Mỗi lần bệnh nhân sẽ nhấp nháy, một tờ trôi chảy nước mắt nên được swabbed ống kính. Điều này có thể được quan sát tốt nhất sau khi fluorescein thuốc nhuộm đã được thấm nhuần. | 132 Chapter 9 Figure 9-10. A loose flat gas permeable lens with pooling at lens edges. Photo by Val Sanders. Figure 9-11. A tight steep gas permeable lens with no dye at lens edges. Photo by Val Sanders. Lens Appearance Each time the patient blinks a smooth sheet of tears should be swabbed over the lens. This may be best observed after fluorescein dye has been instilled. If the tears bead up on the lens or dry spots appear immediately after the blink note this in the record. Lens clouding may have several causes. If the lens surface is not smooth the eye may respond with an increase in mucus or oily discharge. This occurs regardless of lens hygiene. Lenses may also cloud if the patient is mixing incompatible cleaning chemicals. Processes used in lens manufacture can also cause haziness. You can subjectively grade lens clouding using 1 to 4 . Examples lens clear 3 lens clouding. The surface of a rigid lens may become crazed. Have you ever seen an old china plate where the finish on the plate had a network of tiny cracks all over it That is crazing. In a contact lens this can be caused by a plaque on the lens that interferes with wetting and should be noted in the chart. Carefully examine the edges of the lens for chips and nicks and note them in the record. The bottom of the lens may be purposely flattened or truncated to provide for better lens stability. Check the lens surface for cracks and scratches. Scratches may be subjectively graded from 0 to 4 . Contact Lens Evaluation for Nonfitters 133 The segments of a fused or one-piece rigid bifocal contact lens will be visible with the slit lamp. Note where the segment line falls when the lens recenters after a blink. Associated Problems Corneal Hypoxia Corneal hypoxia lack of oxygen in rigid lenses often causes an area of edema that is central rather than diffuse as in soft lenses. This central area is round or oval in the case of astigmatism about to mm across and grayish white. Draw or describe and grade
