Blindness and eye disease in Kenya: ocular status survey results from the Kenya Rural Blindness Prevention Project.

A series of eight regional eye surveys were conducted in Kenya as part of the Kenya Rural Blindness Prevention Project. Each survey consisted of clinical examinations of about 1800 individuals selected by a random cluster sampling technique in geographically distinct and culturally homogeneous rural areas; 13,803 examinations were completed in all. Together these surveys provide the basis for national estimates of the prevalence and aetiology of visual loss and ocular pathology. The results showed that 0.7% of rural Kenyans are blind in the better eye by WHO standards, and another 2.5% suffer significant visual impairment. Rates of visual loss tend to increase five-fold in each 20-year age cohort. Females have higher prevalence of visual loss than males over age 20, and certain geographical areas have markedly higher rates. The commonest cause of both blindness and visual impairment is cataract, accounting for 38% of all visual loss. Trachoma (a localised problem), glaucoma, macular degeneration, and severe refractive errors follow cataract as leading causes of blindness in the better eye. Trauma, corneal scars of various causes, phthisis, and staphyloma are important causes of monocular blindness. Nutritional eye disease does not appear to be a problem of any magnitude in rural Kenya.     

Localised retinal nerve fibre layer defects in chronic experimental high pressure glaucoma in rhesus monkeys

AIM: To evaluate prospectively in an experimental model of chronic high pressure glaucoma whether the concept of a mainly diffuse pattern of optic nerve damage holds true for high pressure glaucoma. METHODS: The study comprised nine eyes of nine rhesus monkeys (Macaca mulatta) with a mean age of 17.7 (SD 3.1) years (range 13-23 years). Experimental glaucoma was produced by multiple applications of argon laser to the trabecular meshwork. Applanation tonometry was regularly performed and fundus photographs, which were taken serially, were used for retinal nerve fibre layer (RNFL) assessment and morphometric optic disc analysis. Six monkeys, in which arterial hypertension and atherosclerosis had additionally been induced several years before elevation of intraocular pressure, did not show any sign of diffuse loss or localised defects of the RNFL before initiation of glaucoma. RESULTS: Compared with the same eyes at baseline, localised RNFL defects had developed in eight (89%) eyes. It included all three eyes (100%) of the monkeys without arterial hypertension/arteriosclerosis, and five of the six monkeys (83%) with arterial hypertension/arteriosclerosis. Four eyes had multiple localised RNFL defects. In all eyes, diffuse RNFL loss was additionally present. CONCLUSIONS: Besides diffuse loss of RNFL, localised RNFL defects were present in almost all eyes of monkeys with chronic experimental high pressure glaucoma. Challenging the concept that a mostly diffuse type of optic neuropathy occurs in high pressure glaucoma, the results suggest that, in high pressure glaucoma, at least a mixture of localised and diffuse pattern of optic nerve damage prevails.     

Scanning laser tomography Z profile signal width as an objective index of macular retinal thickening

AIMS—(i) To evaluate the relation between retinal thickness and the Z profile signal width of a scanning laser tomographer in selected patients exhibiting clinically manifest and circumscribed macular retinal thickening; (ii) to compare the Z profile signal width values of a group of age similar normal subjects with those of the patients with macular retinal thickening; and (iii) to present the methodology underlying the Z profile signal width derivation.
METHODS—Three patients with the following conditions were selected: widespread diabetic macular oedema; localised diabetic macular oedema; and macular hole. The patients were selected because they exhibited clinically manifest and circumscribed macular retinal thickening. Patients underwent fundus photography and a clinical examination which included fundus biomicroscopy. Fourteen age similar normal subjects were also assessed. The Heidelberg retina tomograph (HRT) was utilised to acquire seven topographic images of each macula. Z profile signal width data were analysed using custom software. Signal width was measured at 50% of the maximum intensity.
RESULTS—For each patient with macular retinal thickening, Z profile signal width analysis (after normalisation to reduce the influence of variation in reflectance intensity between successive images) revealed a significant (p<0.0001) localised increase of signal width which agreed with the HRT topographic analysis of retinal height, and also the clinical assessment of retinal thickness. The mean normalised Z profile signal width for the normal subjects (assessed over the whole image) ranged from 0.278 (SD 0.039) to 0.444 (0.063); these values compared with those obtained from patients in areas of macular retinal thickening of 0.761 (0.224) to 0.953 (0.194). Z profile signal width test-retest data for the patient with localised diabetic macular oedema were plus or minus 0.159 which compared with a mean signal width value of 0.761.
CONCLUSION—The evidence of this study, based upon three selected patients with macular retinal thickening and 14 normal subjects, would suggest that Z profile signal width analysis offers a non-invasive, objective, topographic, and reproducible index of macular retinal thickening. Studies employing larger sample sizes are required to determine the true clinical worth of the technique.

Keywords: diabetic macular oedema; retinal thickening; scanning laser tomography; Z profile     

Localised scotomata and types of vascular occlusion in diabetic retinopathy

In 13 patients with diabetic retinopathy who had localised scotomata in the central visual field that could not be ascribed to haemorrhages, cotton wool spots or hard exudates, visual field data obtained by computerised perimetry were correlated with the corresponding retinal morphology as seen on fundus photographs and fluorescein angiograms. It was attempted to relate the retinal areas corresponding to the scotomata to areas of distribution from the central retinal artery. In six cases localised scotomata could be correlated with focal areas of angiographical non-perfusion in the macular area. When these areas of non-perfusion were traversed by a patient vessel, the retinal zone adjacent to this vessel was found to have retained light sensitivity. Two localised scotomata could be correlated with retinal areas peripheral from occluded arterioles, but no scotoma was found which could be related th retinal venous occlusion. In five cases scotomata occurred, however, which could not be related to areas of distribution from the central retinal artery. It is concluded that factors other than retinal vascular occlusion probably are involved in causing impairment of neurosensory function in diabetic maculopathy.     

Using unsupervised learning with variational bayesian mixture of factor analysis to identify patterns of glaucomatous visual field defects

PURPOSE: To determine whether an unsupervised machine learning classifier can identify patterns of visual field loss in standard visual fields consistent with typical patterns learned by decades of human experience. METHODS: Standard perimetry thresholds for 52 locations plus age from one eye of each of 156 patients with glaucomatous optic neuropathy (GON) and 189 eyes of healthy subjects were clustered with an unsupervised machine classifier, variational Bayesian mixture of factor analysis (vbMFA). RESULTS: The vbMFA formed five distinct clusters. Cluster 5 held 186 of 189 fields from normal eyes plus 46 from eyes with GON. These fields were then judged within normal limits by several traditional methods. Each of the other four clusters could be described by the pattern of loss found within it. Cluster 1 (71 GON + 3 normal optic discs) included early, localized defects. A purely diffuse component was rare. Cluster 2 (26 GON) exhibited primarily deep superior hemifield defects, and cluster 3 (10 GON) held deep inferior hemifield defects only or in combination with lesser superior field defects. Cluster 4 (6 GON) showed deep defects in both hemifields. In other words, visual fields within a given cluster had similar patterns of loss that differed from the predominant pattern found in other clusters. The classifier separated the data based solely on the patterns of loss within the fields, without being guided by the diagnosis, placing 98.4% of the healthy eyes within the same cluster and spreading 70.5% of the eyes with GON across the other four clusters, in good agreement with a glaucoma expert and pattern standard deviation. CONCLUSIONS: Without training-based diagnosis (unsupervised learning), the vbMFA identified four important patterns of field loss in eyes with GON in a manner consistent with years of clinical experience.     

Diagnostic value of short-wavelength automated perimetry

Short-wavelength automated perimetry (SWAP) uses a bright yellow background and a large blue stimulus to isolate and measure the sensitivity of short-wavelength-sensitive mechanisms throughout the central 30 degrees visual field. After more than 8 years of cross-sectional and longitudinal studies of patients with early glaucoma, ocular hypertensive patients, and age-matched control subjects, SWAP has been shown to be a sensitive indicator of early damage and progression of loss in glaucoma. It is now available as an optional visual test procedure for routine clinical use on the most widely distributed automated perimeter. Recent investigations have been directed toward refinement and standardization of the technique, as well as toward the influence of other clinical factors (ocular media opacities, macular pigment, statistical interpretation, and related issues) on SWAP. This paper provides a brief overview of current research on SWAP as a clinical diagnostic test procedure.     

Comparison of algorithms for detection of localised nerve fibre layer defects using scanning laser polarimetry

AIMS: To evaluate different algorithms used to analyse retinal nerve fibre layer thickness (RNFL) data obtained by scanning laser polarimetry, in order to compare their relative abilities to discriminate between patients with glaucomatous localised nerve fibre layer defects and normal subjects. METHODS: 48 eyes of 48 glaucomatous patients with localised RNFL defects and 53 eyes of 53 healthy subjects were included in this study. The localised RNFL defects were identified by RNFL photography and/or slit lamp biomicroscopic examination. All patients were submitted to RNFL examination using scanning laser polarimetry (GDx nerve fibre analyser, Laser Diagnostic Technologies, Inc, San Diego, CA, USA). Three methods of analysis of polarimetry data were used: GDx software provided parameters; RNFL thickness measurements in 16 equal sectors around the optic disc (sectoral analysis); and Fourier analysis of the curve of distribution of RNFL thickness measurements. Linear discriminant functions were developed to assess sensitivity and specificity of the sectoral based analysis and Fourier analysis and were compared to the GDx parameters. In addition, areas under the receiver operating characteristic (ROC) curves were compared. RESULTS: At a fixed specificity of 91%, the sensitivity of the linear discriminant function from sectoral data (LDF sectoral) was 81%, with an area under the ROC curve of 0.93. The linear discriminant function from Fourier measures had a comparable performance, with an area under the ROC curve of 0.93, and sensitivity of 71% for specificity at 91%. At the same specificity, the sensitivities of the GDx software provided parameters ranged from 15% to 40%. The areas under the ROC curves for the LDF sectoral and LDF Fourier were significantly greater than the ROC curve area for the single best GDx parameter. CONCLUSION: The sectoral based analysis and the Fourier analysis of RNFL polarimetry data resulted in an improved detection of eyes with glaucomatous localised nerve fibre layer defects compared to the GDx software provided parameters.     

A quantitative scoring technique for panel tests of color vision

Panel tests of color vision (eg FM100-Hue test) lack a common quantitative method for the scoring of cap arrangements. We describe a scoring method applicable to all panel tests that makes use of a novel technique to analyze test cap data, namely the calculation of a moment of inertia from the Color Difference Vectors (CDVs) of any arrangement pattern. Using the Farnsworth D-15 panel, as an example, we specify how to determine CDVs and demonstrate the benefits of calculating a moment of inertia for the analysis of these vectors. Moment of inertia analysis yields three factors which quantify cap arrangements: the first is the confusion angle which identifies the type of color defect; the second is the Confusion index (C-index) which quantifies the degree of color loss relative to a perfect arrangement of caps; and the third is the Selectivity index (S-index) which quantifies the amount of polarity or lack of randomness in a cap arrangement. A retrospective study on the result of 53 normal and 66 congenitally color defective observers is reported and provides normative data. We show that the technique differentiates between different types of color defect and provides useful clinical information regarding a loss of color vision. Likewise, a similar observation is made on a smaller sample of FM100-Hue results. A BASIC computer program is provided for anyone wishing to use the technique.     

Trained artificial neural network for glaucoma diagnosis using visual field data: a comparison with conventional algorithms

PURPOSE: To evaluate and confirm the performance of an artificial neural network (ANN) trained to recognize glaucomatous visual field defects, and compare its diagnostic accuracy with that of other algorithms proposed for the detection of visual field loss. METHODS: SITA Standard 30-2 visual fields, from 100 glaucoma patients and 116 healthy participants, formed the data set. Our ANN was a previously described fully trained network using scored pattern deviation probability maps as input data. Its diagnostic accuracy was compared to that of the Glaucoma Hemifield Test, the Pattern Standard Deviation index at the P<5% and <1%, and also to a technique based on the recognizing clusters of significantly depressed test points. RESULTS: The included tests had early to moderate visual field loss (median MD=-6.16 dB). ANN achieved a sensitivity of 93% at a specificity level of 94% with an area under the receiver operating characteristic curve of 0.984. Glaucoma Hemifield Test attained a sensitivity of 92% at 91% specificity. Pattern Standard Deviation, with a cut off level at P<5% had a sensitivity of 89% with a specificity of 93%, whereas at P<1% the sensitivity and specificity was 72% and 97%, respectively. The cluster algorithm yielded a sensitivity of 95% and a specificity of 82%. CONCLUSIONS: The high diagnostic performance of our ANN based on refined input visual field data was confirmed in this independent sample. Its diagnostic accuracy was slightly to considerably better than that of the compared algorithms. The results indicate the large potential for ANN as an important clinical glaucoma diagnostic tool.     

Corneal sensitivity recovery after photorefractive keratectomy (PRK)

Early developments in specular microscopy and quantitative corneal endothelial image analysis have resulted in the publication of material describing the morphometric appearance of the mosaic in health and disease. A broad range of morphometric parameters, including cell area, diameter, perimeter, form and shape, have been used to describe cellular appearances. In may cases the selection of parameters appears inappropriate and uncoordinated. In this study, the corneal endothelium of 300 normal, traumatised and pathological corneas was photographed using the Keeler Konan Pocklington specular microscope. The morphometric characteristics of endothelial cells were assessed using the Kontron Mop II semi-automated image analysis system. Results indicate that cell density and mean cell area measurements provide the most useful indices of age-related, post-traumatic and pathological cell loss. Cell perimeter and cell diameter measurements add value in those cases where cellular migration occurs in response to trauma or localised cell loss. The quantification of intercellular cell area variability (polymegethism), and cell shape variability (pleomorphism), provides additional data on the effects of physiological stress. Cell form factors, which exhibit large intercellular variability, prove unhelpful when applied to global populations of cells. The authors conclude that in performing quantitative analysis of the corneal endothelium, specular microscopists should apply informed restraint when selecting parameters for morphometric analysis. The relative merits of a range of morphometric parameters are discussed.     

Localised wedge shaped defects of the retinal nerve fibre layer in glaucoma.

Glaucoma can be associated with a diffuse or localised loss of the retinal nerve fibre layer (RNFL). This study evaluated the wedge shaped localised RNFL defects. Red free wide angle RNFL photographs of 421 patients with glaucoma and 193 normal subjects were examined. Localised RNFL defects were described for one eye of the normal group and for 20% of the patients with glaucoma. They were usually located in the inferior temporal and superior temporal fundus regions. Within the glaucoma group, localised RNFL defects occurred most often (p < 0.05) in normal pressure glaucoma, followed by primary open angle glaucoma, and finally secondary open angle glaucoma. They were positively associated with disc haemorrhages. The localised RNFL defects had a high specificity to indicate optic nerve damage. The nerve fibre layer defects occurring more likely in mild rather than advanced glaucoma, they were helpful in the diagnosis of early glaucoma. The association between localised RNFL defects and disc haemorrhages and the varying frequency of localised RNFL defects in different types of glaucoma may be important diagnostically and pathogenetically.     

Ocular findings associated with a 3 base pair deletion in the peripherin-RDS gene in autosomal dominant retinitis pigmentosa.

Affected members of a family with autosomal dominant retinitis pigmentosa were found to have a 3 base pair deletion at codon 118 or 119 of the retinal degeneration slow gene. This mutation causes the loss of a highly conserved cysteine residue in the predicted third transmembrane domain of peripherin-rds, a photo-receptor specific structural glycoprotein localised to both rod and cone outer segment disc membranes. Four of these individuals underwent detailed clinical, psychophysical, and electroretinographic testing in order to characterise their photoreceptor dysfunction. Nyctalopia was reported early in the second decade by all patients. Global rod and cone dysfunction was recorded by the third decade with severe reduction of both photopic and scotopic function by age 30 years. This retinal degeneration slow gene mutation may lead to the primary loss of both rod and cone photo-receptor function.     

Blue on yellow perimetry with scanning laser ophthalmoscopy in patients with age related macular disease

BACKGROUND/AIM: The loss of short wavelength sensitive (SWS) cone mechanism sensitivity is related to severe vision loss in patients with age related maculopathy (ARM). A case-control study of patients with ARM and age matched controls was performed, using blue on yellow static perimetry. METHODS: A bright yellow background at 594 nm isolated the responses of short wavelength cone mechanisms to 458 nm targets. A scanning laser ophthalmoscope produced stimuli and provided real time, simultaneous fundus illumination. The macula was probed with 16 Goldmann IV targets, 1-10 degrees from fixation, using a staircase method. RESULTS: 24 patients with non-exudative ARM were matched to 24 subjects with normal fundus appearance. SWS cone pathway sensitivity for macular targets was significantly reduced in the patients with ARM compared to normals--15.45 (SD 4.56) dB v 17.22 (0.28) dB, respectively (p<0.0005). There was not only a diffuse loss of sensitivity in ARM patients, but also a localised loss of sensitivity over drusen (p<0.025). Neither the mean age, 69 (8) years, nor the mean visual acuity differed between groups, logMAR 0.09 (0.10) v 0.05 (0.06) for ARM patients v normals, respectively. Patients with soft drusen had lower sensitivity than those with hard drusen (p<0.05). CONCLUSION: A loss of SWS cone pathway sensitivity occurred in most patients with early ARM, despite good visual acuity, demonstrating a loss of visual function that cannot be attributed to ageing changes. The loss of sensitivity, despite good visual acuity, included both a diffuse loss and localised losses.     

Study of the visual field using automated static perimetry following panretinal photocoagulation in the diabetic

Twelve diabetic patients presenting a preproliferative or proliferative retinopathy were the subjects of our study. We studied the modifications of the visual fields, analyzed by computerized perimetry, after panretinal photocoagulation (PRP). For every patient, two techniques of PRP were applied, one on each eye: C: conventional scattered treatment, G: cluster technique. Computerized perimetry was done within 10 days prior to PRP, and 4 to 8 weeks following it. We used the Octopus 500, programs 36 (0 to 30 degrees) and 44 (30 to 60 degrees). It appears that, before any treatment, there is a loss of retinal sensitivity excepted the more central zone. Concerning the loss of sensitivity, although the number of laser burns, is greater in group G, there is no statistical significative difference between the 2 groups. Nevertheless, it seems to be more heterogeneous in group G. However, we have not studied the visual field modifications after confluent PRP using up to 5,000 500 micron laser burns. Although there is a loss of retinal sensitivity after PRP, none of our patients complained of it. PRP seems to us to be the only treatment able to prevent the natural evolution of preproliferative or proliferative diabetic retinopathies.     

Retinal thickness at the posterior pole in glaucoma and ocular hypertension

Purpose Ganglion cells with nerve fibre layer contribute a substantial fraction to the retinal thickness (RT). In contrast to the analysis of papillary and peripapillary area, which have shown large variability in normal eyes, the variation of retinal cell density in the perifoveal region is reported to be less than 10%. By measuring RT at the posterior pole we wanted to detect retinal changes due to glaucoma and determine their correspondence with visual field (VF) changes.Methods The Retinal Thickness Analyzer (RTA) was used to obtain RT maps in 21 normal eyes, 20 eyes with ocular hypertension and 22 eyes with glaucoma. A green laser slit (540 nm) of the RTA was focused on the retina at an angle and its intersection with the retina was imaged. The distance between the reflections from the vitreo–retinal and retina–retinal pigment interface is directly proportional to the RT. Five locations covering the central 20° were scanned, generating 10 optical cross sections that were transformed into colour-coded RTA maps. Numerical data were presented for the perifoveal and posterior pole region.Results In glaucomatous eyes significant localised thinning of the retina was present, identified as the increased number of clusters including at least four points that are 2 standard deviations below normal. The minimum retinal thickness was decreased in glaucoma eyes and the perifoveal temporal modulation and perifoveal temporal/nasal asymmetry—indicators of greater RT loss in the temporal and the nasal quadrant, respectively—were higher in eyes with glaucoma, but with overlapping global indices between the groups. In 16 of 22 eyes with glaucoma there was good agreement of RT changes with VF defects. In two eyes with typical glaucomatous damage at the optic disc but without VF defects localised changes of RT were detected.Conclusions The RTA enables objective noninvasive evaluation of the posterior pole and could become helpful in diagnosis of glaucoma before the onset of functional damage. However, at present its clinical usefulness is limited by overlapping values of retinal thickness between normal and glaucomatous populations.     

RAPID FADING OF VISUAL SENSATIONS IN A SUBJECT WITH A PARIETAL — OCCIPITAL TUMOUR

We present psychophysical data measured for a single subject, who had a tumour in the left parieto-occipital region, located by CT scan and post-mortem study. Her vision was characterised by rapid fading during voluntary fixation of images located in the right hemifield. We describe a number of simple tests which we developed in order to study this phenomenon. The results of these tests established that significant loss of function occurs within 1 s of fixation and that the effects associated with fixation are spatially localised. We compare our subject's response characteristics with those observed in other cases of visual disturbance and we examine the possible underlying mechanisms which give rise to the rapid fading of her visual images.     

Some observations on the relation between intraocular pressure reduction and the progression of glaucomatous visual loss.

A quantitative measure of visual field loss associated with kinetic perimetry in chronic open-angle glaucoma (COAG) is discussed. With this new technique an analysis of covariance is applied to a retrospective study of 41 COAG patients. It suggests that reduction of intraocular pressure may not be uniformly effective in controlling the progression of the disease.     

Color mixture thresholds measured on a color television — a new method for analysis,classification and diagnosis of neuro-ophthalmic disease

A color television display can be used to determine color and brightness discrimination thresholds using identical adaptation conditions and experimental technique. The color discrimination threshold is measured by using an equiluminous test spot - i.e. one which differs in color from the surrounding screen but has the same luminance. Because there is no brightness clue, the subject is forced to detect such a spot by using color discrimination. It is shown how color and brightness thresholds may be determined from threshold measurements of different color-mixtures even though it is not known beforehand which stimulus will be equiluminous for the subject. Results are shown for normal subjects, congenital color defectives and for two patients having optic nerve disease who show respectively non-selective and selective loss of color discrimination compared to brightness discrimination. Normal control data are presented, illustrating the effect of eccentricity, optical blur, viewing distance, pupil size and age. It is concluded that the technique is relatively insensitive to moderate variations in these factors and that it is more sensitive in detecting selective color loss than a spectral sensitivity technique which has been described previously.     

Lymphoid proliferations in the orbit: malignant or benign?

Clinical, pathological, and immunological analysis of 20 patients with ocular adnexal lymphoid disease has demonstrated several parameters which are useful for distinguishing malignant from benign lesions. Patients in the fourth or fifth decade of life presenting with an acute history of pain, oedema, epiphora, double vision, and ptosis, with a mass localised in the lacrimal gland area, are more likely to have a pseudolymphoma or a chronic inflammatory lesion than a true non-Hodgkin lymphoma (NHL). It is not possible to obtain a definite diagnosis without surgical intervention, because only three out of nine patients with orbital NHL had evidence of a monoclonal B cell population in peripheral blood on admission to the Orbital Centre. Furthermore it was confirmed that the identification of the various orbital lymphoid infiltrates becomes more distinct when immunological techniques are added to the clinical and histopathological methods of investigation. Multidisciplinary cooperation leads to further improvement of diagnosis and treatment of ocular adnexal lymphoproliferative disease.     

Meibomian gland dysfunction in chronic blepharitis

We examined 57 patients with symptoms of chronic blepharitis using meibomian gland expression, meibography, tear osmolarity, and the Schirmer's test. We also performed meibography on 20 normal patients free of chronic blepharitis. We found that 42 blepharitis patients (74%) had evidence of meibomian gland loss, whereas only four of 20 normal patients (20%) had any gland dropout. We performed cluster analysis on the data from the patients with blepharitis and found that these patients tended to fall into distinct groups with clinically relevant characteristics. We also found that tear osmolarity correlated positively with gland dropout (+0.413) and negatively with excreta volume (-0.499). This study demonstrates that an objective analysis of meibomian gland function may be used to assess chronic blepharitis and define subsets of blepharitis with measurable differences. It also supports the significance of meibomian gland dysfunction on tear osmolarity and the evaporative state of the eye.