Choroidal blood flow in AMD.

In a recent article in this journal, Grunwald et al. 1 quantified foveolar choroidal blood flow using a Laser Doppler flowmeter in patients with nonexudative age-related macular degeneration (AMD). They found that choroidal blood volume and choroidal blood flow in patients with AMD were significantly reduced. However, they found no statistically significant difference for the choroidal blood velocity. In interpreting the results of this study, it has to be kept in mind that laser Doppler flowmetry cannot be used for absolute blood flow measurements because of uncertainty in knowing the volume sampled by the incident laser light and the multiple lightscattering properties of tissue. This results in a lack of specificity in volume examined that is caused by the variable penetration depths of the laser. It is known that eyes with AMD display a thick layer of basal laminar (linear) deposit (BLD). It cannot be excluded that this thick layer of tissue—with unknown optical properties— overlying and within Bruch's membrane affects the sampled volume and alters the scattering behavior of the laser light and thus changes the "Doppler shift power spectrum." This may, after all, result in an inaccurate measurement of flow, which is estimated using the "velocity" and the "volume."