[Corneal sensibility following epikeratophakia]

Neural basis of sensation in intact and injured corneas

A renewed interest in the characteristics and neural basis of corneal and conjunctival sensations is developing in recent years due to the high incidence of discomfort and altered sensitivity of the cornea following refractive surgery, use of contact lenses and dry eyes. Corneal nerves are functionally heterogeneous: about 20% respond exclusively to noxious mechanical forces (mechano-nociceptors); 70% are additionally excited by extreme temperatures, exogenous irritant chemicals and endogenous inflammatory mediators (polymodal nociceptors), and 10% are cold-sensitive and increase their discharge with moderate cooling of the cornea (cold receptors). Each of these types of sensory fibres contributes distinctly to corneal sensations. Mechano-nociceptors mediate, sharp acute pain produced by touching of the cornea. Polymodal nociceptors elicit the sustained irritation and pain that accompany corneal wounding; cold receptors evoke cooling sensations. Depending on the relative activation by the stimulus of each subpopulation of corneal sensory fibres, different subqualities of irritation and pain sensations are evoked. Corneal sensations can be explored experimentally in humans with a gas esthesiometer that applies controlled mechanical, chemical and thermal stimuli to the corneal surface. When the cornea is wounded, corneal nerves are excited and eventually severed in a variable degree and local inflammation is produced. Activated corneal nerves release neuropeptides (SP, CGRP) that contribute to the inflammatory reaction (neurogenic inflammation). They also become sensitized by local inflammatory mediators, such as prostaglandins or bradykinin and thus exhibit spontaneous activity, lowered threshold and enhanced responses to new stimuli. This leads to spontaneous pain and hyperalgesia. Nerves destroyed by injury soon start to regenerate and form microneuromas that exhibit abnormal responsiveness and spontaneous discharges, due to an altered expression of ion channel proteins in the soma and in regenerating nerve terminals. Presumably, this altered excitability is the origin of the lowered sensitivity and the spontaneous pain, dry eye sensations and other disaesthesias reported in patients following refractive surgery.

Corneal Sensitivity 10 Years After Epikeratoplasty

PURPOSE

To assess corneal sensitivity in patients 10 or more years after epikeratoplasty for myopia, aphakia, hyperopia, and keratoconus.

METHODS

A total of 45 eyes of 33 patients (age range at examination 33 to 55 yr) were included in the study. Corneal sensitivity thresholds using an electromagnetic aesthesiometer (Draeger) were measured at various locations on the lenticule and the recipient cornea. Measurements were evaluated regarding the corrective purpose, surgical technique, age, and gender of the patients and the area of measurements.

RESULTS

The mean corneal sensitivity threshold in the center of the epikeratoplasty lenticule was significantly lower than on the peripheral recipient cornea (320.0 +/- 365.1 x 10(-5) N versus 0.1 +/- 0.5 x 10(-5) N). Corneal sensitivity at the 3 and 9 o'clock positions was significantly higher compared to the values at the 6 and 12 o'clock positions on the lenticule. No correlations of these values with age, gender, type, and primary indications for the surgery were observed.

CONCLUSIONS

These results indicate a relative hypesthesia of the epikeratoplasty lenticule as compared to the peripheral host cornea, even 10 years after surgery. In our patients no clinically significant changes were observed that could be attributed to the reduced sensitivity over this follow-up period.

Change in corneal sensitivity following laser in situ keratomileusis

PURPOSE

To evaluate the change in corneal sensitivity following laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Maryknoll Hospital, Pusan, Korea.

METHODS

The corneal sensitivity in 40 eyes (32 patients) was measured before and 1 and 2 weeks and 1, 3, and 6 months after LASIK. Sensitivity measurements were made with an esthesiometer at central, temporal, inferior, nasal, and superior points on the corneal flap. The eyes were grouped by ablation depth: Group 1, corneal ablation depth less than 100 microns; Group 2, corneal ablation depth more than 100 microns.

RESULTS

Corneal sensitivity did not recover to the preoperative level by 6 months after LASIK. Except at the hinge of the corneal flap, the pattern of sensitivity recovery was similar among the various points. Recovery was more rapid at the hinge than at other areas (P < .05). There was a trend toward a greater reduction in corneal sensitivity in Group 2, the group with deeper ablations, than in Group 1.

CONCLUSION

The results suggest that lamellar cutting of the cornea during LASIK impairs corneal sensitivity and that the depth of the corneal ablation affects the extent of corneal sensitivity loss and recovery.

Corneal sensation after cataract and refractive surgery

Most surgical procedures involving the anterior segment of the eye disrupt the normal organization of corneal innervation. Since denervation of the cornea results in impaired epithelial wound healing, increased epithelial permeability, decreased epithelial metabolic activity, and loss of cytoskeletal structures associated with cellular adhesion, it is important to identify the factors that determine the extent of neural regeneration. Mechanisms of corneal nerve damage and studies of corneal nerve fiber loss and reinnervation after cataract and refractive surgery--epikeratophakia, cryokeratomileusis, keratomileusis in situ, photorefractive keratectomy, laser in situ keratomileusis, and phacoemulsification--are reviewed and the decrease in corneal sensitivity, as a measure of corneal destruction and corneal metabolism, after these surgical procedures is compared.

Effect of beam variables on corneal sensitivity after excimer laser photorefractive keratectomy

AIM

To investigate changes in corneal touch sensitivity following excimer laser photorefractive keratectomy (PRK) using different beam configurations.

METHODS

20 subjects were given a unilateral -3.00 D correction with either a 5 mm (26 micrograms, n = 10) or 6 mm (42 micrograms, n = 10) beam diameter. Thirty subjects underwent a unilateral -6.00 D correction with 5 mm (62 micrograms, n = 10), 6 mm (78 micrograms, n = 10), or multizone (62 micrograms, n = 10) treatments. The multizone treatment was 6 mm in diameter with the depth of the 5 mm treatment. Corneal sensitivity was measured using a slit-lamp mounted Cochet-Bonnet aesthesiometer before and at 1, 3, 6, and 12 months after PRK. Stimulus locations included points lying within the ablated zone (central) and outside (peripheral). These were compared with the equivalent locations in control (untreated) eyes.

RESULTS

There was a significant reduction in corneal sensitivity within the central (ablated) zone in all treatment groups after PRK. In most groups a return to full sensitivity was achieved by 6 months with the exception of the multizone treatment group which showed significant corneal hypoaesthesia at 12 months. Peripheral corneal sensitivity was also reduced in this group up to 3 months after the procedure. A comparison between the -3.00 D and -6.00 D treatment groups showed no significant difference. However, combining data from all treatment groups, a significant correlation was found between the interocular difference in central corneal sensitivity and postoperative haze at 3 and 6 months.

CONCLUSIONS

For corrections up to -6.00 D ablation depth and treatment zone diameter do not appear to be clinically important determinants of corneal hypoaesthesia. In contrast, postoperative corneal haze appears to correlate with sensitivity loss.