|Year : 1989 | Volume
| Issue : 1 | Page : 17-19
Specular microscopy of unilateral hard contact lens wearers
Vijay K Dada, Arun K Jain, Manoj R Mehta
Dr.R.P. Centre, AIIMS, New Delhi - 110 029, India
Vijay K Dada
Dr.R.P. Centre, AIIMS, New Delhi - 110 029
Source of Support: None, Conflict of Interest: None
The effects of long term wear of PMMA contact lenses on the corneal endothelium used on a daily wear basis were determined by examining 8 patients who had been prescribed lenses in one eye only. The patients excluded unilateral aphakes. It was found that a significant reduction in the cell density and a change in the cell size and shape was induced by PMMA lenses. These changes reflect an endothelial metabolic stress and emphasize the need to find newer lens materials that are more oxygen permeable and produce little interference with the corneal metabolism.
|How to cite this article:|
Dada VK, Jain AK, Mehta MR. Specular microscopy of unilateral hard contact lens wearers. Indian J Ophthalmol 1989;37:17-9
|How to cite this URL:|
Dada VK, Jain AK, Mehta MR. Specular microscopy of unilateral hard contact lens wearers. Indian J Ophthalmol [serial online] 1989 [cited 2020 Apr 7];37:17-9. Available from: http://www.ijo.in/text.asp?1989/37/1/17/26111
| Introduction|| |
During normal sleep, closure of the lids shuts off the atmospheric oxygen supply to the cornea and partial pressure of oxygen falls . Contact lens induced diurnal variations in the corneal endothelium consists of intracellular stippling, separation of cell boundaries, and small blister like areas separating the cells ,,,, Physical presence of silicone lens alone produced significant alteration in the endothelial appearance.  The aetiology is more complicated than previously suspected, since blebs can be produced independent of precorneal hypoxia. The most important factor is the ability of any stimulus, which induced endothelial changes, to change the pH in or near the corneal endothelial layer. 
Pleomorphism following contact lens wear has been frequently reported ,,. In one study no changes were seen in endothelial cell area. Several reports have demonstrated that PMMA hard contact lens wearers have a greater degree of polymegathism of the corneal endothelium, with cell density remaining in the normal range. ,, We conducted a study, in order to characterize and see the effect of hard contact lens wear on the morphologic characteristics of the corneal endothelium in unilateral hard contact lens wearers. The other eye provided the control.
| Material and methods|| |
A total of 8 patients wearing hard (PMMA) contact lenses in one eye were selected from the contact lens clinic at Dr. R.P. Centre of Ophthalmic Sciences, AIIMS, New Delhi. The other eye was either emmetropic or amblyopic. A contact lens wear history, detailing number of years of wear and average number of hours per day was noted. For each case a control was matched for age (± 2.5 years of age), race and refractive correction (± 1.5 D of spherical equivalent). Patients with matched controls were taken from the refraction clinic at Dr. R.P. Centre. Patients with histories of any ocular disease like Ocular trauma, Glaucoma, Uveitis and eyes with anterior segment surgery were excluded from the study. Also those patients who had suffered attacks of over wear syndrome were excluded from the study. A slit lamp examination was performed on the cornea and the anterior segment and the subjects were excluded if any abnormality was noted. Keratometry and specular microscopy of the central endothelium was performed on both the eyes of the subjects as well as normal controls. Ten to twelve photographs were taken for each cornea. In addition to this corneal thickness was noted with a digital pachymeter attached to the specular microscope. For pleomorphism and polymegathism the pictures obtained from the test eye were compared with those obtained from the fellow eye and also the control group.
| Results|| |
In the control group, the corneal thickness and endothelial cell count of the two eyes of a subject did not reveal any significant variation. Endothelium of the control groups had a relatively regular distribution of cells.
In contact lens group the corneal thickness difference between the two eyes was not statistically significant 0.49±0.023,0.49±0.022. The endothelial cells count, slightly lesser in the eye with contact lens wear, was statistically significant 1900 ± 220 (P<0.0016) 2300 ± 220.
A significant finding was moderate degree of polymegathism and pleomorphism in the eyes with contact lens when compared to the fellow eye. Area of clustering of large and small cells were seen. Groups of four and five sided cells (rosette) pointing towards a central site suggesting cell loss were also seen. Intracellular black-out area, larger areas of shadowing approximately 2-3 cells across with irregular boundaries were observed.
| Discussion|| |
Long lasting changes in the shape and the size of corneal endothelial cells have been reported by a few researchers. ,,, Caldwel et al  used small field specular microscope and reported slight decrease in endothelial cell density with age and with increasing contact lens wear, but they did not observe the bizarre endothelial shapes and changes in cells sizes.
Pathogenesis of the endothelial changes in these hard contact lens wearers remains unknown. It is unclear whether the corneal endothelial polymegathism and pleomorphism are caused by (a) relative long term hypoxia, an anaerobic metabolic shift of the corneal endothelium, build up of toxic byproduct such as lactic acid; or chronic corneal swelling and de- swelling. However, the most likely actinologic factor is that of `chronic hypoxia'. Also, in hard contact lens wearers endothelium is more dependent upon aqueous oxygen than stromal oxygen transmission from the epithelial surface. , Endothelial function may not be affected, as no significant change in corneal thickness was found.
Recent studies by Hirst  and MacRae , noted that longer use of PMMA hard contact lens may be associated with an increase in the degree of morphometric changes of polymegathism and pleomorphism. In our study also we noticed tendency towards polymegathism and pleomorphism.
Endothelial_ cells are known to have the capacity to shift, elongate, expand and rearrange after a minimal endothelial trauma,  which is attributed to be the action of contractile microfilaments within the endothelial cells,  The large cells could be as a result of spreading to fill defects but could also represent the coalescence of cells. , In mechanical endothelial trauma the distribution of cells is either uniform; uniformly mixed with small and large cells adjacent to each other, or with a gradient of change across a corneal meridian.  Partial reduction of polymegathism following removal of the lens could be explained by a boundary shortening mechanism whereby the total cell boundary length tends towards a minimum. 
The finding of smaller than regular cells in the same patient in the other eye, raised the possibility that active mitosis may be occurring in these patients.  Limited mitosis in the adult human cornea has been observed by Laing et al. 
Pre-operative polymegathism may indicate the likelihood of developing irreversible corneal edema than patients without cells size abnormalities.  Endothelial polymegathism has also been associated with a reduced corneal de-swelling rate when the cornea is swollen with a thick hydrogel lens. 
Endothelial pleomorphism, induced by contact lens, may also be indicative of instability or stress of the monolayer. Regular hexagonal pattern is thermodynamically and geometrically the most stable way to fill a plane, , A pleomorphic cells layer may be relatively unstable and more susceptible to stress. Thus morphometric changes associated with the cornea may be an indication that the cornea has less adaptive reserve to meet subsequent stress such as cataract surgery. These changes may precede a decrease in cell density. There may be a considerable lag of time when polymegathism and pleomorphism are noted and when cell density is affected 
Early signs of cell loss maybe present when cells point towards a particular area (Rosette formation). Rosette formation was thought to occur following injury of a single endothelial cell, where by the neighbouring cells radiate towards the centre of the damaged cells.  These areas correspond clearly to areas noted in organ culture when a cell has recently died and the apices of the surrounding cells point toward the site of the dead cells. 
In our study we found statistically significant decreased cell count in the eye with contact lens wear in contradiction to other researchers. ,, It could be due to small field specular microscopy and the cell counting technique  (with its inherent sampling problem) and the cell population described as cell density with no frequency distribution of cell areas performed (frequency of hexagonality).
This study showed that significant endothelial pleomorphism and polymegathism may occur with hard lenses. The other eye in this study acted as an ideal control. No interocular differences of any of the above characteristics were found in matched control groups of anisometropic and amblyopic subjects who did not wear contact lenses.
These changes in endothelium are not completely reversible and are more profound with the greater duration of wear,.  The reason for these changes is not clear but may be related to the cell loss or remodeling from hypoxic stress. Whether functional integrity of endothelium is disturbed in the absence of surgical stress is also unknown. Whether more oxygen permeability in contact lenses will prevent these changes require further studies.
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[Figure - 1], [Figure - 2], [Figure - 3]
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