EyeNet Magazine


 
Ophthalmic Pearls: Cornea
The Nuts and Bolts of Keratoprosthesis, Part Two
By Ricardo M. Santaella, MD, and Natalie A. Afshari, MD
Edited by Ingrid U. Scott, MD, MPH, and Sharon Fekrat, MD
 
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Management of patients with the Boston type 1 keratoprosthesis can be a challenge. Patients are at risk of developing corneal melts, retroprosthesis membranes, glaucoma and retinal detachment. Endophthalmitis is now rare, however.

Management

Bandage contact lenses. Given the elevation of the PMMA front plate, patients who receive a Boston keratoprosthesis require a long-term bandage contact lens (BCL) to allow for improved surface wetting.1,2 A Kontur (Kontur Kontact Lens Co.) 16- or 18-mm BCL works well for this. The BCL is removed and cleaned at every visit and exchanged for a new one every three months. If the BCL is lost, it should be replaced in a timely fashion to prevent corneal melt. For patients in whom the BCL falls out regularly, we often recommend performing a lateral tarsorrhaphy to assist in maintaining the contact lens and in keeping the ocular surface moist.

Medication. The addition of prophylactic topical vancomycin has reduced the incidence of endophthalmitis significantly.3 For this reason, long-term topical antibiotic prophylaxis, including vancomycin 14 mg/ml twice a day in combination with a topical fluoroquinolone once or twice a day, should be continued indefinitely in the patients with a history of autoimmune disease. In others, polymyxin B/trimethoprim (Polytrim) once a day usually suffices. Topical prednisolone acetate is used perioperatively to control inflammation. In select patients, oral doxycycline is added to reduce metalloproteinase activity and corneal melting. Follow-up visits are usually every three or four months if the examination is stable without evidence of melting.

Follow-up exams. At each office visit, patients with a Boston keratoprosthesis should undergo a routine ocular examination with some additional steps. Slit-lamp biomicroscopy should pay particular attention to the ocular surface and the posterior portion of the PMMA cylinder and look for anterior chamber or vitreous cells. The following tests should also be performed.

  • Seidel test for corneal melt or wound leak. At each visit, the BCL is removed and cleaned. The surface is then examined for corneal melt by staining the ocular surface with fluorescein. Attention is given to the anterior plate and tracking of the fluorescein underneath the plate. Tracking underneath the plate can indicate early corneal melt (Figs. 1 and 2). Tracking as far as the transcorneal cylinder usually is associated with aqueous leakage and increases the risk for hypotony and infection.
     
  • Examination for a retroprosthesis membrane. A retroprosthesis membrane (Fig. 2) is a common complication that can be treated easily, if it is diagnosed in time, by using an Nd:YAG laser to perform a laser membranectomy. The Nd:YAG laser is used without a condensing lens and with a maximum power of 2.5 mJ, as the PMMA cylinder may be damaged by the use of excessive power. Regular follow-up visits usually allow for timely diagnosis, and treatment with the Nd:YAG laser. These membranes tend to recur and should be lasered as soon as they are noted. If the membrane cannot be lasered, a pars plana vitrectomy may be necessary to treat the membrane. Topical prednisolone acetate four times daily with a weekly taper is usually added after membranectomy to reduce the recurrence rate. A follow-up appointment in one month is generally adequate to monitor the need for re-treatment.
     
  • Tactile pressure. Patients with a keratoprosthesis may develop a severe form of chronic angle-closure glaucoma that leads to permanent vision loss. And this loss of vision may happen in a short period of time. The common methods of IOP measurement are not feasible in the presence of a keratoprosthesis, as a solid PMMA cylinder has replaced the central cornea. The current method for assessing IOP in these eyes is by tactile tension. Although not as accurate as tonoapplanation, this method works well when comparing the two eyes of the patient. However, the accuracy of tactile tension measurements is decreased in eyes with multiple glaucoma tubes. In these patients, an area devoid of aqueous drainage devices should be palpated and compared with the contralateral eye.
     
  • A 90-D examination. As explained above, glaucoma may be a severe complication in this patient population, and accurate optic nerve assessment is often the only objective measure for progression of optic nerve damage. Funduscopy and the assessment of the peripheral retina in these eyes can often be difficult. However, the posterior pole may be examined by slit-lamp biomicroscopy with a 90-D or similar lens. Some of the newer wide-field cameras are also helpful in assessing the retina (Figs. 3 and 4).
     
  • B-scan ultrasound. When the view of the fundus is inadequate, ultrasonography may be helpful. Also, patients with findings consistent with a leak should undergo an ultrasound to assess for possible choroidal effusions that may not be noted clinically, given the limitations of examining the retinal periphery.

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The Treatment of Corneal Melts

Corneal melts tend to occur at the base of the front plate. When performing a Seidel test, observation of fluorescein moving underneath the front plate is an early sign of melting and should lead the clinician to review reasons why the ocular surface is not being hydrated and to assess if the etiology that originally led to the placement of the Boston keratoprosthesis is still active. Improvement of ocular surface hydration should be attempted and closely followed up. However, once the melting process has begun, it may be difficult to reverse. If a frank leak is noted or suspected (fluorescein tracking into the cylinder and low IOP by palpation), an ultrasound may detect choroidal effusions. Leaks should be corrected surgically.

Surgical correction. Fixing a melt around the cylinder is generally performed either by replacing the Boston keratoprosthesis with a fresh graft and keratoprosthesis as described in Part One or by placing a corneal C-collar around the cylinder. General or retrobulbar anesthesia is used for the C-collar procedure. A tectonic corneal donor is trephinated using a trephine of the same size as in the original surgery, and a 3.0-mm punch is used to remove the central 3.0 mm of cornea from the donor button. Scissors are used to cut a slit in the doughnut-shaped donor button to create a C-shaped collar. The collar’s thickness is debulked by performing a lamellar dissection with scissors. The collar is then wrapped around the front plate on top of the old (melting) corneal button, and a 7.0 or 8.0 silk suture is placed in a figure-eight fashion, with three throws in the knot to achieve adequate tension to slide the C-collar underneath the front plate. This maneuver often requires the use of two needle drivers to achieve suture tension adequate to force the C-collar tissue under the front plate. After the collar has been advanced 360 degrees around the undersurface of the front plate, the C-collar is sewn onto the ocular surface using 9.0 nylon suture. The suture should not be under excess tension, as this will lead to tension on the inner C-collar–PMMA cylinder area, where melting tends to occur.

Subconjunctival antibiotic injections are generally given, and a BCL should be placed to promote ocular surface hydration and healing. We have found success with this technique as compared with amniotic membrane transplantation, gluing or using other materials to patch the leaking sites. Using a fresh graft and keratoprosthesis yields better results; however, obtaining a new keratoprosthesis quickly enough is not always possible.

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Management of Endophthalmitis

The incidence of bacterial endophthalmitis has been nearly eliminated with the chronic use of fortified vancomycin drops.3 The symptoms of bacterial endophthalmitis are usually a painful red eye with decreased vision. Patients with these symptoms are generally found to have a wound leak, and most have been off their prophylactic antibiotic drops. Treatment generally includes closure of the wound leak, intravitreal tap, intravitreal injection of antibiotics and topical antibiotics. Measuring 8.0 mm from the center of the keratoprosthesis will locate the area of the pars plana for intravitreal injection in eyes where identifying the precise limbal anatomy is difficult.

With the addition of prophylactic vancomycin drops, the incidence of endophthalmitis has been significantly reduced; however, fungal infections may still occur. We have treated several eyes with fungal endophthalmitis. They have retained useful vision after intravitreal injection of antifungals in conjunction with three or four months of systemic and topical antifungals.

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Conclusion

The Boston keratoprosthesis has returned vision to many patients blinded by corneal pathology. However, these patients require in-depth exams, routine use of ocular medications, contact lenses and sometimes multiple surgeries. Clinicians will need to learn how to manage these patients as the popularity of this procedure is increasing.
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1 Dohlman, C. H. et al. CLAO J 2002;28(2):72–74.
2 Harissi-Dagher, M. et al. Int Ophthalmol Clin 2008;48(2):43–51.
3 Durand, M. L. and C. H. Dohlman. Cornea 2009;28(8):896–901.
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Dr. Afshari is associate professor of ophthalmology in cornea and refractive surgery, and Dr. Santaella is a Horizon Grant fellow in cornea and refractive surgery. Both are at Duke University Eye Center.

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Part Two of The Nuts and Bolts of Keratoprosthesis discusses postoperative management ranging from corneal melts to endophthalmitis. Last month, Part One covered implantation and patient selection. The authors would like to thank Claes H. Dohlman, MD, for reviewing this article.

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