Eyeworld

EW CORNEA 84 February 2011 A modified procedure in- volving immunofluores- cence staining and imaging has allowed re- searchers to better under- stand the layout of human corneal nerves in the epithelium and stroma, leading to new clinical un- derstanding as well. "The modified technique of tis- sue preparation used for this study al- lowed for observation of new nerve structure features and, for the first time, provided a complete view of the human corneal nerve architec- ture," reported study co-author Nicolas G. Bazan, M.D., Ph.D., di- rector, Neuroscience Center of Excel- lence, ophthalmology department, Louisiana State University Health Sci- ences Center, New Orleans. "Our study reveals that aging decreases the number of central epithelial nerve terminals, and increases the presence of irregular anomalies beneath the basal layer." The study was published online in July 2010 in Experimental Eye Research. New technique, new understanding Dr. Bazan and colleagues analyzed 28 human eyes from 14 donors of different ages and genders using the modified technique. "We developed an approach to generate a three-dimensional map that facilitates the assessment of ep- ithelial nerve density in different corneal areas to define aging and gender influence on human corneal nerve architecture," Dr. Bazan re- ported. "Corneal nerves were stained and consecutive images acquired with a fluorescence microscope, recorded at the same plane, and merged for viewing the complete ep- ithelial and stromal nerve architec- ture." Dr. Bazan divided the eyes into age groups in order to study aging changes on epithelial nerve densi- ties. The age groups consisted of 19- year-old eyes (two eyes), 40- to 54-year-old eyes (eight eyes), 63- to 67-year-old eyes (six eyes), and 75- to 80-year-old eyes (six eyes). "A gradual decrease in central epithelial nerve density concomitant with increasing age was found, and a significant decrease (p<0.01) was ob- served in the corneas of 75- to 80- year-old donors compared to the 19-year-old and the group of 40- to 54-year-old donors," Dr. Bazan re- ported. "In the older corneas (>60 years), irregular lesions of nerve fibers were present in the superficial stroma beneath the basement mem- brane. These lesions were localized around the peripheral cornea in areas where the epithelial nerve bundles emerged. These anomalies were found more frequently in corneas of 75- to 80-year-old donors (17 +/–3.6/cornea) as opposed to those of 60- to 67-year-old donors (7.8 +/–3.0/cornea)." Dr. Bazan also there is a connec- tion between corneal innervation, age, and dry eye. "Clinical studies reported that incidence of dry eye increases significantly with age," Dr. Bazan noted. "The age-related de- crease in corneal innervation dis- rupts the reflex circuits that drive blinking and lacrimation and also diminishes essential corneal nerve- supplied throphic factors, thus lead- ing to diminished corneal sensitivity, dry eye and impaired wound healing." The modified imaging tech- nique found no differences between the distributions of epithelial nerves in the four quadrants, which has clinical implications. "Controversy has arisen in re- gards to the flap position in LASIK surgery procedures," Dr. Bazan noted. "Some ophthalmologists be- lieve a horizontal flap is better than a vertical flap … while others think there is no difference. Our findings that the distributions of both stro- mal nerves and the epithelial nerve bundles are not significantly differ- ent in each quadrant provide direct anatomical evidence to support that the position of the flap is irrele- vant." Still, some ophthalmologists are finding that how the flap is created is very relevant in reducing the risk of post-op dry eye. In one poster presented at the 2009 ASCRS•ASOA Symposium & Congress in San Francisco, Henry Perry, M.D., Ophthalmic Consult- ants of Long Island, Rockville Cen- tre, N.Y., suggested that the ability of the femtosecond laser to cut ellipti- cal flaps "allows use of [a] wider hinge angle for greater flap stability and improved preservation of corneal nerves." Dr. Perry suggesting using sev- eral surgical techniques to reduce dry eye following LASIK, all related to flap creation. "Small elliptical flaps with large hinges reduce the risk of post-opera- tive dry eye," Dr. Perry reported. He recommended customizing the flap to the ablation, using thin planar flaps, and performing bevel-in side cuts to improve corneal nerve appo- sition. In one study, he analyzed how an elliptical flap with a reverse side cut impacted corneal sensation fol- lowing LASIK. Twenty eyes of 10 patients un- derwent LASIK using elliptical/re- verse side-cut flaps (150 degrees) and 4-mm hinges. Twenty eyes of 10 pa- tients also had LASIK with round 70- degree flaps and 3-mm hinges. Mean corneal sensation was bet- ter at 1 week, 1 month, and 3 months in the elliptical/reverse side cut group. "Elliptical flap shapes prevent resection of the vital peripheral corneal fibers, move the hinge pe- ripherally to maximize stromal bed exposure for full delivery of excimer laser ablation, and allow use of [a] wider hinge for greater flap stability and preservation of corneal nerves," Dr. Perry concluded. EW Editors' note: Dr. Bazan has no finan- cial interests related to this study. Dr. Perry has no financial interests related to his poster. Contact information Bazan: 504-599-0877, hbazanl@lsuhsc.edu Perry: hankcornea@aol.com by Matt Young EyeWorld Contributing Editor Modified nerve staining technique leads to new corneal observations Lissamine green is used to determine dry-eye severity. Nicolas G. Bazan, M.D., Ph.D., finds corneal innervation, age, and dry eye are linked Source: James P. McCulley, M.D.

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