Retinal fluorescence lifetime imaging ophthalmoscopy measures depend on the severity of Alzheimer's disease

Purpose: To determine alterations in the retina of patients with Alzheimer's disease (AD) by the newly developed technique of fluorescence lifetime imaging ophthalmoscopy (FLIO) in a pilot study. Methods: FLIO set-up uses a scanning laser ophthalmoscope (HRA2, Heidelberg Engineering, Germany), which was modified by the use of an excitation pulse laser BLD440 (Becker&Hickl, Berlin, Germany) and detection of fluorescence lifetime by time-correlated single photon counting (TCSPC; Becker& Hickl) in two spectral channels (channel 1: 490-560 nm, channel 2: 560-700 nm). Least square fit of three exponential functions was used for fluorescence decay analysis. That resulted in three fluorescent components with lifetimes tau(i), amplitudes alpha(i) and relative contributions Q(i). 16 patients with AD (mean age 77.2 +/- 7.0 years) were investigated. After regular ophthalmic investigation, FLIO examination and OCT examination were performed. Alzheimer-specific clinical data were collected (MMSE, cerebrospinal fluid (CSF) concentration of amyloid-beta (1-42), total-tau and phosphorylated tau181 (p-tau181) protein). Results: The FLIO parameters of the second fluorescent component alpha(2) and Q(2) (channel 2) correlated significantly with MMSE score (Q(2), R = -0.757, p = 0.007; alpha(2), R = -0.618, p = 0.043) as well as p-tau181-protein concentration in CSF (Q(2), R = 0.919, p = 0.009; alpha(2), R = 0.881, p = 0.020) in patients with AD. OCT measurements of retinal nerve fibre layer thickness, optic disc excavation and macular thickness neither correlated with Alzheimer-specific CSF data nor MMSE score. Conclusions: Unlike conventional techniques, such as OCT, the new technique of FLIO revealed changes in the retina of patients with AD in relation to Alzheimer-specific markers in this pilot study.