Optical coherence tomography in mild cognitive impairment - Systematic review and meta-analysis

Thinning of retinal layers, measured using optical coherence tomography (OCT), is associated with some neurodegenerative disorders such as established Alzheimer's disease and multiple sclerosis. The evidence for retinal layer thinning in both mild cognitive impairment (MCI), a precursor of dementia, and delirium, a potential pre-clinical stage of neurodegenerative disorder, is unclear. We performed a systematic review of the associations, in older people, between retinal layer thickness changes (measured using OCT) and delirium or MCI compared to controls (Protocol registration ID (Prospero) CRD42019122165). We did not identify any relevant studies on delirium. This report is therefore a review of retinal nerve layer changes in mild cognitive impairment. Databases were searched using predetermined keywords such as mild cognitive impairment, retinal nerve fibre layer and delirium. Where there were sufficient data, meta-analyses were performed. Twenty-six relevant studies were identified on retinal layer thickness in people with MCI compared to controls. There was significant heterogeneity in the studies for all retinal layers investigated (retinal nerve fibre layer (RNFL), ganglion cell inner plexiform layer (GCIP), foveal thickness and macular volume). Analysis of 17 studies of mean RNFL thickness in MCI (n = 622) compared to controls (n = 1154), irrespective of the type of OCT device, demonstrated a significant thinning in MCI (SMD: - 0·42 and 95 % confidence interval: - 0·68 to - 0·16). This difference was non-significant when studies using only spectral-domain devices were analysed. Subgroup analysis of studies using spectral-domain devices in amnestic MCI diagnosed using comparable criteria, showed statistically significant thinning of RNFL in amnestic MCI (p = 0·02). Meta-analysis of foveal thickness did not show a significant difference between MCI and controls. In conclusion, there is some evidence of an association between retinal nerve fibre layer thinning and MCI. We found no data on the association between RNFL and delirium.