Post-ablation intraocular pressure

Corneal refractive procedures alter the biomechanical properties and physiology of the cornea in ways that render Goldmann applanation tonometry unreliable, but measurements with some of the newer tonometry devices are less affected by the corneal changes, said Miguel Teus MD at a Glaucoma Day session of the XXXIII Congress of the ESCRS in Barcelona, Spain.

“It is well-known that IOP as measured with Goldmann applanation tonometry decreases significantly after these procedures,” said Miguel Teus MD, University of Alcalá, Madrid, Spain.

He noted that corneal laser refractive procedures modify both the central corneal thickness and corneal viscoelastic properties. The viscoelastic changes may be caused by changes in the collagen and extracellular matrix.

Following surface ablation, haze, is the main potential adverse event and it results from an activation of keratinocytes and myofibroblasts that, in turn, causes the synthesis of a new and disorganised extracellular matrix which increases light scatter. However this is easily prevented using mitomycin-C.

Following a LASIK ablation, the cornea loses all of the tensile strength that had been provided by the flap tissue previous to flap creation. All of the cornea’s resistance to deformation beneath the flap is in the residual stromal bed. Research suggests that the risk of ectasia begins to reach significance once the total thickness of the flap and ablated stroma is greater than 40 per cent of the preoperative corneal thickness.

On that basis, various authors have suggested that small-incision lenticule extraction (FLEX) - where only a small side-cut is used - might induce less corneal biomechanical changes than small-inicision lenticule extraction (SMILE), where the lenticule is removed from beneath a flap.

However, in a contralateral eye study involving 35 patients there was no significant difference in the change in Goldmann tonometry IOP between eyes that underwent FLEX and those that underwent SMILE, (Vestergaard et al, J Cataract Refract Surg 2014;40:403411).

That suggests the creation of other factors that reduce corneal rigidity other than decreased corneal thickness. Changes in keratocyte cell density may play a role, since it could result in low collagen synthesis. Fluid in the flap-stroma interface result from sudden increases in IOP may also affect tonometry readings.

He noted that the measurements with the Pascal Dynamic Contour Tonometer (Pascal), The Ocular Response analyzer and the Corvis® ST (Oculus) tonometer have been shown in several studies to have minimal change in their IOP readings after LASIK and are therefore more accurate than Goldman applanation tonometry in patients who have undergone the procedure.

Miguel Teus MD University of Alcalá, Madrid, Spain.

Miguelteus@gmail.com