EXERCISE-INDUCED ACUTE CHANGES IN SYSTOLIC BLOOD PRESSURE DO NOT ALTER CHOROIDAL THICKNESS AS MEASURED BY A PORTABLE SPECTRAL-DOMAIN OPTICAL COHERENCE TOMOGRAPHY DEVICE

The ability to image the choroid successfully using optical coherence tomography (OCT) has not been possible until recently because of its location and the attenuation of the OCT signal by the retinal pigment epithelium (RPE). Recent studies1-5 report successful examination and measurement of choroidal thickness in healthy and disease states using the Heidelberg Spectralis (Heidelberg Engineering, Heidelberg, Germany), RTVue (Optovue, Inc, Fremont, CA), and Cirrus HD-OCT (Carl Zeiss Meditec, Inc, Dublin, CA) spectral-domain (SD) OCT instruments. The choroid serves many functions. It is responsible for supplying oxygen and nutrients to the RPE and the retina up to the inner nuclear layer. It is also believed to play a role in temperature regulation and waste product removal.6 Thus, a structurally and functionally normal choroidal vasculature is essential for the function of the retina. Abnormal choroidal blood volume or compromised flow or both can result in photoreceptor dysfunction and death.7 The choroid circulation displays one of the highest rates of blood flow in the body.8 In fact, per tissue gram, the choroid has four times more blood flowing through it than does the cortex of the kidney.9 Most of the blood in the eye is found in the choriocapillaris.8 In a vascular disease such as diabetes, the choroid shows evidence of degeneration, as suggested by previous studies.7 The choroicapillaris layer is highly innervated by both sympathetic and parasympathetic nerves.10 These nerves are thought to play a role in the regulation of choroidal blood flow.8 In addition to sympathetic regulation, autoregulation is another mechanism by which blood flow can be controlled.8 Moreover, there are nonvascular smooth muscles in the choroid that are thought to play a role in altering the thickness of the choroid under the influence of local mediators.11 Little is known about the regulation of choroidal thickness and its role in different diseases. There are vasoactive substances both endogenous and exogenous that have been shown to alter the thickness of the choroid.8 Vance et al12 demonstrated that sildenafil citrate ingestion produced a significant increase in choroidal thickness, as measured by SD-OCT. Intravenous acetazolamide increases choroidal thickness,13 while nitric oxide released locally has been shown to play a role in increasing choroidal thickness by relaxing both vascular and nonvascular smooth muscles.14 There are a number of other vasoactive substances with potential influence on the choroid, and their role in disease states is still to be elucidated.15 The choroid thickness can be altered by at least 3 mechanisms: 1) local signaling that either stimulates or inhibits the fluid pumping capacity of the RPE and therefore altering the amount of fluid flux from the retina into the choroid16; 2) an increase or decrease in vascular permeability of the choroicapillaris that affects the amount of protein that leaks out of these vessels, which in turn changes the oncotic pressure in the choroid17; and 3) dilation or constriction of both the vascular and nonvascular smooth muscles.18 Previous studies19,20 have demonstrated that increases in blood pressure (BP) induced by isometric exercise were not linearly associated with an increase in ocular perfusion indicating a degree of autoregulation. The present study was designed to determine whether acute changes in systemic BP after exercise alter choroidal thickness.