TONOMETRY

Goldmann applanation tonometry does not overestimate intraocular pressure (iOP) following manual Descemet stripping endothelial keratoplasty (DsEK), as might be expected given much thicker central corneas after surgery, reported Dipanjan Pal MBBs of Priyamvada Birla Aravind Eye hospital, Kolkata, india, at the 2011 annual meeting of the AAO. Contrary to studies in normal eyes, Goldmann measures of post-DsEK eyes in a recent study did not correlate with central corneal thickness, he noted.

However, differences between mean iOP measured by dynamic contour tonometry (DCT)and Goldmann in post-surgery eyes were similar to those in a normal control group, with DCT readings running a little over 2.0 mmhg higher than Goldmann readings in both groups. These results suggest that Goldmann may be suitable for monitoring pressure after DsEK. however, results from the two methods were not comparable, and Dr Pal recommended that they should not be used interchangeably.

Goldmann vs DCT
DsEK involves selective replacement of damaged corneal endothelium, and typically results in increased corneal thickness. Goldmann tonometry, which calculates iOP based on the pressure required to applanate the cornea based on a modified imbert-Fick law, has been shown to depend on central corneal thickness. Correction factors may be needed for accurate iOP readings with thicker corneas. DCT is a novel technique that is less influenced by central thickness and curvature of the cornea. Dr Pal's purpose was to compare their performance in DsEK and normal eyes from the same patient cohort.

In a case control study conducted between March 2009 and November 2010, Dr Pal and colleagues compared Goldmann and DCT readings between eyes that had undergone manual DsEK and normal eyes in the same patient cohort. DsEK group inclusion criteria included clear cornea and post-op status at least three months after surgery. Eyes treated with anti-glaucoma medication, eyes that had ocular surgeries other than cataract extraction, eyes with ocular trauma, eyes with more than 3.0 D astigmatism, eyes with poor visual outcomes and eyes with poor quality scores on DCT were excluded.

A total of 46 DSEK eyes in 42 patients qualified. These were compared with 35 non-DSEK eyes in 35 patients, with one-eyed patients and those with corneal oedema excluded. All patients were Asian Indians with mean age of 60.2 years. Measurements were conducted at a mean 10.15 months after surgery. Indications for DSEK were bullous keratopathy for 33, Fuchs’ endothelial dystrophy for 12 and iridocorneal endothelial syndrome for one. For each patient three Goldmann and three acceptable quality DCT scores were averaged and compared. Central corneal thickness results were also averaged from three measurements.

In the DSEK group, mean Goldmann IOP measured 19.40 mmHg, or a mean 2.12 mmHg less than the DCT mean of 21.50 mmHg (p<0.0001), and mean central corneal thickness was 624.5 microns. Mean Goldmann IOP value in the unoperated control group 15.12 mmHg, or a mean 2.36 mmHg less than the 17.60 mmHg DCT mean (p<0.05), and mean central corneal thickness was 518.41 microns. The higher mean readings for DCT is consistent with previous published reports, Dr Pal noted. Various studies have found that DCT measures were higher than Goldmann by a mean of 0.4 to 7.7 mmHg. (Doyle A et al. J Glaucoma 2005; 14:288-92; Schneider E et al. J Glaucoma 2006; 15:2-6).

Interestingly, IOP universally measured higher in DSEK eyes by both the methods. Analysis showed a strong positive linear correlation between Goldmann and DCT IOP measurements in both DSEK (r=0.935; p<0.0001) and controls (r=0.882; p<0.0001). However, while previous studies have found an inverse correlation between central corneal thickness and Goldmann IOP readings for normal eyes, as did this one, this study found no significant correlation between Goldmann or DCT IOP and corneal thickness in DSEK eyes. Both Goldmann and DCT showed higher mean IOP values in DSEK eyes, and the differences were independent of central corneal thickness, Dr Pal concluded.