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Influence of Corneal Thickness on Intraocular Pressure Measurements in Patients with Glaucoma

Vernaleken B., Palmowski A., Ruprecht K. W.,
Universitätskliniken des Saarlandes, Augenklinik und Poliklinik (Homburg/Saar)

Purpose: In previous studies we reported on differences in multifocal ERG recordings of patients with high (HTG) and low tension glaucoma (LTG)1,2. However, it has been suggested3, that the differences in intraocular pressure (IOP) as measured with the Goldmann tonometer could be due to a thinner cornea in LTG patients. Thus IOP itself may not allow a subdivision of POAG into LTG and HTG. Therefore we studied the influence of corneal thickness on the IOP (Goldmann) measured in our patients with both HTG and LTG.
Method: 12 eyes with HTG (highest measured IOP > 21mmHg) and 16 eyes with LTG (highest measured IOP < 21mmHg) underwent tonometry, ultrasonic pachimetry and static visual field testing to confirm glaucoma.
Results: Both groups compared well to one another in regard to visual field parameters. The mean MD was 9.18 (SD 4.58) in LTG, and 6.46 (SD 4.33) in HTG. Mean IOP prior to pachimetry was 13.75 mmHg in LTG and 19.75 mmHg in HTG. The mean corneal thickness was 0.556 mm (SD 0.039 mm) in LTG, and 0.545 mm (SD 0.052 mm) in HTG.
Conclusions: Corneal thickness was similar in LTG and HTG. Therefore, in our patients, IOP differs between LTG and HTG independent of corneal thickness, a finding that is also supported by other authors4. Thus the changes we observed in the MFERGs1 in LTG cannot be explained by a high IOP as in HTG, but rather points to additional underlying pathomechanisms such as an impaired retinal circulation.
1) Allgayer R, Palmowski AM, Heinemann-Vernaleken B, Ruprecht KW. Slow-stimulated multifocal ERG in high and low tension glaucoma. Invest Ophtalmol Vis Sci 2001, Vol.42, No.4: S147. 2) Palmowski A-M, Allgayer R, Heinemann-Vernaleken B. The multifocal ERG in open angle glaucoma--a comparison of high and low contrast recordings in high- and low-tension open angle glaucoma. Doc-Ophthalmol. 2000 Jul; 101(1): 35-49. 3) Sha S, Chatterje A, Mathai M, Kelly S, Kwartz J, Henson D, McLeod D. Relationship between corneal thickness and measured intraocular pressure in a general ophthalmology clinic. Ophthalmology 1999 (106) 2154-60. 4) Peplinski L, Torkelson K. Normal-tension glaucoma and central corneal thickness. Optom-Vis-Sci. 1999 Aug; 76(8): 596-8