity of dementia.

Abstract

In response to the Letter to the Editor by Tas et al. (2014), we would like to add some new information from our study in patients with Alzheimer disease (AD). In a previous article (Bambo et al. 2014), we described retinal nerve fibre layer (RNFL) thickness differences between 57 patients with mild–moderate AD and 57 healthy controls based on the two most commonly available spectral-domain optical coherence tomography (SD-OCT) devices. Significant thinning of the superior and inferior RNFL in the Cirrus OCT (Carl Zeiss, Meditec, Inc., Dublin, Ireland) and significant thinning of the inferior and infero-temporal RNFL in the Spectralis OCT (Heidelberg Engineering, Inc., Carlsbad, CA, USA) were observed. Visual function tests were performed in both groups, including best-corrected visual acuity with an ETDRS chart, contrast sensitivity with a CSV-1000E test and Pelli-Robson chart, and colour vision using Farnsworth 15D and Lanthony 15D tests of Vision Color Recorder software (Optical Diagnosis, Inc., Beusichem, the Netherlands). Correlations between visual function results and RNFL thicknesses with the severity of the AD (measured by Mini-Mental State Examination [MMSE] score and disease duration) were tested in the AD patient group using Pearson's correlation analysis. We found moderate negative correlations between the Pelli-Robson and CSV 1000E results with disease duration (−0.586; p = 0.017 and −0.565; p = 0.028). Risacher et al. (2013) reported that contrast sensitivity disturbances are present even in the early stages of AD (even in mildly cognitively impaired patients); they further demonstrated a correlation between contrast sensitivity (the FDT-2 24-2 VF contrast sensitivity test) and the MMSE score. We also found a strong significantly positive correlation between the Colour Confusion Index (CCI) with the Lanthony 15D test and disease duration (0.973, p = 0.027). The CCI was developed by Bowman (1982) and assesses the severity of dyschromatopsia. A CCI score higher than 1 indicates altered colour vision perception; the higher the score, the worse the condition. Other authors, such as Pache et al. (2003), revealed colour vision impairments in mild to severe patients with AD compared with a control group using Ishihara charts and the PV-16 test, but found no statistical correlation. The Lanthony 15D test, however, is more sensitive than other tests for detecting colour vision impairment. On the other hand, only superior RNFL thickness with the classic glaucoma application of the Spectralis OCT device was significantly negatively correlated with disease duration (−0.551; p < 0.001). Other tests revealed only a slight correlation. In conclusion, we found significant correlations between visual function tests and RNFL thickness with the severity of AD (measured by disease duration). The correlations between the MMSE score and different visual function tests, however, were slight. Colour vision impairments are strongly correlated with disease duration; at this point, therefore, colour vision impairment could be a good biomarker for diagnosis and follow-up of AD. Gundogan et al. (2007) also found that colour vision assessment is better correlated than RNFL thickness with disease activity in other neurodegenerative diseases, such as multiple sclerosis. We only recruited patients with mild to moderate AD, so more studies including patients with mild cognitive impairment and severe AD and with a larger sample size are necessary. Visual function tests, especially colour vision assessment, seem to be more related with neurodegenerative disease progression than other digital imaging techniques such as OCT. Visual impairments are often among the earliest complaints of patients with AD; accordingly, visual function tests (especially colour vision assessment) could facilitate evaluation of suspected AD.