Aspects of control of the retinal and choroidal circulations in normal and diabetic human subjects

This thesis examines aspects of the control of the retinal and choroidal circulations, with particular reference to autoregulation and to regulation mediated by the autonomic nervous system. Handgrip isometric exercise was used to induce an acute rise of mean arterial pressure (MAP) in the region of 20%. The response of the human retinal circulation to acute MAP elevation by isometric exercise was investigated in 9 normal subjects. Autoregulation was demonstrated by an 8.4% rise in flow in response to a 34% rise in retinal perfusion pressure (p=0.0007). The variable in velocity change derived from veins (SD 3.4%) was lower than that from arteries (SD 12.1%), which makes venous measurements more useful for obtaining statistically reliable results from these techniques. The response of the diabetic human retinal circulation to acute MAP elevation by isometric exercise was investigated in 19 diabetic subjects. Measurements were performed at normoglycaemia and hyperglycaemia. The conductance of the retinal circulation, defined as flow/perfusion pressure, was calculated. Conductance was significantly elevated at hyperglycaemia (p0.007). Conductance decreased significantly (p<0.05) in response to elevation of blood pressure both at normoglycaemia and hyperglycaemia. The conductance change per unit perfusion pressure change was significantly smaller in diabetics than in normal subjects. Autonomic neuropathy had no effect on conductance nor on the change in conductance in response to isometric exercise.21 diabetic subjects and 13 control subjects had intraocular pressure (IOP) measured at rest and again during acute MAP elevation by isometric exercise. In normal subjects there were no detectable changes in IOP. There was a rise in IOP in a significant proportion of diabetic subjects, both at normoglycaemia (p=0.007) and hyperglycaemia (p=0.002).