Selfless Patient Allowed PRK to Enter the Clinic

In 1988, Marguerite B McDonald MD, a young faculty member at Louisiana State University (LSU) in New Orleans, US, performed the first in-human excimer laser refractive procedure. The patient, Alberta Cassady, had advanced ocular melanoma and was scheduled to undergo exenteration of the eye in 11 days.

“Alberta asked her oculoplastic surgeon if anyone could use her eye for research. If it were not for this selfless thought, we may never have moved beyond doing only preclinical research,” Dr McDonald emphasised.

While the FDA had refused to allow initiation of a blind eye clinical trial, the agency granted permission to perform photorefractive keratectomy (PRK) on Ms Cassady, whose eye was healthy aside from the tumour and emmetropic with 20/20 vision. 

“We performed a 4.5 D myopic ablation. Findings from daily exams until the exenteration and on post-exenteration examination showed the refractive and anatomic results were perfect,” Dr McDonald said. 

“We named our research lab after Ms Cassady, but I think it would also be fitting for laser vision correction procedures to be called Cassady procedures instead. Without Ms Cassady, we might still be doing monkey studies.”

Dr McDonald’s interest in using the excimer laser to reshape the cornea arose in 1983 after reading the American Journal of Ophthalmology paper “Excimer laser surgery of the cornea” by Stephen Trokel MD, and colleagues. Dr McDonald had recently joined the LSU ophthalmology department after completing a coveted cornea fellowship under the mentorship of Herbert E Kaufman MD and was the leader of the department’s refractive team. 

“What I found most intriguing in this paper describing removal of corneal tissue without collateral thermal damage was a finding suggesting the laser might be used for refractive surgery,” she explained.

After contacting Dr Trokel about exploring this idea and enticing him with the idea that the LSU primate centre offered access to animal eyes, Dr McDonald teamed up with Dr Trokel, who was already working with Charles Munnerlyn PhD. Stephen Klyce PhD also joined the team, using his new colour-coded corneal topography maps to help guide the effort. Live animal research began after countless plastic blocks and animal cadaver eyes were ablated using a wide beam industrial laser, but the initial in vivo outcomes were poor as the eyes developed significant central corneal scars. Not giving up, Dr McDonald postulated that changing the laser diaphragm stops from 5 to 40 could solve the problem. She was right.

Once they treated Ms Cassady, the team was granted FDA permission to begin treating blind eyes. Fear the laser could leak toxic gas into the environment required that the procedures be performed in an outdoor trailer situated next to a trash compactor away from campus buildings. Concerned that shaking of the laser induced by the operating compactor was compromising surgical outcomes and unable to get the sanitation engineers to stop the compactor during surgeries, Dr McDonald set out to prove her point.

“Much to my amazement and embarrassment, I found procedures performed when the compactor was running had better outcomes,” she said. “Realising that the vibrating laser created blended zones between spots and therefore smoother ablations, we increased the number of diaphragm stops to 120 and achieved better results.” 

Dr McDonald recognises she is a key figure in the development of excimer laser refractive surgery. But, she also emphasises she had the good fortune to work with a tremendous team and is honoured knowing that their seminal work has improved quality of life for so many people.