Corneal Wound Hydration
Clear corneal incisions (CCIs) are now the most common method of wound construction in cataract surgery. Despite the many advantages they offer there is an apparent increase in the rates of post-operative endophthalmitis which parallels the increase in popularity of CCIs. Although the wounds are designed to be self-sealing, studies have revealed wound gaping allows the inflow of extraocular contents and therefore potentially pathogens. Stromal hydration is now a popular technique used in an attempt to improve the integrity of the wound and reduce the risk of endophthalmitis. A review of current evidence shows that stromal hydration may lead to increase corneal thickness for up to 2 weeks and may reduce the inflow of extraocular contents. However stromal hydration has not been shown to reduce wound gaping and is associated with the risk of Descemet’s membrane detachment and cannula slippage. Despite stromal hydration there is evidence that wound leak still occurs with external manipulation, therefore alternatives such as ocular bandages have been proposed. No studies have been conducted assessing whether stromal hydration ultimately reduces the rates of post-operative endophthalmitis, therefore it remains down to the individual surgeon to evaluate the evidence for and against.
Clear corneal incisions (CCIs) have now superseded scleral and limbal incisions as the preferred method of wound construction in cataract surgery . The benefits of well-constructed CCIs include decreased operating time and induced astigmatism as they should not require suturing. However, doubts about the integrity of such wounds have arisen due to the apparent increased incidence of endophthalmitis that appeared to parallel the increased popularity of CCIs  . Studies supporting this potential causative relationship have shown CCIs suffer from endothelial and epithelial gaping and misalignment, allowing inflow of fluid. Similar inflow of bacteria is plausible and would increase the risk of endophthalmitis. Wound leak on the first post-operative day has been associated with a 44-fold increased risk of endophthalmitis. Postoperative intraocular pressure has been shown to potentially fluctuate to low levels and allow changes in wound morphology with loss of wound edge apposition and the potential for the passage of fluid. Cadaveric models have shown passage of India ink via this route and an interventional case series demonstrated that extraocular fluid may enter the anterior chamber with provoked intraocular pressure variations after surgery, mimicking the effect of a patient touching the incision area . Stromal hydration of CCIs via different methods has been promoted in an attempt to improve sealing and prevent entry of pathogens. In our review we attempt to evaluate and summarise the evidence for and against stromal hydration of CCIs.
What is stromal hydration?
Stromal hydration is hypothesised to assist wound sealing by thickening the corneal tissue around the wound site, forcing the roof of the wound down into the floor with enhanced endothelial pumping. This pump mechanism effectively pulls the roof of the wound into the floor . The original technique of stromal hydration described by Fine et al in 1993 involves forcefully injecting balanced salt solution into the walls of CCIs using a blunt cannula. This usually involves the lateral walls of the CCI but there are recommendations that the roof is also hydrated . Alternative techniques of stromal hydration have also been suggested, hypothesised to create a better seal. A variation to the conventional hydration approach involves using a 30 gauge needle to hydrate corneal stroma immediately anterior and central to the main CCI . Suzuki et al suggested hydration with the irrigation port during phacoemulsification . They postulate that this technique does not disturb wound architecture or damage surrounding structures such as Descemet membrane. Another technique, colloquially known as “the Wong way”, describes the creation of a supra-incisional pocket overlying the internal aspect of the CCI which is hydrated at the end of the surgery .
Mifflin et al (2012) compared anterior stromal pocket hydration technique with the conventional method (including CCI roof hydration) in 66 eyes . Although none of the wounds spontaneously leaked, with direct pressure on the posterior lip in the incision there was a significantly reduced rate of wound leak with anterior stromal pocket hydration. The relevance of this study to clinical practise is limited as artificial direct pressure was asserted to test the wound, smaller incisions were not tested and the immediate post-operative period was only considered. This new method also is not without additional risks, if the hydration occurs too superficial epithelial defects or erosions could occur leading to possible recurrent epithelial erosion syndrome or scarring. It has been hypothesised that this technique could lead to a greater astigmatism effect, however in this study there was no significant difference between the two groups.
Does wound hydration prevent endophthalmitis?
Endophthalmitis has a multifactorial aetiology. Wound hydration alone would never completely eradicate the risk of subsequent endophthalmitis but aim to reduce this risk. Despite the popularity of wound hydration, no studies have directly compared the incidence of endophthalmitis with and without stromal hydration. Low incidence of endophthalmitis with modern cataract surgery makes it difficult to obtain sufficient numbers to detect a statistically significant result to answer this question. Instead, research is currently focused on whether stromal hydration improves the architecture of the wound and prevents ingress of extraocular contents; hypothesising that this would lead to a reduced incidence of endophthalmitis.
Does wound hyration improve wound architecture?
Optical coherence tomography (OCT) has been used in attempt to evaluate the changes in wound architecture due to stromal hydration. Calladine et al found increased corneal thickness and wound length (from corneal endothelium to epithelium) with stromal hydration 1 hour post-surgery . Fine et al (2007) showed this increase in thickness persisted 24 hours post operatively and Fukuda et al found a significantly thicker cornea up to 2 weeks post-operatively  . These studies had a simple and experimental style of methodology. Fine et al gives little details of his methods (including no number of patients) and only evaluates incisions made by one surgeon. Fukuda et al also only study wounds from one surgeon. However, their sample size included 30 eyes randomly assigned to receive stromal hydration or not. The randomisation process was not clearly described. Calladine et al also assessed and randomised 30 eyes. They had 3 different surgeons, who were blinded to the randomisation until immediately before wound hydration was required. Calladine et al and Fukuda et al found stromal hydration had no effect on reducing endothelial gaping . Fukuda also found there was no improvement with epithelial gaping or misalignment of the wound. Bang et al also investigated CCIs in 60 eyes and found that with 2.2 mm wounds stromal hydration lead to a thinner cornea and increased the incidence of endothelial gaping . Calladine found that stromal hydration leads to a borderline clinical significant increased IOP and postulated that this effect may be due to a better seal . This is significant as previous studies shown an association between low IOP and endothelial gaping and loss of coadaptation of CCI . However, it was not clear whether the increased IOP was due to a better wound seal or better filling of the anterior chamber initially following stromal hydration. Subsequently Fukuda et al found no difference in IOP with stromal hydration ). Early case series have noted that stromal hydration may be effective in reducing the ingress of extraocular contents. Herretes initially reported reduction in ingress of blood-stained fluid into the anterior chamber with SH in a case series . This involved the observation of only 8 patients before and after stromal hydration by 2 independent observers. They applied external pressure onto the scleral side of the wound to simulate blinking. Whether this pressure was an accurate representation of the forces applied to the eye post-operatively is questionable. Vasavada subsequently showed a reduction in trypan blue ingress in eyes randomly assigned to receive SH  . This evaluated wounds made by only one surgeon. Randomisation was attempted, however this was with sealed envelopes rather than random number generator/computerised methods. This study only evaluated ingress through the wound immediately following surgery and not long-term wound integrity. When external manipulation to simulate forced blinking is applied, Masket et al showed that wound leakage still occurred in 67% of CCIs despite stromal hydration. Masket used a calibrated force gauge in order to attempt to standardise and replicate the normal fluctuations in IOP post-operatively. There was no control to compare wounds with and without hydration.
The evidence for and against stromal hydration is extremely limited, mainly due to the small sample sizes of the studies and primitive methodologies.
There are no studies to investigate whether stromal hydration prevents the egress of fluid from the anterior chamber. Hypothetically this would lead to ocular hypotony and therefore poor wound architecture with the potential for ingress of microbes.
There is evidence that stromal hydration may reduce the inflow of extraocular contents, however it is clear that wound leakage does occur despite hydration. OCT evidence to support improved wound architecture is poor and conflicting. Also it is not clear whether improved wound architecture would equate to a better seal and reduction in endophthalmitis.
What are the complications of wound hydration?
Stromal hydration may lead to increased rates of Descemet’s membrane detachment and cannula slippage. Calladine et al found higher rates of detachment of Descemet’s membrane on OCT in eyes with stromal hydration than without (64% compared to 25%) . However other factors that may contribute to increased detachment were not accounted for such as experience of surgeon, grade of cataract and patient compliance. Descemet’s membrane detachment may lead to impaired wound healing due to impairment of the endothelial pump mechanism. However, Bang et al and Fukuda et al both found no significant difference in Descemet’s membrane detachment on OCT in their respective investigations.
Our literature review found 9 reported cases of slippage of cannula (both luer-lock and slip-lock) during stromal hydration causing damage to intraocular structures . Injuries included corneal perforation, retinal detachment, vitreous haemorrhage, hyphaema, iris laceration and corneal contusion. 3 cases involved slip-lock cannulas, 3 reported luer-lock and 3 did not specify. Rumelt et al retrospectively reviewed 15 years of anterior segment surgery to identify an incidence of 0.88 per 1000 procedures per year for injuries caused by cannula release. They made several suggestions to prevent inadvertent cannula release including: double-checking the cannula is secured; using luer-lock syringes; holding the base of the cannula and avoiding aiming the tip towards the posterior segment.
What are the alternatives?
If the evidence of stromal hydration is limited, surgeons may wish to employ alternative techniques to reduce the risk of endophthalmitis. Sutures are often used to create a better seal for a corneal wound especially if there are doubts over its integrity. Sutures in the cornea can lead to astigmatism, foreign body sensation, inflammation, tissue necrosis and require removal. New technologies and techniques have emerged as an alternative to sutures and stromal hydration; such as adhesives, bandages and the use of femtosecond laser. Cyanoacrylate glue traditionally used in the management of cases such as corneal perforation can also be used in cataract wound closure, withstanding significant rises in IOP. It can result in inflammation, increased infection and foreign body sensation as it is toxic and inflexible  . Fibrin tissue glue is more flexible, non-toxic and biodegradable and has been shown to produce a better seal than sutures in CCIs . It does carry the potential for transmission of prions and viruses from donors.
Polyethylene glycol (PEG)-based products such as bandages have been shown to withstand high IOP when tested in cadaver eyes in . Harvey et al showed the liquid adhesive ocular bandage to have better wound closure than stromal hydration alone, less astigmatism than sutured wounds and less foreign body sensation than either stromal hydration or sutures . They found that stromal hydration would be an unnecessary step if an ocular bandage was used .
Femtosecond laser-assisted cataract surgery (FLACS) is emerging as a promising technology in wound construction with its ability to provide stable and reproducible incisions. FLACS causes less epithelial wound gaping, incision-site Descemet membrane detachment, endothelial misalignment and variability in astigmatism than manually created incisions . Whether this method leads to a better wound seal and reduction in post-operative endophthalmitis is yet to be investigated.
Stromal hydration of CCIs during cataract surgery is now standard practise for many ophthalmologists. The intention is to improve wound integrity and therefore prevent ingress of extraocular contents and endophthalmitis. Research so far has suggested that stromal hydration may reduce the ingress of extraocular fluid and that it can increase the thickness and wound length for up to 2 weeks. However, several OCT studies have shown no difference in wound gaping or misalignment compared to wounds without hydration and in some wounds OCT revealed significantly thinner cornea in hydrated corneas after the operation day. Hydrating the cornea may be associated with higher rates of Descemet’s membrane detachment and case reports revealed incidences of significant trauma due to cannula slippage.
Despite stromal hydration, it is evident that CCIs still allow wound leak especially with external manipulation. Various alternatives have been suggested in attempts to improve wound integrity such as anterior pocket stromal hydration and the use of ocular adhesives. Current evidence is not definitive as to whether stromal hydration is beneficial. No studies have been conducted assessing whether stromal hydration ultimately reduces the rates of post-operative endophthalmitis. Although alternatives such as ocular bandages may give a better seal it also still not known whether this translates to a reduced risk of endophthalmitis and therefore justifies the additional cost. Considering that stromal hydration is such a widely used technique, it remains the choice of the individual surgeon to balance any potential complications with the perceived benefits.
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