Ocular blood flow measurement.

Many techniques have been devised to measure the haemodynamics of the human and animal eye. In this perspective these are outlined and their use in ophthalmic investigation summarised. Some have exploited the ability of an observer to directly visualise the retinal vasculature by optical means, others have been designed to study the haemodynamics ofthe invisible parts of the eye such as the choroid, optic nerve head, and ciliary body. Although useful in ophthalmic investigation, none have satisfied all of the requirements of the researchers in this field and most have not achieved regular use in clinical practice. In any examination of blood flow a multitude of variables must be studied (Table 1). The interrelation ofthese variables must be determined while considering physical or physiological principles (Table 2) which are often not strictly applicable to the vasculature for example, the Hagen Poiseuille law was described for a rigid tube and not for elastic walled tubes such as blood vessels. In the human, study ofthe circulation is further hindered by the requirement for a noninvasive and safe method for obtaining measurements. Study of the haemodynamics must be performed if we are to understand the mechanisms leading to the large variety of vascular diseases which affect the eye. The blood flow to the eye is of particular interest because: (1) Many localised and systemic disorders affect the vasculature of the eye. (2) The eye has unusual haemodynamic properties because the tissues are subjected to a high intraocular pressure. (3) Ocular blood flow is autoregulated for example, during changes in retinal illumination, blood pressure, or posture. (4) Pharmacological agents which are routinely used in systemic and ocular diseases may affect the blood supply of the eye.