Giant vacuole survival time and implications for aqueous humor outflow.

PURPOSE: A quantitative study was conducted to estimate the survival time of giant vacuoles of the endothelial wall of Schlemm's canal once perfusion pressure has dropped to zero. Vacuoles are known to be sensitive to intraocular pressure and could serve as real-time markers of aqueous pressure in the juxtacanalicular tissue if their survival time is short once pressure is lowered. METHODS: Six pairs of healthy human eyes were perfused with phosphate-buffered saline. One eye was then fixed by perfusion, whereas the fellow eye was dissected into wedges and fixed by immersion at intervals ranging from 3 to 120 minutes after perfusion had been stopped. Light microscopy was used to determine the number of giant vacuoles on the inner and outer walls of Schlemm's canal. RESULTS: A 70% decrease in giant vacuole counts was found 3 minutes after discontinuation of saline perfusion when compared with perfusion-fixed fellow eyes (5.9 +/- 4.0 vacuoles per histologic section versus 19.1 +/- 9.0; P = 0.02). Vacuole counts continued to decrease with time so that by 120 minutes at zero pressure, vacuole counts were 12% of those in the perfusion-fixed eyes (88% decrease; P = 0.003). Giant vacuoles occurred on the inner and outer walls of the canal, but were more numerous on the inner wall. CONCLUSIONS: Most giant vacuoles have a short survival time, less than 3 minutes, once perfusion pressure decreases to zero. This suggests that they can respond rapidly to intraocular pressure changes and thus can serve as markers of aqueous pressure differences across the inner and outer walls of Schlemm's canal.

A comparison of different embedding media on the ultrastructure of the trabecular meshwork.

PURPOSE: To determine whether the preservation of the extracellular matrix and the ultrastructural appearance of the trabecular meshwork are affected by different histologic processing protocols and embedding media. Conventionally used epoxy resins such as Araldite require complete dehydration of tissue, while the acrylic resin LR White requires only partial dehydration, better preserves tissue antigens, and has been reported to preserve more of the extracellular matrix. METHODS: Seven human eyes ranging in age from 2 months to 78 years were dissected and tissue samples from each eye processed and embedded in both Araldite and LR White media. The ultrastructure of the trabecular cells and the extracellular matrix of the meshwork was compared between media. The preservation of the extracellular matrix in the juxtacanalicular region was determined by measuring the amount of material immediately underlying Schlemm(1)s canal. Immunoelectron microscopy was used to determine the composition of this material. RESULTS: Araldite provided better resolution of ultrastructural details than freshly polymerized LR White. After a period of ripening for several months, however, resolution of tissue details in LR White improved. No significant quantitative difference was found in the amount of extracellular matrix underlying Schlemm's canal when comparing the two media. Neither post-mortem time to fixation (up to 31 hr), donor age, nor immersion vs. perfusion fixation technique affected the amount of extracellular material present in the comparison of the two embedding media. Immunogold labeling of the extracellular material within the juxtacanalicular tissue revealed the presence of collagen IV, laminin, and fibronectin in the basement membrane region immediately underlying the inner wall, and also in scattered patches within the juxtacanalicular tissue. CONCLUSIONS: Despite the less rigorous processing required for LR White than epoxy embedding, neither the appearance nor amount of the extracellular material was affected by the different embedding protocols. Prolonged post-mortem time to fixation did not affect the appearance nor amount of extracellular matrix. Immunolabeling revealed that the extracellular matrix of the juxtacanalicular tissue contains components of basement membrane material.

Giant vacuoles are found preferentially near collector channels.

PURPOSE: To determine whether giant vacuoles form preferentially near collector channels or over regions of optically empty space within the juxtacanalicular tissue (JCT). METHODS: To assess the relationship between giant vacuoles and collector channels, six eyes were perfused with phosphate-buffered saline (PBS) at 20 mm Hg and then fixed by perfusion. Serial sections were cut in the frontal plane and light microscopy used to count the number of giant vacuoles per length of Schlemm's canal. The number of giant vacuoles between two adjacent collector channels was determined. To assess the relationship between giant vacuoles and the ultrastructure of the JCT, an additional seven eyes were perfused with PBS at 10 mm Hg, fixed by perfusion, and examined by transmission electron microscopy. The ultrastructural components of the JCT were quantitated with an image analysis system. RESULTS: Twice as many giant vacuoles were present in regions underlying collector channels as in regions between channels (giant vacuoles per histologic section: 14.0 +/- 1.7 versus 7.3 +/- 0.8, P: = 0.01). Giant vacuoles occurred on both the inner and outer walls of the canal but were more numerous on the inner wall (9.1 +/- 1.0 versus 2.6 +/- 0.4, P: < 0.001). No significant increase in optically empty space was found in the JCT regions underlying giant vacuoles compared with regions with no vacuoles (50.7% +/- 2.3% versus 47.3% +/- 2.5%, P: = 0.09). Examination of the amount of optically empty space immediately adjacent (within 1 microm) to the inner wall endothelial cells of the canal did not reveal a significant difference between regions under vacuoles and regions without giant vacuoles. CONCLUSIONS: Giant vacuoles are found preferentially near collector channels, indicating that aqueous flow across the inner wall is sensitive to downstream pressure. The variability in giant vacuole distribution noted in previous studies is in part due to the distance of the vacuoles from the collector channels. No distinct findings in the JCT were associated with the presence of giant vacuoles.

Wide QRS tachycardia refractory to pharmacologic interventions and cardioversion.

The case of a 43-year-old man with an idiopathic wide QRS tachycardia resistant to adenosine, lidocaine, procainamide, and multiple electric shocks is described. The tachycardia eventually stopped after a pacing wire was introduced in the right ventricle, inducing a run of rapid ventricular tachycardia. The possible causes of resistance to medications and cardioversion, and alternative treatment modalities are reviewed.

Topical tetracaine with bandage soft contact lens pain control after photorefractive keratectomy.

PURPOSE: A major disadvantage of photorefractive keratectomy (PRK) is pain and discomfort in the first three postoperative days. We tried to assess the efficacy and safety to the cornea of a limited amount of topical tetracaine given to patients for use when needed to manage severe pain. METHODS: Sixty-nine eyes of 49 patients who underwent PRK between June 21, 1996 and June 15, 1998 by a single surgeon were prospectively included. Approximately 10 drops of commercial, non-preserved 0.5% tetracaine were given to patients to use when needed to control severe pain. A bandage soft contact lens was applied. Patients were examined at 1 and 3 days after surgery, at which time corneal re-epithelization was assessed and the number of tetracaine drops used was noted. No systemic analgesic or topical non-steroidal anti-inflammatory was prescribed. RESULTS: All eyes healed within 3 days. The mean number of drops of tetracaine used was 2.3 drops over 3 days, although in 33 eyes (48%) the patient did not use any tetracaine. There was no correlation between the attempted correction in diopters and the number of drops used. No significant difference was found in the number of drops used in the second eye of patients who had both eyes treated. CONCLUSIONS: Limited use of topical anesthetics is an effective and safe analgesic option after PRK. Use of tetracaine in this protocol did not prolong the time to re-epithelialization. Giving only a limited amount of tetracaine to patients prevents abuse and toxicity to the cornea while managing severe pain.