Figure 1. Cataract in human eye Image: Rakesh Ahuja MD
Dr Brendan Cronin
MBBS(Hons) DipOphthSci BCom LLB FRANZCO
Director of Education, Queensland Eye Institute
The incidences of both glaucoma and cataracts increase with age. This means that patients with both pathologies become increasingly common as we live longer, and with new technology we detect glaucoma earlier.
For a patient with glaucoma, a visually significant cataract that warrants surgery now opens up a multitude of treatments, benefits and potential complications that need to be explored.
The first and simplest case to discuss is primary angle closure glaucoma and phacomorphic glaucoma. If a patient still has a significant degree of iridotrabecular contact despite a patent peripheral iridotomy, then a lensectomy is the appropriate next step in their treatment. Removing the crystalline lens with a thickness of 4-6 mm and replacing it with an artificial lens with a thickness of approximately 1 mm opens up the angle significantly.
If there has been iridotrabecular contact for a prolonged period of time, then there may be peripheral anterior synechae (PAS). These adhesions between the iris and trabecular meshwork create a physical barrier to aqueous outflow and in the longer term, cause scarring and failure of the trabecular meshwork.
In patients with PAS, gonio-synechiolysis can be performed at the same time as the cataract extraction. This involves physically peeling the iris off the trabecular meshwork using the surgical gonioscopy lens to visualise the angle. If the meshwork has not been scarred, then this may restore aqueous outflow and a normal IOP.
Cataract surgery as a treatment for glaucoma
Numerous studies have shown that cataract surgery alone may lower the IOP by up to 6 mmHg even when the angle is completely open prior to surgery. This effect can last for up to two years. The mechanism is unknown and unfortunately, we cannot predict which patients will have this IOP lowering effect. Nonetheless, cataract surgery alone may sometimes significantly improve a patient’s IOP.
Micro invasive glaucoma surgery (MIGS) is an extremely promising new development in glaucoma surgery. The two different MIGS devices that are available in Australia are stents that bypass the trabecular meshwork and allow aqueous to flow straight into the canal of Schlemm. The stents therefore bypass the trabecular meshwork, which is the presumed barrier to the efficient outflow of aqueous.
The stents are less than 1 mm in cross section and can be seen only on gonioscopy. They can be inserted when cataract surgery is performed. As the stents drain into the canal of Schlemm, their effect will be limited by the episcleral venous pressure. This means that unlike a trabeculectomy, they cannot lead to hypotony.
The outcomes of MIGS may be potentially better, safer and more predictable than those of traditional glaucoma surgery.
Specific issues for patients with glaucoma
The vast majority of patients on ocular anti-hypertensives use prostaglandin drops. Years ago, these were stopped in the peri-operative period as they were thought to increase the risk of cystoid macular oedema; however, this theory has been discredited.
Generally, all glaucoma drops with the exception of pilocarpine should be continued up to the day prior to cataract surgery. They should be continued post-operatively with the caveat that they may be stopped or modified if the cataract operation lowers the IOP.
Post-operatively, all patients will need to use steroid eye-drops. Although the rate of steroid response is higher in patients with glaucoma, they are generally used only four times per day for one month, so a significant rise in IOP from the steroids is unlikely.
If the steroids drops are stopped too early and the patient develops cystoid macular oedema, then steroid drops may be required for many months. If the patient is a known severe steroid responder, then additional medications may be needed to control the increase in IOP over the short term.
Preserving the ocular surface is an important part of post-operative management in glaucoma patients. Many surgeons routinely use three different drops after cataract surgery: an antibiotic, a steroid and an NSAID such as Acular or Voltaren. Combined with the patient’s glaucoma drops, both the medications themselves and their preservatives can damage the ocular surface quite significantly.
It is important to change the patient to preservative-free medications wherever possible to avoid causing unnecessary damage to the ocular surface. Even with non-preserved medications, many glaucoma patients will develop a moderate transient epitheliopathy. This should settle with non-preserved lubricants once the additional post-operative drops are ceased.
Cataract surgery in patients who already have a functioning trabeculectomy
With MIGS, selective laser trabeculoplasty and prostaglandin analogues, it is reasonably rare for a patient needing cataract surgery to have already had a trabeculectomy. Unfortunately, even a perfectly functioning trabeculectomy will sometimes fail after routine cataract surgery.
This is thought to be due to post-operative inflammation. Generally, the surgeon will administer a sub-conjunctival injection of the anti-metabolite 5-Fluorouracil at the end of the cataract surgery and use intensive post-operative steroids to reduce the risk of these failures.
To avoid these complications , if there are no contra-indications, cataract surgery will often be combined with a trabeculectomy to avoid issues in later years.
Cataract surgery in end-stage glaucoma
Patients who have very severe or end-stage optic nerve damage are at risk of ‘snuffing out’ their vision after cataract surgery. The risk of this is particularly increased if there is a ‘macular split’ on the visual fields. There are multiple potential causes of this but the ultimate mechanism is further damage to an already perilously damaged optic nerve.
Intra-operative or post-operative IOP spikes, or damage from a higher intraorbital pressure (as opposed to intraocular) from a peri-bulbar anaesthetic may all be contributing factors. As with all patients with glaucoma, meticulous control of these factors is vital to ensuring a good outcome. These patients are often given oral acetazolamide for two to five days post-operatively to avoid pressure spikes. To reduce intro-orbital pressure spikes from the anaesthetic, topical or general anaesthesia may be needed for the procedure.
A patient with severe glaucoma may require a trabeculectomy or a glaucoma drainage tube in the future. The surgeon must take meticulous care of the conjunctiva intra-operatively to preserve it for any future surgery.
Cataract surgery alone may be very beneficial for patients with glaucoma; however, with MIGS, cataract surgery may have even more benefits. Despite the advantages of cataract surgery, it is vital that all of the potential complications are anticipated and avoided wherever possible.
Glaucoma patients need close monitoring post-operatively and even if their IOP drops enough so that they can stop drops altogether, they still require follow-up in the long term.