IOP measurement after LASIK may be unreliable

By Stefanie Petrou-Binder MD

BERLIN - Intraocular pressure (IOP) in LASIK-treated eyes may be much higher than it seems, according to Burkhard Dick MD.
He took a critical look at the reliability of glaucoma diagnostic tools at the annual Congress of the German Ophthalmologic Society in Berlin.
"The ophthalmologist must be aware that his post-LASIK patient may have an increased IOP which the standard diagnostic instruments measure as inaccurately low.

"Knowing this, it is his responsibility to establish by just how much the pressure measurement is off. This is of particular significance in the LASIK treated myopic patients whose central corneal contours have been changed," Dr Dick said.
He added that changes in the cornea which alter the efficacy of tonometric instruments are especially dangerous in myopes who are already at a higher risk from develop glaucoma.

According to Dr Dick, precise glaucoma diagnostics have a heightened relevance in the context of the rising popularity of refractive surgery in Germany.
Myopic patients constitute a quarter of glaucoma cases. They are therefore at an increased risk from developing glaucoma after undergoing LASIK procedure.

Several studies have revealed that tonometric applanation measurements have overestimated or underestimated IOPs depending on the original corneal thickness and shape, which varies from person to person and on the size of the ablation zone.
Tonometric instruments are calibrated according to predetermined physiological standards. Standardised physiological corneal parameters such as the corneal thickness, flexibility and perfect spherical shape inevitably vary from eye to eye.

They therefore do not quite represent the patient's individual corneal measurements to begin with and can certainly no longer be applied after refractive surgery, Dr Dick said.
He stressed that tonometric instruments were conceived for application onto the central cornea, which is the precise area ablated for myopia correction.
The central cornea is made thinner and flatter. LASIK can also dry the cornea, rendering it less flexible.

"The question is are tonometric values reliable? When in doubt, we perform postoperative 'temporal' Goldman tonometry in Mainz, making sure the patient looks straight ahead, doesn't adduct and that the measurement is made 1.5 mm to 2.0 mm from the corneal centre.
"The peripheral corneal structure and thickness are often unchanged, allowing peripheral measurements to be more accurate as studies implementing the Tonopen have shown," he stressed.

Dr Dick reported that IOP readings after myopia correction might be off by 2.0 mm Hg to 5.0 mm Hg. These values represent not only physiological differences among patients but also the increasingly larger ablation zones.
Diverse studies have shown that the use of various tonometres can be responsible for up to 30% of reported errors.

Measurements with the same instrument by different investigators have been shown to vary by up to 3.0 mm Hg.
Moreover, clinical experience shows that IOP can vary intra-individually up to 7.0 mm Hg during the course of the day. All these factors help to account for inconsistencies in IOP measurements and contribute to the current confusion felt by ophthalmologists.

By how much are measurements off with reference to the level of the ablation zone? As a rule of thumb, and due to a lack of any proven guidelines, Dr Dick suggests there is almost
1.0 mm Hg deviation per 31 microns ablation depth which represents about
2.5 D.

Dr Dick asserted that along with the fundamental alteration of the thickness and structure of the corneal contour, the forces exerted on the cornea during refractive surgical procedures put additional pressure on and within the eye.

The suction of the corneal surface through the corneal microkeratome ring prior to flap cutting and the shock waves created within the eye from high-speed keratectomy increase IOP sometimes to values exceeding 120 mm Hg, he said.
He cautioned his fellow ophthalmologists to be aware that patients who receive topical steroids for diffuse interstitial keratitis may present with a paradoxical hypotonia after LASIK.

In myopic patients especially, steroid-induced glaucoma increases IOP. It is caused by an increase in the interface fluid which paradoxically keeps the IOP low. As many publications substantiate, this state can bring about glaucomatous damage without detection.

Although tonometry is a mainstay in glaucoma diagnostics, it detects only around 50% of glaucoma cases. Perimetry and papillary diagnostics therefore play a critical role in recognising the signs that indicate glaucomatous change that go beyond increased IOP.
Digital optic nerve head measurements were compared in a study involving 40 eyes (20 patients) using confocal scanning laser-polarimetry (SLP), confocal scanning laser topography (SLT) and optical coherence tomography from Zeiss (OCT).
The study revealed that under less than optimal corneal conditions, these methods gave inconsistent results, particularly with SLP without corneal compensation. In contrast, SLT and OCT were more reliable.

At the University of Mainz Eye Clinic, Germany, the same physician carries out the preoperative and postoperative IOP measurements on LASIK patients at the same place and at a similar time if the patient has a positive glaucoma family history or is at higher risk from glaucoma.
Patients receive a 'LASIK passport' in which both preoperative and postoperative refractive values are listed, as well as the corneal radii and the IOP.

"At our institution, the presence of a glaucoma surgery is an indication to hold back on a LASIK procedure. We recommend ophthalmic surgeons to remember that IOP is usually underestimated after LASIK and that the informative value of SLP is also therefore limited," Dr Dick said.

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