IOP Measurement

Asymmetry in IOP measurements. Researchers at the University of California, San Diego, evaluated interocular pressure asymmetry over a 24-hour period in 53 healthy older patients and 41 glaucomatous older patients. Subjects' IOPs were measured every two hours for 24 hours, both sitting and supine during waking hours, and in the supine position at night.

Researchers found that the mean IOP in the right eye of healthy subjects was consistently higher than mean IOP in the left eye—a significant difference (18.3 ±2.1 vs. 17.9 ±2.2 mmHg). However, no significant difference between left and right IOP was found in glaucoma patients. Also, the difference was accentuated in the supine position in healthy subjects, but not in glaucoma patients.⁴⁸⁷³

Comparing Goldmann and dynamic contour tonometry. Researchers at Moorfields Eye Hospital in London studied 130 eyes to compare IOP measurements made by Goldmann applanation tonometry (GAT) and Pascal dynamic contour tonometry (DCT). They found that both central corneal thickness and subject age affected GAT significantly more than DCT; differences between the two devices ranged up to 3 mmHg between the youngest and oldest eyes and between the thinnest and thickest corneas. Also, the relationship between GAT-measured IOP and CCT was significant; this wasn't the case for DCT. However, the DCT showed greater variability than the GAT.⁴⁸³⁸

Ocular characteristics' impact on IOP measurement. A study conducted at the Wilmer Eye Institute compared the influence of different ocular characteristics on IOP measurements made by three instruments: Goldmann applanation; the Tonopen; and the Reichert Non-Contact tonometer (NCT). The study involved 230 patients with glaucoma, ocular hypertension, or suspected glaucoma; IOP measurements were correlated to axial length, corneal curvature, corneal thickness and corneal hysteresis (a measure of bioelasticity).

The authors found that high average IOP was associated with a thicker cornea and steeper corneal curvature, but not axial length. They also found that the lowest and highest hysteresis values were associated with higher IOP values than mid-range hysteresis. Overall, Goldmann tonometry was least affected by corneal thickness, the NCT was least affected by corneal curvature, and the Tonopen was least affected by hysteresis.⁴⁸⁶⁸

Diurnal IOP shifts, upright and supine. A prospective clinical trial at the University of Dresden in Germany evaluated IOP in 30 glaucoma patients and 50 healthy controls every four hours, day and night, using Goldmann and Perkins tonometers (the former in upright position, the latter in both upright and supine positions).

Perkins tonometry produced IOP readings about 1 mmHg lower than Goldmann at noon, 8 p.m. and midnight in both groups. Perhaps more significantly, diurnal IOP change was similar in both groups when measured upright, and increased in both groups when supine—but the difference was greater in glaucoma patients. The authors hypothesize that this might indicate faulty regulation of fluid shift in glaucoma patients.⁴⁸³²

Imaging

ONH photography: film vs. digital. A study conducted at the New Jersey Medical School and West Virginia University compared glaucoma specialists' assessment of traditional simultaneous stereo photographs of optic nerves with their assessment of corresponding digital images. Pictures were obtained using a Nidek 3-DX stereo disc camera with standard 35-mm film and a new 6.1 megapixel digital camera. Digital images were evaluated directly from a computer monitor; digital and film images were presented at the same viewing size and luminance.

A study found no significant differences in optic nerve head evaluation or image quality between traditional simultaneous stereo photographs and digital images presented on a computer monitor. The latter has several practical advantages.

The study found no significant differences in ONH evaluation or in ratings of image quality between film and digital images. The study authors note that digital photography has several advantages over film, including instant feedback on image quality, ease of electronic storage, and use in telemedicine.⁴⁸¹⁵

Comparing GDx VCC, HRT I, and stereo photographs. In a study conducted at the Rotterdam Eye Hospital in the Netherlands, one eye each of 40 healthy subjects, 48 glaucoma patients, and six ocular hypertension patients was measured with scanning laser polarimetry and confocal scanning laser ophthalmoscopy. Researchers also obtained simultaneous stereoscopic optic-disc photographs. The data were analyzed by four glaucoma specialists, four general ophthalmologists, four residents in ophthalmology, and four optometrists.

The SLP Nerve Fiber Indicator proved to be most accurate as a diagnostic tool, followed by CSLO's Bathija linear discriminant function. Notably, automated analysis of measurements with the GDx VCC and HRT I had a higher diagnostic accuracy than classification of stereoscopic optic-disc photographs by glaucoma specialists. The intra- and interobserver agreement for optic-disc analysis was only moderate to good.⁴⁸⁰⁹

Using baseline parameters to predict POAG. A multicenter study ancillary to the Ocular Hypertension Treatment Study evaluated 865 eyes of 438 participants to determine whether baseline confocal scanning laser ophthalmoscopy parameters (Heidelberg Retina Tomograph) were associated with later development of POAG endpoints.

Several baseline topographic optic-disc measurements alone, or combined with clinical and demographic factors, were significantly associated with the development of POAG, including larger cup-to-disc area ratio, cup depth, height contour, cup volume, reference-plane height, smaller rim area, ratio of rim area to disc area, and rim volume. In addition, classification as "outside normal limits" by the HRT and the Moorfields Regression Analysis were significantly associated with the development of POAG. The predictive value ranged from 14 percent for HRT classification and MRA (overall), to 40 percent for MRA (temporal superior).³⁶³¹

Assessing early glaucoma using RNFL asymmetry. A University of Southern California study evaluated 687 normals and 37 patients with early glaucomatous damage for degree of asymmetry in retinal nerve fiber layer thickness between the right and left eyes. Subjects underwent a complete visual examination, including Stratus OCT scans of the peripapillary RNFL.

In normals the mean RNFL asymmetry was 4.5 ±3.7 µm, but patients with early glaucoma had a mean overall RNFL asymmetry of 6.4 ±6.9 µm, a significant difference. Although there were no consistent associations between RNFL asymmetry and age, gender, axial length, or spherical equivalent refractive error, the asymmetry in the superior RNFL was 48 percent greater in early glaucomatous patients than in normal subjects. The study authors conclude that asymmetry in the Stratus OCT-measured RNFL may be an early sign of glaucomatous optic-nerve damage.²⁵²¹

Pseudo red-free images vs. full color and conventional red-free. A randomized, masked study conducted at the University of Pittsburgh School of Medicine evaluated a new software program that converts color optic disc photographs into pseudo red-free images (forced RF) by removing the red-channel information and halving the intensity of the blue channel, to see whether this inexpensive approach could aid detection of nerve fiber bundle defects.

A new software program converts color optic disc photographs into pseudo red-free images by removing the red-channel information and halving the intensity of the blue.

Thirteen images of 10 subjects (three normal, eight glaucoma patients with focal defects, and two glaucoma patients with diffuse damage) were studied by two specialists and two residents; the shape of optic disk was masked by a monochromatic circle. On average, forced RF helped glaucoma specialists find NFBD in one eye (7.7 percent), while it helped residents find NFBD in two eyes (15.4 percent). All participants agreed that forced RF images were significantly easier to evaluate than full-color images, although they also agreed that conventional RF images were superior to forced-RF images. The authors conclude that forced-RF images may be useful when conventional RF is not available, or when assessing old archived disc photos.²⁴⁹²

Anterior segment OCT vs. gonioscopy. A multicenter study in Singapore and the United States is evaluating the anterior segment optical coherence tomograph (Carl Zeiss Meditec) to see how well it detects occludable angles. (The AS-OCT provides rapid, noncontact imaging of the drainage angle.) Patients at the National University Hospital in Singapore are undergoing imaging with the AS-OCT and gonioscopy using the Goldmann two-mirror lens.

Preliminary data from 54 eyes (29 patients) shows that the AS-OCT detected narrow or closed angles in temporal quadrants in 24 out of 28   rated occludable by gonioscopy (85.7 percent); 40 out of 46 inferior angles (87 percent); and 17 (89.5 percent) out of 19 nasal quadrants. Overall, the AS-OCT correctly identified 81 out of 93 occludable angle quadrants (87.1 percent) as narrow or closed.¹⁴⁵

Glaucoma Surgery

Use of nonpenetrating surgery: survey results. An e-mail survey of members of the American Glaucoma Society regarding their use of viscocanalostomy and/or nonpenetrating deep sclerectomy (NDS) found that, in general, most of the surveyed surgeons prefer trabeculectomy. Sixteen percent of the respondents perform NDS, but only in cases of early glaucoma; the number of NDS procedures performed per year ranged from two to 150. Seven of the 82 respondents said they have performed NDS in the past, but stopped because they were not impressed by the resulting IOP reduction, the limited indications for these procedures, the unpredictable failures that occurred, or the need for repeated laser goniopuncture postoperatively.¹⁰⁴

Comparing deep sclerectomy and trabeculectomy. A seven-year prospective, randomized, investigator-masked clinical trial at the University of Parma and University of Brescia in Italy compared the long-term outcomes of trabeculectomy and deep sclerectomy in 79 eyes of 79 patients. Sixty-two percent of the 41 patients randomized to deep sclerectomy also received YAG-laser goniopuncture during follow-up. Trabeculectomy controlled IOP better than deep sclerectomy seven years after surgery, but deep sclerectomy's success rate increased significantly when it was converted to a penetrating procedure using postoperative YAG-laser goniopuncture. Also, deep sclerectomy was associated with a lower incidence of cataract extraction.¹²¹⁷

CCT's impact on SLT. A retrospective chart review of all selective laser trabeculoplasty procedures performed at the University Eye Clinic in Dallas (53 patients/81 eyes) found that patients with thin corneas generally showed a greater reduction in IOP. Researchers noted, however, that when the patients were divided into three groups by corneal thickness, the trend was only statistically significant when comparing those in the thickest cornea group with those in the thinnest group.¹⁰⁷

Outcomes with the ExPress miniature glaucoma shunt. Researchers at the University of Tennessee Health Science Center implanted the ExPress miniature glaucoma implant under a partial-thickness scleral flap with Mitomycin C in 52 eyes of 50 patients who were candidates for trabeculectomy, to study the procedure's complication rate and efficacy in lowering IOP. Fifteen of the eyes also underwent cataract surgery and IOL implantation.

At six months, IOP dropped from a preop baseline of 26.3 ±10.4 mmHg to 11.2 ±3.8, and 95.8 percent of eyes had an IOP between 4 and 22 mmHg. The mean number of medications was reduced to 0.4 ±1.0 at last follow-up. Three eyes (5.8 percent) developed choroidal detachments that resolved spontaneously. The authors conclude that this procedure is a safe and effective alternative to trabeculectomy.⁷¹

Comparing one- and two-site phacotrabeculectomy. A prospective, randomized, two-year clinical trial involving 80 patients at the University of Toronto is comparing the effectiveness of one- and two-site phacotrabeculectomy. Preliminary results have found no statistical difference in IOP between the two groups at three, six and 12 months of follow-up, or in the number of glaucoma medications in use (which decreased from a mean of 2.9 to zero for the one-site group, and from 2.9 to 0.1 for the two-site group). Researchers also failed to find any statistical difference in visual acuity improvement, but noted a trend to lower IOP in the two-site group.¹⁰⁵

Long-term impact of 5-FU. Researchers in Singapore and London conducted a six-year, multicenter, prospective, randomized, masked trial to evaluate the long-term effect of a five-minute application of 5-Fluorouracil (50 mg/ml) during trabeculectomy on Asian patients. One hundred fifteen patients received 5-FU; 120 received placebo. The long-term data showed no significant differences in complications between groups, but a significant difference in successful outcomes, defined in terms of IOP. With a mean follow-up of 54 ±15.1 months, 83/115 of the 5-FU subjects exceeded 14 mmHg, compared with 101/120 in the placebo group; 57/115 of the 5-FU subjects exceeded 17 mmHg, compared with 73/120 of the placebo group.¹²¹⁹

Resolving encapsulated filtering blebs. Researchers at Baylor College of Medicine and the University of Texas conducted a retrospective case study of 39 eyes that had developed an encapsulated bleb following trabeculectomy, to determine how well different treatments aided resolution. Subjects were POAG patients with no prior history of neovascular or uveitic glaucoma, bleb revision or combined surgical procedures.

Of the 39 eyes, encapsulation was resolved with medical management in 29 (74.4 percent). In 20 of those eyes, Pred Forte was tapered (11 with  multi-drug therapy; nine with Pred Forte taper alone). The authors conclude that bleb encapsulation following trabeculectomy responds well to tapering topical corticosteroids, alone or in conjunction with IOP-lowering agents. They also note that lowering IOP through steroid withdrawal appears to be more beneficial than maintaining steroid use to prevent inflammation.³⁷⁷⁹

Evaluating a novel glaucoma surgery. A study conducted at the State University of New York at Binghamton evaluated a new method of shunting aqueous into the suprachoroidal space as a way of lowering IOP. Fifty-six glaucomatous eyes received an internal tube silicone shunt, 15 shunting directly from the anterior chamber, 41 shunting from a deep scleral lake. At baseline, the mean IOP value in the anterior-chamber shunted eyes was 33.2 ±9.8 mmHg, on 3.4 meds; after 6.1 ±3.2 months it was 13.6 ±5.1 mmHg, on 1.7 meds. Also, IOP after a topical carbonic anhydrase inhibitor was 11.2 ±4.1 mmHg; after timolol, 11.3 ±4.2 mmHg.

A new surgical treatment that shunts aqueous from the anterior chamber into the suprachoroidal space appears to be an effective way to lower IOP.

The author concludes that shunting aqueous humor from the anterior chamber to the suprachoroidal space is an effective way to lower IOP and enhance the effect of a topical CAI or timolol. He suggests that the tube could also improve topical meds' access to the posterior uvea, to treat chorio-retinal diseases.¹²²³