ORIGINAL ARTICLE Tonopen Versus Goldmann Applanation Tonometry for Detecting Restrictive Thyroid Eye Disease Imran Rahman, M.R.C.Ophth., Paul S. Cannon, M.R.C.Ophth., M.R.C.P.I., and S. Ahmed Sadiq, D.O., F.R.C.Ophth., F.R.C.S., D.M. Royal Manchester Eye Hospital, Manchester, United Kingdom Purpose: In thyroid eye disease (TED), intraocular pressure (IOP) measurements are taken in both the primary and upgaze positions to elicit restrictive muscle disease. The aim of this study was to assess whether the IOP when measured with the eyes in upgaze (the Goldmann applanation tonometer [GAT] head ap- planating the inferior/peripheral cornea) is similar when compared with the central corneal IOP in upgaze using the Tono-Pen XL. Methods: IOP was measured with the GAT on the central cornea in primary gaze and on the inferior cornea in upgaze. IOP was measured with the Tono-Pen XL in the central cornea in both primary gaze and upgaze. The outcome measure was the difference in IOP readings between the GAT and the Tono-Pen XL for patients with restrictive TED. Results: Fifty-two patients were included in the study; 31 patients with restrictive TED and 21 control patients. In the control group, there was no significant difference in Tono-Pen XL and GAT readings for both primary gaze and upgaze (p = 0.99). Both instruments detected an increase in IOP with upgaze in patients with restrictive TED compared with controls (p = 0.0006). There was no significant difference between the 2 instruments’ readings in upgaze for patients with restrictive TED (p = 0.39). Conclusions: Both the GAT and the Tono-Pen XL can be used to establish IOP in patients with restrictive TED. (Ophthal Plast Reconstr Surg 2010;26:36–38) T he difference in intraocular pressure (IOP) between the primary position and upgaze can indicate restrictive eye disease, particularly in patients suffering with thyroid eye disease (TED). 1 There are limitations to the principles of accurate applanation tonometry in modern day practice in which accuracy is paramount. Because the Goldmann applana- tion tonometer (GAT) is slit-lamp mounted, it will applanate the inferior/peripheral cornea when the eyes are in upgaze. The Tono-Pen XL (Medtronic Solan, Jacksonville, FL, U.S.A.) operates with a smaller surface area than the GAT. It is handheld and can measure the central cornea despite the posi- tion of gaze. The Tono-Pen XL uses a measuring principle similar to the Mackay-Marg tonometer. 2 The aim of this prospective study was to determine whether the GAT and the Tono-Pen XL were equivalent when used to detect restrictive muscle disease by comparing readings in the primary and upgaze positions. METHODS This was a prospective study conducted over a 6-month period. All consecutive patients with a documented history of restrictive TED as recorded by an orthoptic assessment and who attended the oculo- plastic clinic of a single surgeon (S.A.S.) were recruited in the restric- tive TED group. The control patients were recruited from the general clinic of the same surgeon in a consecutive fashion during the study period. Exclusion criteria included all individuals with concurrent eye disease requiring treatment (e.g., glaucoma, corneal transplants), pa- tients under the age of 16 years, and those unwilling to participate following consent. Ethics approval was obtained that followed the tenets of the Declaration of Helsinki. Randomization was performed using a random number producing algorithm (www.graphpad.com) to randomize eyes to the left or right eye in the control patients, as only one eye was used for the IOP measurements. If the restrictive eye disease was unilateral, this eye was used in the study. In the case that both eyes were restrictive, randomization as outlined above was applied. Primary gaze was defined as the gaze adopted when the patient focused on an object slightly below the line of vision, to mimic the -5° position of downgaze, because this is the position at which the superior rectus exerts no tension on the globe. Upgaze was defined as the maximum gaze adopted when the patient focused on a point on the ceiling without any head movement. Four investigators were involved in performing the measure- ments, 2 measured the patients with TED and 2 measured the controls. Initially the cornea was anesthetized with lidocaine 4% combined with fluorescein 0.25% (Chauvin Pharmaceuticals, Montpellier, France). In both groups of patients, the IOP was always initially measured using the GAT (with the patient’s chin above and in front of the chin rest on the slit lamp) in the primary position of gaze and in upgaze in close succession on each patient. We recorded 2 measurements on the central cornea (primary position) and 2 on upgaze, where the GAT was usually positioned on the inferior, peripheral cornea. Then Tono-Pen XL tonometry was used on the central cornea in both positions (primary and upgaze) to measure the IOP. The instrument was calibrated according to the manufacturer’s instructions and took an average of 3 readings in each position of gaze provided the variance was 5% for each position. This order was adopted to reduce any chance of bias of one method influencing the readings of the other method. We felt that the Tonopen should be performed after the GAT because its readings are read off the display, and therefore less likely to be influenced by the GAT readings. Statistical Analysis. We planned a study of continuous response variable from matched pairs of study subjects. Prior data indicated that the difference in the response of matched pairs was normally distrib- uted with a standard deviation of 6. If the true difference in the mean response of matched pairs was 4 mm Hg (predicted by us as the Accepted for publication April 22, 2009. No authors have any financial/conflicting interests to disclose. Address correspondence and reprint requests to Paul S. Cannon, M.R.C.Ophth., M.R.C.P.I., Manchester Royal Eye Hospital, Oxford Road, Manchester, M13 WH, United Kingdom. E-mail: pscan05@yahoo.co.uk DOI: 10.1097/IOP.0b013e3181b80fcb Ophthal Plast Reconstr Surg, Vol. 26, No. 1, 2010 36