PubMed Health⌕ Search

Biomedical subjects

P H House

Publications and source records attributed to P H House.

17 recordsLinked to original sources
PubMed2000

Methods and limits of digital image compression of retinal images for telemedicine.

PURPOSE: To investigate image compression of digital retinal images and the effect of various levels of compression on the quality of the images. METHODS: JPEG (Joint Photographic Experts Group) and Wavelet image compression techniques were applied in five different levels to 11 eyes with subtle retinal abnormalities and to 4 normal eyes. Image quality was assessed by four different methods: calculation of the root mean square (RMS) error between the original and compressed image, determining the level of arteriole branching, identification of retinal abnormalities by experienced observers, and a subjective assessment of overall image quality. To verify the techniques used and findings, a second set of retinal images was assessed by calculation of RMS error and overall image quality. RESULTS: Plots and tabulations of the data as a function of the final image size showed that when the original image size of 1.5 MB was reduced to 29 KB using JPEG compression, there was no serious degradation in quality. The smallest Wavelet compressed images in this study (15 KB) were generally still of acceptable quality. CONCLUSIONS: For situations where digital image transmission time and costs should be minimized, Wavelet image compression to 15 KB is recommended, although there is a slight cost of computational time. Where computational time should be minimized, and to remain compatible with other imaging systems, the use of JPEG compression to 29 KB is an excellent alternative.

Humans↗
PubMed1999

Comparison of conventional and high-pass resolution perimetry in a prospective study of patients with glaucoma and healthy controls.

OBJECTIVE: To determine whether high-pass resolution perimetry detected glaucomatous visual field progression earlier than conventional perimetry. METHODS: In a prospective longitudinal study, we observed 113 patients with open-angle glaucoma and with early to moderate visual field damage and 119 healthy control subjects. Each subject underwent testing at 6-month intervals using conventional and high-pass resolution perimetry (program 30-2 of the Humphrey Field Analyzer [Humphrey Instruments, Inc, San Leandro, Calif] and the Ring program of the Ophthimus perimeter [Hi-Tech Vision, Göteborg, Sweden], respectively). Our predetermined criterion for progression with conventional perimetry was the presence of at least 4 overlapping nonedge locations outside the fifth percentile for test-retest variability of threshold deviations (defined by the Glaucoma Change Probability Analysis of the Statpac 2 program) in 2 of 3 consecutive visual fields. We employed the identical criterion for progression with high-pass resolution perimetry using our own test-retest variability data. We repeated this procedure in the controls to measure the false-positive rate of progression. RESULTS: Patients were observed for a median of 4.5 years and 11 examinations with each technique. Fifty-seven patients (50.4%) did not show progression with either technique. Twenty-four patients (21.2%) showed progression with high-pass resolution perimetry alone, whereas 6 (5.3%) showed progression with conventional perimetry alone. Of the remaining 26 patients (23.0%) who showed progression with both techniques, 14 (54%) showed progression with high-pass resolution perimetry first (median, 12 months earlier); 5 (19%), with conventional perimetry first (median, 6 months earlier); and 7 (27%), with both techniques at the same time. Controls were observed for a median of 5 years and 11 examinations with each technique. One control (0.8%) showed progression with high-pass resolution perimetry. CONCLUSIONS: Our results suggest that high-pass resolution perimetry detects glaucomatous visual field progression earlier than conventional perimetry in most patients with progression.

Adolescent↗
PubMed1998

A personal computer-based method of stereo chronometry for measuring neuroretinal rim width: a pilot study.

PURPOSE: Quantifying neuroretinal rim width from optic disc photographs to study glaucomatous progression requires precise, reproducible measurements. We explored the use of a personal computer with three-dimensional capabilities to enhance measurement accuracy and repeatability. METHODS: A simultaneous stereo retinal camera was used to serially photograph the optic discs often glaucomatous eyes with a mean interval of 18 months. The neuroretinal rims were qualitatively assessed as either stable (five) or narrowing (five). The images were measured using software written for an IBM compatible personal computer The measurements were compared from the earliest to the most recent image and the five narrowed rims were correctly identified. RESULTS: This simple, low-cost method allowed both three-dimensional viewing and measurement of the optic disc rim from simultaneous stereo images. CONCLUSIONS: This method is, potentially, more sensitive in identifying glaucomatous rim changes than conventional, subjective image comparison techniques.

Glaucoma↗
PubMed1998

The correlation between cerebrospinal fluid pressure and retrolaminar tissue pressure.

PURPOSE: To measure the effects of cerebrospinal fluid pressure (CSFp) on retrolaminar tissue pressure (RLTp) and the translaminar pressure gradient (TLPG), particularly at low CSFp, which is the normal situation in erect posture. METHODS: Micropipettes coupled to a servonull pressure system were passed into eyes of anesthetized dogs to the optic disc and advanced in steps through the lamina cribrosa to the optic nerve subarachnoid space (ONSAS), while pressure measurements were taken. Cerebrospinal fluid pressure and intraocular pressure (IOP) were monitored and controlled. The TLPG was measured at varying IOPs and CSFps. The RLTp and ONSAS pressure (ONSASp) were measured at varying CSFps. In separate experiments, the optic nerve dura was incised, and pressure measurements were taken across the pia mater. RESULTS: The TLPG was strongly correlated to the difference between IOP and CSFp (r=0.93; n=18) when CSFp was more than zero. Mean RLTp was 3.7+/-0.2 mm Hg (SEM; n=15) when CSFp was 0 mm Hg. The ONSASp and RLTp were largely dependent on the presence of CSFp higher than break point pressures of -0.5 mm Hg and 1.33 mm Hg, respectively. However, below these break points, RLTp (slope 0.07) and ONSASp (slope 0.18) were little influenced by CSFp. Separate measurements across the pia mater revealed that 95% of the pressure drop occurred within 100 microm of the pial surface. CONCLUSIONS: The TLPG and RLTp are dependent on CSFp when CSFp is more than -0.5 mm Hg. Below this level, there is no hydrostatic continuity between the intracranial and optic nerve subarachnoid space. In this range, RLTp is stable and is little influenced by CSFp changes.

Animals↗
PubMed1993

The effect of lens opacity on the resolution visual field in normal subjects.

The sensitivity of the visual field declines with age. The density of the crystalline lens increases with age and may be partly responsible for the decline in sensitivity. To determine whether lens opacity influences the mean threshold of the visual field, 82 normal subjects were enrolled. All had their lens opacity measured with the Interzeag Lens Opacity Meter 701 and the visual field assessed with the Ring Resolution Perimeter. Lens opacity was highly correlated with age, while mean visual field threshold was also correlated with age but showed much greater variation. Multiple regression failed to show any influence of lens opacity measurements on the mean field threshold after age had been accounted for.

Adult↗
PubMed1991

Diffuse and localized glaucomatous field loss in light-sense, flicker and resolution perimetry.

A total of 75 eyes in 75 patients with different types of glaucoma (21 eyes with low-tension glaucoma, 49 with primary open-angle glaucoma and 5 with pigmentary glaucoma) were examined by automated light-sense, flicker and resolution perimetry. All fields were classified in a masked fashion as being "normal" (N) or as having "diffuse loss" (D), "localized loss" (L) or "diffuse plus localized loss" (DL). The frequency distributions for the various field loss categories were plotted against the highest intraocular pressure ever reported in the patients' records. The frequency distribution for the purely localized defects showed a peak at 20 mmHg and were markedly skewed to low pressure values, whereas those for both diffuse plus localized damage and purely diffuse loss peaked at about 30 mmHg. The data suggest that diffuse field loss may be an indicator of pressure-induced damage.

Adult↗
PubMed1991

Intratest variability in conventional and high-pass resolution perimetry.

The authors studied the effects of threshold, age, and visual field location on intratest variability in 11 normal subjects between the ages of 15 and 50 years (mean, 35.37 years). The subjects were tested with a conventional manual (Tübinger) perimeter and a high-pass resolution (Ring) perimeter at the following locations: (0 degrees, 30 degrees), (0 degrees, 15 degrees), (0 degrees, -15 degrees), and (0 degrees, -30 degrees). Frequency-of-seeing curves were constructed to obtain accurate estimates of threshold and intratest variability. Although location did not have a significant effect on intratest variability in conventional perimetry, intratest variability increased with threshold (P = 0.031) and age (P = 0.012). The results with resolution perimetry contrasted sharply and showed that neither threshold, age, nor location had a significant effect on intratest variability (P greater than 0.225). Although the thresholds with the two types of perimetry were correlated, intratest variability was not. These results show that resolution perimetry may be able to bypass some of the limitations encountered in obtaining thresholds with conventional perimetry, thereby providing more reliable information.

Adolescent↗
PubMed1991

Choice of anesthetic agents for peribulbar anesthesia.

To determine the effectiveness of various combinations of lidocaine, epinephrine, and hyaluronidase with bupivacaine, a prospective, randomized, masked study on 117 patients having cataract extraction under local anesthesia was undertaken. All patients were given a 9 ml peribulbar block by one surgeon, and a graded assessment of analgesia and akinesia was made in a masked fashion one hour later. A mixture using all these agents gave significantly better results than any of the combinations (P = .001). No single agent was shown to be the major determinant of effectiveness. The result suggests a synergistic effect found only when all the agents are combined.

Anesthesia, Local↗
PubMed1991

Evaluation of sensitivity and specificity of spatial resolution and Humphrey automated perimetry in pseudotumor cerebri patients and normal subjects.

To determine the sensitivity and specificity of high-pass resolution perimetry ("ring test"), 18 patients with pseudotumor cerebri (PTC) and 18 age-matched controls were examined with the Humphrey program 24-2 and the ring test. Goldmann perimetry also was done to determine if defects found with the ring test were present with another method. Testing with Humphrey perimetry revealed defects in 15 PTC patients and four control subjects; with the ring test, 13 PTC patients and two control subjects had abnormalities. The disturbed areas in the control subjects with both automated tests were not reproducible. Humphrey perimetry had a sensitivity of 83% and the ring test, 72%. The specificities were Humphrey perimetry, 78% and the ring test, 89%. These differences were not statistically significant. Qualitative assessment of the presence and extent of damage using the pointwise probability plots and graphically displayed raw data showed good correlation of the tests in 11 of the 18 patients. The lack of correlation in four of the patients was caused by the presence of a generalized depression or a peripheral contraction on the Humphrey test; this defect, not present on retesting, may have been related to fatigue or poor motivation. The ring test is a sensitive and specific perimetric technique in patients with PTC.

Adult↗
PubMed1991

[Diffuse and localized glaucomatous visual field changes in light sense, flicker and visual acuity perimetry. Evidence of pressure damage].

75 eyes of 75 patients with different types of glaucoma (21 eyes with normal-tension glaucoma, 49 eyes with primary open-angle glaucoma and 5 eyes with pigmentary glaucoma) were examined by automated light-sense, flicker and resolution perimetry. All fields were classified as being "normal" (N) or having "diffuse loss" (D), "localized loss" (L) and "diffuse plus localized loss" (DL) in masked fashion. The frequency distributions for the various field loss categories against the highest intraocular pressure ever reported in the patients' records were plotted. The frequency distributions for the purely localized defects show a peak at 20 mmHg and are markedly skewed to low pressure values, while the diffuse plus localized damage and the purely diffuse loss, however, show a peak at about 30 mmHg. The data support the hypothesis that diffuse field loss may be an indicator of pressure-induced damage.

Adult↗
PubMed1990

The effect of refractive blur on the visual field using the ring perimeter.

To determine the effect of optically induced blur on the visual field measured with high pass spatially filtered targets, 10 normal subjects had field examinations with 0 diopter + 1.00 diopter or + 2.00 diopter of overcorrection in the cyclopleged state. All subjects showed a significant loss of sensitivity when defocussed, but not with pupil dilation. Mean thresholds for the central field rose from 4.79 +/- 0.68 dB (Mean +/- SD) with the image focussed and pupil dilated, to 7.16 +/- 0.72 dB with two diopters of image blur (P less than 0.001). The effects of defocussing did not differ significantly between the central and peripheral parts of the field. The great influence of defocussing the image on sensitivity should be considered during clinical perimetry as the 2.37 dB decline shown would place the group's mean sensitivity below the fifth percentile for age corrected normals.

Adult↗
PubMed1990

Control of intraocular pressure after cataract extraction.

We carried out a prospective randomized study in 172 patients undergoing cataract extraction and lens implantation to compare the effects of pilocarpine gel, 1% acetylcholine chloride and 0.01% carbachol on early postoperative intraocular pressure (IOP), to determine the effect of sodium hyaluronate on IOP and to compare the effects of 0.01% carbachol (full-strength) and 0.005% carbachol (half-strength) on IOP, pupil size and brow ache. IOP was measured 3, 6, 9 and 24 hours after surgery, and in the full- and half-strength carbachol groups pupil size and subjective complaints of brow ache were recorded. The mean IOP 3 and 6 hours after surgery was significantly lower in all the treatment groups than in the control groups. At 9 and 24 hours it was significantly lower only in the carbachol groups. The use of sodium hyaluronate was not found to affect the postoperative IOP. There was no difference in postoperative IOP or miosis between the full- and half-strength carbachol groups, but fewer patients in the half-strength group than in the full-strength group reported brow ache at 9 and 24 hours. The results suggest that carbachol is the most effective agent currently available for the management of IOP after cataract extraction.

Acetylcholine↗
PubMed1988

An evaluation of the Hyde surgical astigmatic ruler.

The Hyde Ruler, a semi-quantitative intraoperative keratometric device, was compared with the Bausch and Lomb keratometer by taking 50 paired measurements of induced astigmatism on a model cornea. The correlation between the two instruments was 0.884, and regression of the Hyde Ruler estimates on the Bausch and Lomb keratometer readings produced a slope of 0.732 with an intercept of -0.150 D. From the regression equation, a correction was applied to the Hyde Ruler estimates, resulting in 68.2% of the latter falling within 1.22 D or less of the Bausch and Lomb readings. We consider this accuracy to be acceptable for a semi-quantitative intraoperative keratometer, and should allow appropriate suture tensioning during cataract surgery to aid in early visual rehabilitation of the patient.

Astigmatism↗
PubMed1987

Postoperative astigmatism using the McIntyre needle.

We performed 30 consecutive cataract extractions using two different needle types to assess postoperative astigmatism. In each case, we used a standard corneal incision followed by an extracapsular cataract extraction and the insertion of a posterior chamber intraocular lens. The incisions in group A were closed with the McIntyre needle and in group B with a standard curve needle. We compared the postoperative astigmatism in each group with sutures in at 3 weeks, and when removed selectively by 12 weeks, and found no significant difference.

Astigmatism↗
PubMed1986

Ocular leprosy: a case report and discussion of the pathology.

Although leprosy is considered rare in Australia outside the Northern Aboriginal population, its presence within Indian and Southeast Asian migrant groups must be considered. A case of ocular leprosy is presented in which the definitive diagnosis was delayed because lid changes had been diagnosed as senile ectropion. The pathology of ocular leprosy is discussed, together with relevant therapeutic considerations.

Aged↗