Pseudophakic or Aphakic Bullous Keratopathy: Management Strategies and Prevention
 

Corneal edema resulting from cataract extraction is called pseudophakic or aphakic bullous keratopathy.  Pathologically, bullous keratopathy is caused by changes in the corneal endothelium, allowing the cornea to be in an abnormal state of hydration.  As endothelial cells are damaged,, the remaining cells rearrange themselves to cover the posterior corneal surface.  These cells are shaped iregularly and enlarged.  The Descemet membrane is produced in increased amounts by any pathologic process that affects the endothelial cells, resulting in cornea guttata.  As the endothelium becomes increasingly unable to act as a pump to deturgesce the cornea, the stroma begins to swell, especialy in the central cornea.  As the stroma swells, the cornea thickens and folds are seen in the Descemet membrane.  The edema may fluctuate in response to changing intraocular pressure.  Epithelial edema manifests as fluid accumulation between the basal epithelial cells.  As more fluid accumulates, blisters, and then bullae, develop.

Patients with bullous keratopathy demonstrate decreased visual acuity and pain or discomfort.  Pain or discomfort associated with corneal edema results from exposure of corneal nerves to an often noxious environment.  As the edema progresses and bullae are formed, rupture of bullae results in pain, photophobia, and epiphora.

Surgical trauma, most commonly during cataract extraction, can damage the endothelium, causing a period of postoperative edema that resolves in most cases.  Most corneas recover, but some do not.  Knowledge of the preoperative status of corneal endothelium may help to reduce this complication. Preoperative clinical specular microscopy is used to examine the quality and quantity of endothelial cells.  Significant correlation has been found between variation in cell size and the development of postoperative corneal edema. Endothelium with a greater degree of pleomorphism reacts more adversely to intraocular surgery and requires a longer time for corneal deturgescence.  The type of cataract surgery also has an impact on how much trauma occurs to the endothelium.  Routine uncomplicated phacoemulsification surgery results in 9% endothelial cell loss 1 year postoperatively, while an average of 16% endothelial cell loss is associated with phakic anterior chamber intraocular lens surgery.

Regardless of surgery type and whether an intraocular lens is implanted, continuing endothelial loss greater than the usual 1% per year occurs in patients who have had cataract extraction.  Corneal edema usually develops within 1 year after the endothelial cell density falls below 500 cells/mm, but no absolute lower limit to the number of cells has been found to be associated with stromal edema.  The type of lens implanted also is significant in determining the amount of endothelial cell loss over time.  persistent low-grade inflammation and intermittent contact of the implant with the corneal endothelium may cause PBK.

While mechanical trauma to the endothelium during surgery is considered to be the most significant factor influencing postoperative corneal edema, other factors can adversely affect the endothelium.  Toxic substances used to disinfect instruments may inadvertently be introduced into the eye, if inadequate rinsing of instruments allows some of the substances to remain in the small lumens of the instruments. Water, and not saline, should be used to rinse the instruments.  Viscoelastics can reduce touch between the cornea and the intraocular lens during lens insertion, and they can deepen the anterior chamber to minimize endothelial damage in the event of a shallow anterior chamber.  Reusable cannulas with viscoelastic can result in toxic residues being introduced into the eye; therefore, disposable cannulas should be used whenever possible.  Viscoelastics are used routinely to maintain anterior chamber depth, to protect the endothelium, and to facilitate placement of intraocular lenses.

Therapy of pseudophakic and aphakic bullous keratopathy is performed to reduce discomfort or to increase visual acuity.  The corneal edema associated with bullous keratopathy is chronic and usually non-inflammatory.  A number of treatment options are available.  The reduction of intraocular pressure is an important treatment for corneal edema, because increased intraocular pressure can compromise endothelial function and lead to epithelial edema and further endothelial damage.  Epithelial edema often can be managed with topical hypertonic agents such as sodium chloride (5%) ointment or drops.  Hydrophilic contact lenses, on an extended-wear basis, can be used to decrease pain associated with epithelial bullae.  It is thought that the lens acts as an effective precorneal protective layer and shields the abnormal epithelium from the environment and prevents bullae from bursting.  In the presence of low grade inflammation, topical steroids can be useful, since low-grade anterior uveitis, not infrequently, is associated with chronic corneal edema.

Surgical treatments for bullous keratopathy include conjunctival flap, cauterization of the Bowman layer, anterior stromal micropuncture, excimer laser phototherpeutic keratectomy (PTK), annular keratotomy, and penetrating keratoplasty.  A conjunctival flap is an excellent manner to decrease painful symptoms in eyes with painful bullous keratopathy.  Recently, amniotic membrane has been used successfully to cover swollen corneas and to decrease pain.  Neither of these procedures tends to improve the vision.  Cauterization of the Bowman layer is performed for pain relief.  This procedure is thought to produce a dense fibrous barrier between the corneal stroma and the epithelium that fluid cannot permeate into the epithelial cells and produce bullous changes.  Anterior stromal micropuncture and excimer laser PTK also have been used with some success to cause scarring of the superficial cornea and to decrease painful symptoms. Annular keratotomy has been used to treat the pain associated with bullous keratopathy in eyes with poor visual potential.  A partial-thickness corneal incision is made with a trephine and relieves pain by severing branches of corneal ciliary nerves to decrease corneal sensation.  Penetrating keratoplasty is the only surgical treatment that relieves pain, while attempting to restore visual acuity. In patients with pseudophakic bullous keratopathy, the intraocular lens may be removed or exchanged at the time of transplant.  Displaced lenses causing recurrent uveitis, closed loop, or anterior chamber iris supported lenses generally should be removed.

Surgical techniques of cataract extraction have resulted in a reduction in the number of bullous keratopathy cases; however, bullous keratopathy still constitutes a major indication of penetrating keratoplasty. Penetrating keratoplasty techniques also have improved, but cystoid macular edema associated with previous intraocular surgery may limit its effectiveness in improving visual acuity.  The decision to proceed with penetrating keratoplasty must be made while fully aware of the risks of infection, secondary glaucoma, and graft rejection, but it still remains the treatment most likely to markedly improve visual acuity.

Sudesh Kumar Arya
Deptt. of Opthalmology
Govt. Medical College & Hospital, Chandigarh

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