There is current interest in determining whether reduced ocular blood flow is partly responsible for various eye diseases, such as glaucoma and age related macular degeneration. Proponents of the vascular pathogenetic hypothesis of open angle glaucoma (OAG) claim that chronic or intermittent decrease in blood flow to the optic nerve is partially responsible for the glaucomatous optic neuropathy. For example, the incidence of OAG has been reported to be higher in patients with lower perfusion pressure (blood pressure minus IOP).11 Progression of visual field loss has been related to greater than normal nocturnal decrease in systemic blood pressure.12,13 Reduced blood flow has been observed by various methods in larger ocular arteries and nerve head microcirculation in patients with POAG and NTG.14–17
Accordingly, there is much interest in measuring ocular blood flow in humans non-invasively. The OBF tonometer reflects the main—pulsatile—portion of the choroidal blood flow, which in turn is the main contributor of ocular blood flow.18,19 Thus, it may be used to study total ocular blood flow and blood flow to the optic nerve head.
We found a strong correlation between severe stenosis of the ICA and the inability of the OBF machine to record the pulsatile IOP. In certain situations, listed in the operating manual of the OBF tonometer, the machine may not record the pulsatile IOP of the eye. These include cardiac arrhythmia, poor patient fixation, dry eyes, and nystagmus. According to our results, significant stenosis of the ICA may be added to this list. After ruling out the above situations, in 64.7% of cases with significant ICA stenosis OBF could not be recorded. Presumably, the pulsatile component of blood flow to these eyes is severely reduced. Further research is needed to confirm this assumption and examine its implications for ocular disease.
Even more significant from a clinical standpoint, when OBF could not be recorded, there was a 79% chance of significant ICA stenosis on the ipsilateral side. This figure undoubtedly reflects the fact that our patients were recruited from the vascular ultrasound clinic and is higher than would be expected in the general population. Nevertheless, clinicians should consider performing cardiovascular examinations, specifically carotid ultrasound, in patients in whom pulsatile OBF can not be measured. Since the use of the OBF machine is fast and straightforward, it might even be incorporated into a screening programme of cardiovascular disease in high risk populations.
When we analysed only those eyes with recordable POBF, there was no correlation between POBF and ipsilateral ICA stenosis. Presumably, ocular blood flow is controlled by local autoregulation, and in cases of chronic severe stenosis may be maintained by collateral circulation. In this regard, it is interesting that Harris and co-workers reported that in glaucoma patients with stable visual fields, nocturnal ocular blood flow remains unchanged despite dips in systemic blood pressure.20,21 In addition, invasive animal studies have shown that retinal and choroidal blood flow is maintained when systemic blood pressure or IOP are changed.22,23
The clinical relation between POBF and extra cranial blood vessels has not been thoroughly studied. There is one case report where an inter-eye difference in measured PA was attributed to a stenotic innominate artery, with equal values measured following the dilation of the stenotic vessel.24 Two cases were reported by Clarideg and James where POBF was measured before and after surgery for ICA stenosis.25 In one case POBF was unrecordable before surgery and 173 µl/min postoperatively. In the second case POBF increased significantly from 280 to 758 µl/min. In our study we observed the normalisation of POBF in five cases following ICA surgery after being unrecordable preoperatively.
POBF values in our study were somewhat higher than previously reported in healthy eyes. Mori et al reported POBF values ranging from 290.7–1201.6 µl/min with a mean of 593.3 µl/min in 80 normal subjects using the Langham OBF tonometer (Langham Ophthalmic Technologies, Timonium, MD, USA).26 Yang et al reported a range of 306–1645 with a mean of 775.9 µl/min in 83 healthy eyes using the OBF tonograph (OBF Laboratories, UK Ltd, Wilts, UK) which is similar to the one we used.3 POBF values in the present study ranged 667–2095 with a mean of 970.72 µl/min.
In conclusion, we have demonstrated that inability to record POBF in healthy eyes may be related to severe stenosis of the ICA, and that severe ICA stenosis may signify a marked reduction in the pulsatile component of ocular blood flow. Clinicians using the OBF tonometer should be aware of this association and consider its implications for the general health of their patients. The significance of carotid artery stenosis to ocular blood flow in health and disease requires further investigation.