A new look at ophthalmodynamometry
It was with great interest that I read the article by Jonas.1 It is indeed noteworthy that efforts are being undertaken for reviving ophthalmodynamometry. This is especially pertinent as this method is, despite the many modern techniques published within the past 20 years, the only way to gain insights regarding the pressure present at the eye’s central retinal vessels. A direct measuring device has demonstrated the usefulness of this examination.2 Nevertheless, Jonas’s recent article raises some questions that must be clarified in light of additional articles that will soon be published by the same author as well as for those who are no longer familiar with ophthalmodynamometry.
The device, as it is described, appears to measure the appositional force exerted by the examiner’s hand on the contact lens placed upon the eye. One has to clearly distinguish between the appositional force and any pressure within the eye itself. In no way does the former reflect any circulatory parameter of the eye, certainly not a collapse pressure. Moreover, this measurement is strongly dependent on the IOP which, in these experiments, seems not to have been considered. The collapse pressure itself strongly depends on the IOP—that is, the tissue pressure around the central retinal vessels, a matter of debate for many years.3 Since the examination itself lowers the IOP considerably, any repetition of the examination must yield a new value. Therefore, it has been recommended to repeat the examination only after a longer rest period for the eye. If, as cited in this study, the examination has been repeated nine times in succession, the results should show a linear decrease of the distribution curve.
Pulsations of the central retinal vein express the fact that the IOP oscillates at levels near the cerebrospinal fluid pressure, a difference in the order of 1–2 mm Hg.4 We wonder whether the set-up and the method are accurate enough to document such subtle changes.
In order to obtain answers to questions concerning the circulation of the central retinal vessels in the diseases mentioned in the article, it would be necessary to convert the force in the corresponding pressure values. I look forward to reading the articles the author cited in the reference list and to clarification of the points raised in this letter.
