Ophthalmodynamometric measurement of orbital tissue pressure in thyroid‐associated orbitopathy (2004)

Sir, One of the most severe complications of thyroid-associated orbitopathy is compression of the optic nerve due to increased orbital tissue pressure. It may therefore be useful to measure orbital tissue pressure in order to determine when it might be too high for the optic nerve and when treatment should be initiated. The central retinal vein runs through the orbital space and is ophthalmoscopically assessable; it was therefore the purpose of the present study to determine central retinal vein collapse pressure in patients with thyroid-associated orbitopathy in an attempt to obtain an estimate of the orbital tissue pressure. This clinical, observational, comparative, non-randomized study included three patients who presented consecutively with endocrine ophthalmopathy (aged 57.1 ± 8.7 years) and 40 control subjects (50 eyes) (aged 64.8 ± 15.6 years) who attended the hospital because of cataract or refractive problems. Ophthalmodynamometry was performed in all patients included in the study. The ophthalmodynamometer (Meditron GmbH, Völklingen, Germany) consisted of a conventional Goldmann contact lens fitted with a pressure sensor at its outer margin where the Goldmann contact lens is usually held during an ophthalmoscopic examination. By applying slight and increasing pressure onto the contact lens, the optic nerve head could be continuously observed ophthalmoscopic ally. When the central retinal vein and artery, respectively, or one of the vessel branches on the optic disc surface started to show pulsations, the value given by the pressure sensor was noted as the diastolic collapse pressure. The method, which has already been described in detail (Jonas 2003), adhered to the tenets of the Declaration of Helsinki. Central retinal vein collapse pressure was significantly higher (p = 0.009, Mann–Whitney U-test) in the study group than in the control group (30.8 ± 22.7 relative units versus 5.1 ± 8.4 relative units). Central retinal artery collapse pressure did not vary significantly (p = 0.34) between the two groups (64.1 ± 10.5 relative units versus 74.2 ± 18.8 relative units). As the central retinal vein passes through the optic nerve meninges and the orbital space, and indirectly via the ophthalmic vein through the brain, it has an intimate anatomical relationship with the cerebrovascular system, cerebrospinal fluid system and orbital tissue. Ophthalmodyna mometric examination of the central retinal vessels can therefore be important in the diagnosis of neurological, neuro-ophthalmological and orbital diseases (Rios-Montenegro, Anderson & David 1973; Entenmann et al. 1997; Hartmann & Meyer-Schwickerath 2000; Jonas 2003). Confirming a previous study (Hartmann & Meyer-Schwickerath 2000), the results of the present study suggest that central retinal vein collapse pressure as measured by the new technique of ophthalmodynamometry may be higher in patients with thyroid-associated orbitopathy than in normal subjects. It may be taken as a hint of increased orbital tissue pressure. Future studies might compare the measurements obtained by this modified technique of ophthalmodynamometry with those obtained by direct orbital manometry of the orbital tissue (Riemann et al. 1999). This may make it potentially possible to use ophthalmodynamometry to monitor orbital tissue pressure in patients with endocrine ophthalmopathy.