Preoperative astigmatism (regular or irregular) has to be considered when planning cataract, corneal or refractive surgery. In the case of physiological astigmatism we can plan a neutral effect or we can try to limit the effect of the induction of astigmatism the surgical procedure planned, or use it to our benefit.
Important Preoperative Assessments
Important measures in the preoperative evaluation are: unaided and aided visual acuity, autorefraction, manifest and cycloplegic refractions, keratometry, computerized corneal topography and endothelial cell count.
Endothelial changes should be assessed, since Fuchs’ endothelial dystrophy, traumatic breaks in Descemet’s membrane, and other causes of endothelial dysfunction may preclude stromal ablation.
Pupil size measured in different degrees of illumination is essential. Young patients with large pupils should be advised of potential glare, halo and night driving problems following refractive surgery. One has to weigh the benefits versus these difficulties in highly myopic patients requiring small optic zones to correct that much myopia. In patients with myopia over –10 D and large pupils, refractive surgery may give rise to visual discomfort. The same applies to phakic IOL implantation when using very thin, foldable lenses with a small diameter optic.
Biomicroscopy
The presence and quality of the tear film should be evaluated and specific tests, such as Schirmer’s and tear film breakup time (BUT) done. Corneal clarity, and absence of conjunctival pathology which could perhaps cause difficulty with fixation of the suction ring in LASIK, should be noted. Tonometry is necessary to exclude undiagnosed ocular hypertension or glaucoma and to establish a preoperative intraocular pressure (IOP) level.
The examination of the anterior segment for lens changes, such as early posterior subcapsular cataract, should be done both to document its existence prior to surgery from a medical/legal point of view, and to suggest the possibility of lens extraction
by phacoemulsification, with (or without) implantation of an intraocular lens (IOL), as an alternative method of refractive correction.
Detection of Corneal Problems
One of the most important principles of laser ablation with the excimer laser is that the currentinstruments, will superimpose the ablation on any irregularity present at the time of surgery. The two most common sources of preoperative irregularity are warpage from rigid or gas-permeable lenses. Even soft lenses can produce limited warpage in some patients. If the topography shows significant irregularity or an abnormal pattern (for example steeping of the topography of the inferior cornea relative to the superior), we continue to monitor the topography at one- to two-month intervals until the topography is normal and stable.
Some irregular astigmatism may occasionally be noted during examination. The same strategy of following the corneal topography over time should be used. However, it is very rare that soft contact lens-induced warpage would take more than one to two months to resolve.
Keratoconus patients are frequently detected in a busy kerato-refractive practice. Studies have shown that two to 10 percent of the patients who are seen for preoperative screening will have inferior steepening of the corneal topography consistent with the patient being a keratoconus suspect or having frank keratoconus. This very high incidence is due to self-selection. These patients are typically not satisfied with their quality of vision with glasses or contact lens.
Dry Eye Patients
A major problem following LASIK is the reduction in tearing that is attributable to the corneal nerves being severed during production of the flap. This interrupts the normal cornea-central nervous system-lacrimal gland reflex arc that modulates even basal tear production. It also temporarily interrupts the known neurotrophic effect of the corneal nerves on the epithelium. Patients with manifest dry eye will be even more likely to have major symptoms and signs after surgery. Usually, these patients do get better over time, but they often need to use frequent tears and ointments without preservatives to get through those first few months.