Glaucoma-related implant changes care plan

March 1, 2010

Glaucoma is common in eyes with a keratoprosthesis implanted, and this increases the challenge of disease management, particularly IOP measurement, according to one expert.

Key Points

Chicago-Glaucoma is common in eyes with a keratoprosthesis implanted, and this increases the challenge of disease management, particularly IOP measurement, according to Thasarat S. Vajaranant, MD.

Routine IOP monitoring is inadequate to follow glaucoma progression in these patients, and ancillary testing is recommended, said Dr. Vajaranant, assistant professor of ophthalmology, Glaucoma Service, Department of Ophthalmology and Visual Sciences, University of Illinois at Chicago.

Two types of FDA-approved keratoprostheses are available in the United States: the Boston Keratoprosthesis (KPro) and the AlphaCor (Addition Technology). The Boston Keratoprosthesis, developed by Claes Dohlman, MD, of the Massachusetts Eye and Ear Infirmary, is made of PMMA and consists of front and back plates. The AlphaCor is a one-piece device composed of hydrophilic polymer poly (2-hydroethyl methacrylate).

In at least two-thirds of patients accepted for implantation of the KPro, glaucoma has been preoperatively diagnosed, and the disease can be aggravated by the implantation surgery.

Commenting first on IOP measurement, Dr. Vajaranant said the most common method in eyes with the KPro implant is finger palpation over the upper lid, but that other techniques also are useful.

"We recommend palpation and compare it [with] the IOP measurement in the fellow eye," Dr. Vajaranant said. "In addition, we found that the scleral pneumotonometer can be used, but it is less accurate when IOP is high. Our group, led by Jose de la Cruz, MD, director of artificial cornea services at the University of Illinois at Chicago, also monitors our patients with the KPro implant with other ancillary tests, including visual field, optic nerve photography, and imaging such as optical coherence tomography (OCT)."

She added that automated visual field testing can be performed in some cases, and Goldmann visual field testing can be performed in most cases. The visual field extends for up to 70° in the type 1 KPro and 40° in the type 2 device.

She also said that when using OCT in patients with glaucoma and those with KPro implants, the signals through PMMA are reduced for macular scans but are acceptable for retinal nerve fiber layer scans.

When patients with a Boston KPro implant also require a glaucoma implant, several considerations arise.

"First, a crowded anterior chamber in eyes with the KPro [implant] is demonstrated by anterior segment OCT and ultrasound biomicroscopy," Dr. Vajaranant said. "There is not much space in the anterior chamber to place a glaucoma tube.

"Furthermore, the tube cannot be visualized after KPro implantation," he said. "Accordingly, we prefer pars plana tube placement in [those] eyes."

Another consideration is the frequency of significant conjunctival scarring in candidates for the KPro implant, resulting from underlying conditions and/or from multiple previous ocular surgeries. Dr. Vajaranant recommended using a corneal or corneoscleral patch graft from corneal rim tissue.

Optimal contact lens fitting also is essential in eyes with KPro implants to maintain corneal graft hydration, prevent corneal melt, and optimize visual rehabilitation.

FYIThasarat S. Vajaranant, MD
Phone: 312/996-7030
E-mail: thasarat@uic.edu

Dr. Vajaranant does not have any financial disclosures relevant to this topic.