CAIRNS' TRABECULECTOMY

 

The original objective of glaucoma surgery was to allow aqueous humor to exit more freely either through the sclera or into the suprachoroidal space. The former came to be called, generically, a glaucoma filtering procedure.

In his seminal paper published in 1968, John Edward Cairns described the goal of trabeculectomy as excising a short length of the Canal of Schlemm, with its trabecular adnexa, thus leaving two cut ends opening directly into the aqueous humor, with no trabecular tissue remaining as a barrier at that point, and restoring the integrity of the corneoscleral coat over the area of the excision. 

John Edward Cairns

Although naming this procedure trabeculectomy was appropriate (because the trabecular meshwork was removed to open Schlemm’s canal), the procedure might also have been accurately called canalostomy.

Cairns hoped that cutting open the edges of Schlemm’s canal would allow aqueous to leave without producing a filtering bleb; however, it became apparent that Cairns’ ‘‘trabeculectomy’’ only worked when a filtering bleb developed.

Pathology later showed fibrotic closure of the cut ends of Schlemm’s canal. Additionally, the presence of Schlemm’s canal in the trabeculectomy specimen did not correlate with outcomes.

The procedure that started as a ‘‘trabeculectomy’’ worked as a guarded filtration procedure, but the terminology remained unchanged.

Cairns reported 17 ‘‘trabeculectomy’’ procedures on eyes with uncontrolled glaucoma. After creating a flap starting in the cornea 2--3 mm anterior to the limbus and hinged on the sclera, a deep-scleral lamella containing Schlemm’s canal and trabeculum was excised and the flap was sutured firmly. A peripheral iridectomy was performed in seven patients. Seven of 17 patients developed a bleb, and histologic evaluation of the excised tissue in eight patients showed trabecular tissue.

Control of IOP for 10--14 weeks without subconjunctival drainage of aqueous humor occurred in about two-thirds of the patients. Cairns’ explanation for the reduction of IOP was the flow of aqueous through the open ends of Schlemm’s canal.

Further studies by Cairns and other investigators led to the conclusion that Cairns’ trabeculectomy was a ‘‘guarded filtering procedure’’.

Bleb formation occurred in most successful cases, histological studies on the excised block failed to show trabecular meshwork and patency of cut ends of Schlemm’s canal was observed in only a minority. In a few patients, the cut edges of Schlemm’s canal remained open. In these cases, compression of superficial scleral veins years after the surgery caused the passage of blood through the cut edges of Schlemm’s canal and into the anterior chamber. This finding indicated that at least some of Cairns’ trabeculectomies functioned as he intended.