Enter the reverend: introduction to and application of Bayes' theorem in clinical ophthalmology.

BACKGROUND: Ophthalmic practice utilizes numerous diagnostic tests, some of which are used to screen for disease. Interpretation of test results and many clinical management issues are actually problems in inverse probability that can be solved using Bayes' theorem. DESIGN: Use two-by-two tables to understand Bayes' theorem and apply it to clinical examples. SAMPLES: Specific examples of the utility of Bayes' theorem in diagnosis and management. METHODS: Two-by-two tables are used to introduce concepts and understand the theorem. The application in interpretation of diagnostic tests is explained. Clinical examples demonstrate its potential use in making management decisions. MAIN OUTCOME MEASURE: Positive predictive value and conditional probability. RESULTS: The theorem demonstrates the futility of testing when prior probability of disease is low. Application to untreated ocular hypertension demonstrates that the estimate of glaucomatous optic neuropathy is similar to that obtained from the Ocular Hypertension Treatment Study. Similar calculations are used to predict the risk of acute angle closure in a primary angle closure suspect, the risk of pupillary block in a diabetic undergoing cataract surgery, and the probability that an observed decrease in intraocular pressure is due to the medication that has been started. The examples demonstrate how data required for management can at times be easily obtained from available information. CONCLUSIONS: Knowledge of Bayes' theorem helps in interpreting test results and supports the clinical teaching that testing for conditions with a low prevalence has a poor predictive value. In some clinical situations Bayes' theorem can be used to calculate vital data required for patient management.

Heidelberg Retinal Tomograph (HRT 2) parameters in primary open angle glaucoma and primary angle closure glaucoma: a comparative study in an Indian population.

AIMS: To compare the Heidelberg Retinal Tomograph (HRT 2) parameters in patients with primary open angle glaucoma (POAG) and primary angle closure glaucoma (PACG) in an Indian population. MATERIALS AND METHODS: Two groups of patients were recruited: group I comprised 78 eyes (78 patients) with POAG and group II 58 eyes (58 patients) with PACG. Based on visual field defects detected by automated perimetry, the groups were further classified into early, moderate, and severe glaucoma. All patients underwent a complete ophthalmic examination as well as an HRT 2 examination. The HRT parameters for the two groups were compared and the sensitivity and specificity of the parameters calculated. RESULTS: All HRT parameters were similar in both groups: 85.9% (95% CI: 78.2, 93.6) of POAG and 81% (95% CI: 72.3, 89.3) of PACG had a positive F. S. Mikeleberg (FSM) discriminant function. Considering outside normal limits and borderline as abnormal, the Moorefield regression analysis had 83.3% (95% CI: 75, 91.6) sensitivity in POAG and 75.9% (95% CI: 64.9, 86.9) in PACG. In early POAG, the FSM discriminant function had a sensitivity of 74.3% (95% CI: 59.8, 88.8) compared to 58.3% (95% CI: 38.9, 77.7) for early PACG. The cup shape measure (p = 0.018) and the Moorfield regression analysis (p = 0.011) had significantly higher sensitivity for early POAG than for early PACG: cup shape measure sensitivity 62.9% (95% CI 46.9, 78.9) for early POAG versus 33.3% (95% CI: 14.4, 52.2) for early PACG and Moorefield regression analysis 74.3% (95% CI: 59.8, 88.8) versus 45.8% (95% CI: 39.9, 65.7). CONCLUSIONS: HRT has moderate sensitivity in the detection of damage leading to glaucomatous field defects. The sensitivity of HRT for early PACG appears to be less than that for early POAG. This may indicate a difference in pathophysiology.

Comparison between phacoemulsification and the Blumenthal technique of manual small-incision cataract surgery combined with trabeculectomy.

OBJECTIVE: To compare results of filtration combined with either phacoemulsification or the Blumenthal technique of manual small-incision cataract surgery. MATERIALS AND METHODS: Records of glaucoma "triple" operations from March 1997 to May 2000 were reviewed. Seventy-eight eyes (70 patients) that underwent phaco-triple were compared with 86 eyes (80 patients) that underwent the Blumenthal technique of manual small-incision cataract surgery combined with filtration (Blumenthal triple). Three minutes of 0.4 mgs/ml Mitomycin was used in all eyes. Posterior chamber IOLs were implanted through 5.5-mm incisions in both groups. Outcome measures were intraocular pressure (IOP) reduction and achievement of target IOP. Fourteen patients who underwent phacoemulsification-triple in one eye and Blumenthal triple in the other eye were also evaluated separately. RESULTS: The minimum follow-up period was 6 months (range 6-30 months). At last follow-up review, mean reduction in IOP was 17.7 mm Hg (+/- 9.3 mm Hg) in the phaco group and 17.1 mm Hg (+/- 10 mm Hg) in the Blumenthal group. At last visit, target IOP was achieved in 75.6% of the phaco group and 73% of the Blumenthal group. There was no significant difference between groups in IOP reduction or achievement of target IOP. In the 14 patients who had undergone phaco-triple in one eye and Blumenthal-triple in the other, there was no inter-eye difference in IOP reduction. CONCLUSIONS: In this small retrospective study we could not demonstrate a difference in IOP outcomes between the two procedures.

Population-based screening versus case detection.

India has a large burden of blindness and population-based screening is a strategy commonly employed to detect disease and prevent morbidity. However, not all diseases are amenable to screening. This communication examines the issue of "population-based screening" versus "case detection" in the Indian scenario. Using the example of glaucoma, it demonstrates that given the poor infrastructure, for a "rare" disease, case detection is more effective than population-based screening.

The incidence of vitreous loss and visual outcome in patients undergoing cataract surgery in a teaching hospital.

PURPOSE: To determine the incidence of vitreous loss in patients undergoing cataract surgery and the visual outcome in a tertiary teaching hospital. METHODS: Hospital records of 2095 consecutive patients undergoing cataract surgery between July 1999 and June 2000 were reviewed in this non-concurrent cohort study. Incidence and visual outcome of vitreous loss managed using standard vitrectomy techniques were assessed for different cataract surgical techniques (extracapsular, Blumenthal technique and phacoemulsification) as well as at different levels of surgical training. The outcome was compared with matched cases without vitreous loss (controls). RESULTS: Vitreous loss occurred in 160 of 2095 eyes (7.63%; CI -7 to 9.3): 8.3% for ECCE, 8.1% for the Blumenthal technique and 5% with phacoemulsification. Vision > or = 6/18 was achieved in 85% of cases and 95% of controls. For experienced surgeons, 95% of the cases and controls had vision > or = 6/18. 5.8% of cases and 0.7% of controls had vision < 6/60. One patient in each group was blind following cataract surgery; both had operable cataracts in the fellow eye. CONCLUSIONS: The vitreous loss rate in this tertiary teaching hospital is relatively high. This complication, managed with standard surgical techniques, is compatible with good visual outcome. In eyes with vitreous loss, the final visual acuity achieved by experienced surgeons was similar to that in uncomplicated cases.