• Cornea
• degeneration
• dystrophy
• intraocular pressure

The management of corneal endothelial dysfunction has undergone a paradigm shift in the last decade, from the gold standard of penetrating keratoplasty (PK) through to pure endothelial cell transplantation. Posterior lamellar keratoplasty,1 2 deep lamellar endothelial keratoplasty (DLEK),3 Descemet's stripping endothelial keratoplasty (DSEK),4 Descemet's stripping automated endothelial keratoplasty (DSAEK)5 and Descemet's membrane endothelial keratoplasty (DMEK)6 are all variations on the theme of replacing diseased recipient endothelium with healthy donor endothelium, and are collectively referred to as endothelial keratoplasty (EK).

The major advantage of EK over PK is the rapid visual recovery, with minimal change in corneal shape and refraction. Bahar et al found that many patients undergoing EK achieved stable postoperative best corrected visual acuity (BCVA) as early as 1 month after surgery.7 DSEK or DSAEK causes little or no change in corneal topography or mean refractive cylinder compared with PK, which also stabilises quickly, hence accounting for a faster visual recovery.8

With refinement of the techniques of EK, better BCVA outcomes have been reported. While only 49% of the earlier series of DLEK cases of Terry and Ousley achieved 20/40 or better,9 Chen et al, more recently, reported a mean BCVA of 20/30 in DSAEK patients without visual comorbidity.10 Visual outcome results after DSAEK or DSEK are now comparable to, or exceed, those reported in large series of PK.11 12 However, despite apparently clear grafts, not all eyes achieve what would be considered their full visual potential—that is, 20/20 or better. The potential reasons for this are many and varied.

The donor/host interface is a significant factor in determining the final visual acuity after EK. Poor BCVA results after DLEK are probably due to an irregular host interface resulting from a manual dissection. Such an interface causes irregular light ray transmission and therefore can degrade optical quality. This suboptimal final BCVA after DLEK has previously been reported by Terry and Ousley.9 Descemet's stripping provides a smooth recipient refractive surface, as well as reducing the chance of interface haze (by minimising stromal trauma) and hence may overcome this problem.

The technique of dissection of donor tissue also has a bearing on the BCVA. A smoother donor surface should result in better visual outcomes by virtue of a regular refracting surface. Manual dissection of donor tissue, while effective, cannot provide the smoothest refracting surface. The stromal trauma sustained can result in stromal reaction and haze as well. An automated microkeratome or femtosecond laser dissected donor should overcome these disadvantages. This point was supported by Price and Price, who found that the use of the microkeratome accelerated visual recovery compared with manually dissected tissue.13 While 79% of the microkeratome group achieved a BCVA of 20/40, only 69% of the hand dissected group managed this level. In addition, as eye bank pre-cut donor tissue becomes popular, it is good to know that a randomised control trial found that such tissue provided similar visual outcomes to, and compared well with, surgeon-dissected tissue for DSAEK.14

Timing of surgery is also likely to influence the visual outcome. Hecker et al found a 54–63% reduction in keratocytes in the anterior stroma in Fuchs endothelial dystrophy that required keratoplasty.15 This is probably to lead to permanent structural changes of the corneal stroma. Hence cases of longstanding bullous keratopathy may not achieve their full BCVA potential. Terry, drawing on experience from 450 cases of EK, suggests that light scatter from the irregular anterior surface in longstanding cases of oedema is a factor leading to reduction in BCVA.16 In addition, he also observed that better preoperative BCVA resulted in better postoperative BCVA after EK. Interestingly, younger host age has been found to correlate with better BCVA after DSEK—both Terry and Price observed this relationship in their series.16 17

When cataract surgery is planned simultaneously or sequentially in patients undergoing EK, the selection of intraocular lens (IOL) will have a bearing on the vision. The IOL power should compensate for 1–1.25 dioptres of induced hypermetropia.18 However, when thinking about BCVA, Koh et al and Muftuoglu et al found that DSAEK induces greater higher-order aberrations than PK.19 20 The introduction of a graft lenticule of different corneal curvature is thought to be the cause. Koh et al speculate that the graft/host interface in DSAEK induces substantial negative spherical aberration of the posterior cornea surface.19 An aberration-free IOL is therefore preferred to counteract the negative spherical aberration induced by the donor tissue, as it is difficult to predict the exact level of higher-order aberration after surgery. It is rational to assume that a multifocal IOL may deteriorate optical quality further in EK.

It has been questioned whether donor corneal thickness is relevant to the postoperative BCVA. While Nieuwendaal et al found no correlation,21 Neff et al found that BCVA after DSAEK was dependent on the donor lamellar thickness.22 Separately, Van der Muellen et al, evaluating visual quality (in the form of stray light or disability glare), found that stray light improvement after DSAEK correlated with postoperative central corneal thickness.23 It is interesting to examine the visual outcome results after DMEK. Recent studies report a BCVA of 20/25 or better in more than three-quarters of patients after DMEK.24 25 It is postulated that the thin Descemet's membrane graft restores the anatomical structure of the posterior corneal surface almost to preoperative state and therefore there is little disruption of the refractive state of the eye. However, the technical difficulty, with high rate of graft dislocation and donor tissue loss while stripping Descemet's membrane, limits the widespread adoption of DMEK.26

Shinton et al, in the February issue of the journal, report the largest cohort to date, exploring the relationship between graft thickness and BCVA at different time points after DSEK.27 In this series of 70 consecutive cases, graft thickness decreased progressively after surgery. However, this did not correlate with the BCVA at the last follow-up. Subgroup analysis comparing results with thin (<100 μm) grafts showed no difference in BCVA. Even though graft thickness had stabilised by month 1, BCVA took longer to reach full potential. The mean BCVA of 20/46 is comparable to other series of manually dissected donor tissues.

The factors that determine final BCVA after EK are becoming clearer with time. Within the same short space of time, EK has become the procedure of choice for endothelial dysfunction. While Shinton et al rightly suggest manually dissected donor tissue as a viable option, an automated keratome cut has been shown to give the patient the best chance of achieving a level of BCVA that is of driving standard. In this day and age, a socioeconomic model would easily justify the expense of an automated keratome, either within the eye banks or at individual higher-volume sites, just to be able to achieve this improved BCVA, and one could and should question the lack its availability in the presence of improved patient outcomes.