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G M Baxter, T H Williamson, G McKillop, G N Dutton; Color Doppler ultrasound of orbital and optic nerve blood flow: effects of posture and timolol 0.5%.. Invest. Ophthalmol. Vis. Sci. 1992;33(3):604-610.
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Color Doppler ultrasound allows simultaneous imaging with real-time ultrasound and superimposed color-coded vascular flow, allowing visualization of vessels previously beyond the resolution of conventional imaging, such as those in the orbit. With this technique, 20 healthy volunteers were studied. Three regional vessels named 1, 2, and 3 were identified. No significant difference in maximum or minimum blood velocity or resistive index was detected between vessels 1 and 2, although significant differences were noted between both these vessels and vessel 3 (P less than 0.01 and P less than 0.001, respectively). These regional variations are unaffected by small but significant rises in pulse (P less than 0.05) and diastolic blood pressure (P less than 0.01) induced by postural change. No significant change within each vessel was recorded in response to posture, reflecting autoregulation within these vessels. Using a similar technique, 10 healthy volunteers were studied at baseline and at 2 hr and 3 d following the unilateral instillation of 0.5% timolol eye drops. A fall in resistive index was recorded in vessel 3 for both eyes (P less than 0.05, timolol administered eye; P less than 0.01 timolol-free eye). This effect was independent of any simultaneous fall in intraocular pressure that occurred only in the eye receiving timolol drops (P less than 0.01). These results support the presence of B receptors in the vessels at the optic nerve head (vessel 3). A fall in resistive index should not compromise the blood supply in this region, and may even increase it.(ABSTRACT TRUNCATED AT 250 WORDS)
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