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Hertel exophthalmometry: the most appropriate measuring technique
  1. Tennent Institute of Ophthalmology
  2. Western Infirmary, Glasgow
  1. Niall P O’Donnell, Southport General Infirmary, Scarisbrick New Road, Southport, Merseyside PR8 6P.

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Editor,—The common method used in Hertel exophthalmometry involves aligning the corneal apex with the reference cone on the exophthalmometer and noting the position of this alignment on the millimetre scale (method 1). Keeler, the manufacturers of the exophthalmometer, recommend that the reference cone is aligned with the reference line on the millimetre scale and the position of the anterior surface of the cornea on the millimetre scale noted (method 2).

Our study, in which two masked observers of differing experience examined 50 patients using these two methods, was set up to compare the results obtained using the two methods, determine if they could be used interchangeably, and whether we could recommend one method over the other.

Readings obtained by each observer from each eye with one method were compared with the readings of that same observer from the same eye using the different measurement method. A Wilcoxon statistical analysis was performed. Observer 2 was the more experienced.

There was a statistically significant difference between the readings made by observer 2. This is most probably clinically relevant and does show that both methods of measurement cannot be used interchangeably.

There was a 60–70% (method 1) and 30–40% (method 2) interobserver percentage agreement for readings from each eye and each measurement method. Agreement was present if the interobserver readings were within 1 millimetre of each other. These results may reflect the fact that both observers were more familiar with method 1. A follow up study, once the observers were more familiar with the Keeler method, should show an improvement in the agreement rate if this was the case.

We believe this study shows that a failure to make clear the measurement method used in exophthalmometry can lead to clinically significant false positive results which could have serious implications in the management of these patients.

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