Decrease in Tear Secretion and Corneal Sensitivity After Laser In Situ Keratomileusis

Abstract

Purpose. 

To evaluate tear secretion and corneal sensitivity after laser in situ keratomileusis (LASIK) for the correction of myopia.

Methods. 

In a prospective study, 48 consecutive eyes (24 patients) underwent LASIK to correct myopia ranging from −3.5 to −12.25 diopters. Tear secretion tested by the tear function index and corneal sensitivity tested using the Cochet-Bonnet esthesiometer were evaluated preoperatively and 1 week and 1, 3, 6, and 9 months postoperatively.

Results. 

Tear secretion and corneal sensitivity after LASIK were reduced during the first 3 months after surgery (p < 0.001). Tear secretion returned to its preoperative values only after 9 months. Tear secretion and corneal sensitivity were more depressed in long-term contact lens wearers preoperatively and 6 months after surgery (p < 0.05).

Conclusion. 

In the correction of myopia, tear secretion was depressed after LASIK during the first 6 months after surgery.

After laser in situ keratomileusis (LASIK), patients usually report dry eye symptoms. Dry eye arises from a series of etiologies. ¹ Normal corneal sensitivity is important to normal corneal function. The ocular surface (cornea, conjunctiva, accessory lacrimal glands, and Meibomian glands), the main lacrimal gland, and the interconnecting neural reflex loops comprise a functional unit whose parts acts together. ² Tear secretion is in part, if not wholly, reflexive in origin. When the afferent nerves of the ocular surface (trigeminal nerve) are stimulated in a normal individual, a reflex results in immediate blinking and secretion of tears. Sensory loss causes decreased tear secretion and, when bilateral, reduces the blink rate. ³

Significantly decreased sensitivity may be caused by certain disorders, including diabetes and herpes keratitis. Contact lens wear can also induce corneal hypesthesia. ⁴ Various refractive surgical procedures have been associated with marked postoperative hypesthesia. ^5–7 Perez-Santonja et al. ⁷ demonstrated a more depressed corneal sensitivity after LASIK than after photorefractive keratectomy. Only after 6 months did corneal sensitivity return to its preoperative values. In this study, we aimed to evaluate tear secretion and corneal sensitivity after LASIK for correction of myopia.

METHODS

Of 24 patients who underwent LASIK, 48 consecutive eyes were prospectively studied. All procedures were performed by the same surgeon (J.M.B.C.). Patient selection criteria were age 21 to 45 years; stable myopia −3 to −13 diopters (D), astigmatism less than 2.5 D, intolerance of contact lenses or unwillingness to wear spectacles and/or contact lenses, normal anterior segment, normal peripheral retina, and no general health problems, previous ocular surgery, corneal diseases, glaucoma, or history of ocular trauma. Informed consent was obtained from all patients after they received a detailed description of the surgical procedures and their known risks.

Pre-and postoperative basic ocular examination included visual acuity, manifest and cycloplegic refraction, slit-lamp biomicroscopy, applanation tonometry, pachymetry, videokeratography, indirect ophthalmoscopy, tear secretion, and corneal sensitivity testing. Patients were asked to discontinue contact lens use 3 weeks before the surgery. Postoperative tear secretion and corneal sensitivity testing were conducted at 1 week and at 1, 3, 6, and 9 months.

All laser in situ procedures were performed with the Bausch & Lomb Hansatome microkeratome (Claremont, CA, U.S.A.) and the Keratom Multi-Scan Schwind excimer laser (Kleinostheim, Germany). The procedure was done with topical anesthesia of 0.4% oxybuprocaine. The flap diameter created by the microkeratome was 8.5 mm and its thickness was 160 μm. A 6-mm single-zone ablation was used for all of the patients. Ciprofloxacin 0.3% and fluorometholone 0.1% eyedrops were instilled four time a day for the first week.

The Schirmer values with anesthesia and the tear clearance rate ⁸ were measured 5 minutes after instilling a 10-μL drop of 0.5% fluorescein and 0.4% oxybuprocaine hydrochloride into the conjunctival sac. A standard Schirmer test strip then was placed for another 5 minutes. The length of the wet portion was measured and the intensity of its staining was compared with the standard strip colors for the Schirmer test with anesthesia or tear-clearance rate respectively. The tear-clearance rate was determined by the rate at which the color of the fluorescein dye faded and was graded as 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, or 1/256. The tear function index (TFI) was defined as the value of the Schirmer test with anesthesia divided by the tear clearance rate. ⁹ All tests were conducted in a quiet room of relatively constant temperature (22°C) and humidity.

Corneal sensitivity was tested using the Cochet-Bonnet esthesiometer (Luneau, Paris, France). The instrument consists of a nylon monofilament 0.12 mm in diameter with a variable length of 0 to 62 mm so that the pressure applied to against the cornea can be 11 to 200 mg/0.0113 mm². The test is performed with the patient positioned at the slit-lamp. The instrument is advanced perpendicular to the corneal surface until contact is made. If the patient feels the filament, the response will be considered positive. The test was started at the maximal length, approximately 62 mm, which is the lowest possible pressure. A bend in the filament gave an objective measurement of contact. If no response was obtained at 62 mm, the length was reduced to 55 mm, and thereafter it was reduced in 5-mm increments until a positive response was obtained. At each length, three measurements were performed at the center of the cornea (inside the ablated area). The longest filament length with which a positive response was obtained from the patient was considered to be the corneal sensitivity threshold. All measurements were performed by the same observer (J.M.B.C.) before any drops were placed in the eye. We are aware that using the Cochet-Bonnet esthesiometer has limitations, particularly at the upper end of the scale. If the patients feels the filament at the maximum length (62 mm), the response will be considered positive at 62 mm, when the real corneal sensitivity could be 62 mm or higher. However, normal corneal sensitivity is usually approximately 62 mm, and the Cochet-Bonnet esthesiometer was the one most used instruments in previous studies. ^5–7

Both parametric and nonparametric statistical analyses were done, based on the distribution of the data under consideration. Measurements for right and left eyes of each subject were averaged for the analysis. Differences for continuous variables were tested with the Student t test for normally distributed data, and the Mann-Whitney and Wilcoxon rank sum tests for non-normally distributed data. Differences for categoric variables were tested with the Fisher exact test for independence. Correlations between continuous variables were obtained using the Spearman correlation coefficient. Differences were considered statistically significant when p values were less than 0.05.

RESULTS

The mean patient age was 29.3 ± 6.6 years (mean ± SD). Of all, 14 patients were female and 10 patients were male. Both eyes of the patients were included in the study. Mean preoperative refraction was −7.6 ± 2.3 D (range, −3.5 to −12.25 D) and the mean preoperative pachymetry was 535 ± 31 μm (range, 484 to 572 μm). All patients were available at each of the postoperative follow-ups. Of all, 12 patients were long-term contact lens wearers (more than 5 years) and 12 were not contact lens wearers. Mean preoperative and postoperative TFI and corneal sensitivity values are shown in Table 1.

TABLE 1: Mean ± SD preoperative and postoperative TFI and corneal sensitivity values

There was an important decrease in TFI at 1 week and 1 month (p < 0.001) after surgery. TFI showed some recovery at 3 months, although the difference was still significant (p < 0.001). At 6 months, TFI nearly returned to its preoperative values (p = 0.07). TFI returned to preoperative values at 9 months after surgery (p = 0.77).

There was a deep decrease in corneal sensitivity at 1 week and 1 month (p < 0.001) after surgery. Corneal sensitivity showed some recovery at 3 months (p < 0.001) and returned to nearly preoperative values at 6 months after surgery (p = 0.20). At 9 months, corneal sensitivity returned to its preoperative values (p = 0.98). A statistical correlation was found between TFI and corneal sensitivity before and after surgery (p < 0.05). No statistical correlation was found between depth of ablation (as determined by attempted correction) and postoperative TFI and corneal sensitivity at any point of the follow-up (p > 0.05). No statistical correlation was found between age and postoperative TFI and corneal sensitivity at any point of the follow-up (p > 0.05).

There was no significant difference in preoperative refraction, pachymetry, and gender between the long-term contact lens wearers and noncontact lens wearers. There was a significant difference in preoperative TFI and corneal sensitivity when long-term contact lens wearers and noncontact lens wearers were compared (TFI: 231 ± 514 vs. 272 ± 505, p < 0.05; corneal sensitivity: 58.7 ± 5.8 vs. 62.0 ± 0.0, p < 0.05). No differences were found between both groups at 1 week and at 1 and 3 months. However, 6 months after surgery, TFI and corneal sensitivity values were lower in the long-term contact lens wearers than in the noncontact lens wearers (TFI: 220 ± 485 vs. 248 ± 345, p < 0.05; corneal sensitivity: 56.5 ± 7.1 vs. 59.1 ± 3.5, p < 0.05). No differences were observed between both groups at 9 months after surgery.

DISCUSSION

The primary function of tears is to serve as a first line of defense to protect the ocular surface against microbial and toxic agents. Tears supply necessary ingredients (e.g., vitamin A and lysozymes) and remove harmful substances (e.g., inflammatory cytokines and allergens). Any imbalance between tear secretion, evaporation and drainage can impair the precorneal tear film and, thus, can result in dry eye. Dry eyes may be assigned to two major classes: tear-deficient dry eye and evaporative dry eye. ¹ Tear-deficient dry eye requires the demonstration of defective lacrimal production. Secreted by the lacrimal gland and swept over the ocular surface by blinking, tears either evaporate or migrate to the inner canthus and ultimately to the nose. Tear dynamics thus are determined by three factors: production or secretion, evaporation, and drainage. Although we cannot measure tear secretion independently and directly, the TFI incorporates the Schirmer and the tear clearance rate tests and eliminates the influence by the forces of tear drainage. The higher the TFI, the higher tear secretion. ⁹

Corneal sensitivity is mediated by axon terminals of the long ciliary nerves. Seventy to eighty large radial branches enter the cornea at the midstromal level. As these nerves course to the center of the cornea, they branch horizontally and vertically, giving rise to the dense subepithelial plexus beneath Bowman layer. ¹⁰ Normal corneal sensitivity is essential to normal structure and function. Although some variations in corneal sensitivity are normal, significantly decreased sensitivity may be caused by diabetes, herpes simplex, contact lenses, or by some surgical and medical ocular treatments. ⁴ Corneal hypesthesia compromises the protective blinking reflex, delays epithelial wound healing, and is associated with decrease tear secretion. Stimulation of the ocular surface initiates neural signals, resulting in aqueous tear secretion. ² Corneal sensitivity is very acute, centrally processed, and interpreted solely as pain. ¹¹ The so-called basal tearing results from continuous stimulation of the corneal surface by environmental factors, even though these signals occur below the level of perception in normal individuals. ³

Several corneal and refractive surgical procedures have been associated with a marked loss of normal sensitivity. A return of corneal sensitivity to normal levels within 12 months after penetrating keratoplasty has been reported. ¹² Radial keratotomy can also reduce corneal sensitivity as long as 1 year postoperatively. ⁵ After photorefractive keratectomy, patients recover corneal sensitivity within the first 3 postoperative months. ¹³

LASIK to correct myopia is a new technique that has generated high expectations among refractive surgeons. Preliminary studies found that LASIK offers excellent results in the correction of moderate and severe myopia, with few complications in experienced hands. ^14–16 In LASIK, a corneal flap (with the corneal epithelium, Bowman layer, and anterior stroma) is created using a microkeratome. During this procedure, the superficial stromal nerves are cut in the flap margin, and the nerves in the stromal bed under the flap are subsequently exposed to excimer laser photoablation. Linna et al. ¹⁷ have shown in rabbit cornea that at 5 months after LASIK, the epithelial, subepithelial, and anterior stromal innervation had gained an almost normal nerve density and architecture. Perez-Santonja et al. ⁷ in a series of 17 eyes after LASIK for the correction of myopia, found that corneal sensitivity was nearly normal 6 months after surgery. Our results show a deep decrease in corneal sensitivity at 1 week and at 1 and 3 months after LASIK. Although some recovery was present at 6 months, corneal sensitivity returned to its preoperatively values at 9 months.

Reduced corneal sensitivity facilitates dry eye by two mechanisms: sensory loss causes decreased tear secretion ³ and, when bilateral, reduces the blink rate. Infrequent blinking is associated with ocular drying due to increased tear evaporation. Therefore, dry eye in LASIK may be assigned to the two major classes: tear-deficient and evaporative dry eye. ¹ Our study shows that tear secretion after LASIK for the correction of myopia is reduced during the first 6 months after surgery and that it returned to its preoperative values only after 9 months. Comparing long-term contact lens wearers with noncontact lens wearers, corneal sensitivity and tear secretion was more depressed preoperatively and at 6 months after LASIK in long-term contact lens wearers, although no differences were found at 9 months. Loss of corneal sensitivity is a feature of contact lens wear and it has been proposed as a mechanism for dry aye associated with long-term contact lens wear. ¹⁸ Alternative suggestions for the mechanism of decreased tearing may also relate to changes in the shape of the ocular surface and its relationship to the upper lid possibly resulting in increased evaporative tear loss. In conclusion, in the correction of myopia, tear secretion is reduced after LASIK. Only after 9 months is tear secretion recovered. These results indicate the importance of artificial tears usage in the LASIK patients.