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Biomedical subjects

W H Morgan

Publications and source records attributed to W H Morgan.

At least 19 recordsLinked to original sources
PubMed2005

Confocal laser Doppler flowmeter measurements in a controlled flow environment in an isolated perfused eye.

The aim of this study was to improve our ability to interpret and validate Heidelberg Retina Flowmeter (HRF) flow images by recording flow measurements from specific regions of the retinal vasculature by taking advantage of the ability to precisely regulate perfusion flow in an isolated eye preparation. The retinal vasculature in 16 isolated perfused pig eyes was perfused with a 50%/50% Krebs/RBC solution at known flow rates ranging from 0 to 300 microl min(-1). At each flow rate, HRF images were obtained at a location approximately two disc diameters from the disc. After HRF image acquisition, the retinal vasculature was perfused with fluorescein isothiocyanate for fluorescence microscopy. Using the standard HRF software and a 10 x 10pixel measurement window, flow rates were measured from a retinal artery, vein, arteriole, venule, and the retinal capillary bed and a capillary-free-zone. The relationship between HRF measured flow and perfusion flow in the different measurement locations was determined. At zero perfusion flow the measured HRF flow was consistently greater than zero ( approximately 170 arbitrary units (AU)), and not significantly different at each measurement location except for the retinal vein, which had a significantly higher HRF flow value ( approximately 230AU). At higher perfusion flow rates the flow signal from the larger vascular elements (arteries and veins) increased rapidly thereafter to reach several thousand AU at a total perfusate flow of 50 microlmin(-1) and increased less rapidly at higher flow rates. In arterioles, the HRF flow was more linear over a broader range of perfusate flow rates but the peak flow signal was an order of magnitude smaller than that from the retinal artery. Both the linearity and magnitude of the flow signal in venules was less than that in arterioles. In capillary areas and in the capillary free zone, the HRF flow showed only a very weak relationship to perfusion flow when compared to the background noise. The choice of location for HRF flow analysis greatly influences the ability of the technique to measure changes in retinal blood flow. The major arteries and veins provide the strongest signal and greatest signal to noise ratio. However, the retinal arterioles produce an HRF signal that is more linear over a wider range of perfusate flow rates.

Animals↗
PubMed2001

A pilot study of in vivo venous pressures in the pig retinal circulation.

Retinal venous pressure was examined in six pigs using a balanced servo-nulling micropuncture technique. The mean transmural venous pressure was 0.95 mmHg (SD 1.50 mmHg). The transmural venous pressure was lower at the optic disc than away from the disc (0.32 +/- 1.46 mmHg vs 1.69 +/- 1.19 mmHg, P < 0.01). At the disc a transmural pressure of zero or less (0 to -0.5 mmHg) was demonstrated in 10/21 (48%) of the disc readings. Over an intraocular pressure range of 15-26 mmHg there was a strong correlation between intraocular pressure and retinal venous pressure (Pearson coefficient r = 0.92). The results are compatible with the Starling resistor theory of venous outflow from the eye.

Animals↗
PubMed2000

Computer-assisted planimetry associated with Sturge-Weber syndrome.

Photographs of the optic nerve head (optic disc) in the eye are used for the clinical assessment of the disease glaucoma. These images are usually subjectively assessed by a clinician. A case of Sturge-Weber Syndrome which includes glaucoma as a symptom, is presented here. Narrowing of the rim of the optic disc was measured using custom-made measurement software confirming glaucomatous progression. To maximize the information obtained from optic disc images, low cost software can assist with quantifying disc parameters aiding clinical interpretation.

Adolescent↗
PubMed1998

The distribution of angioarchitectural changes within the vicinity of the arteriovenous crossing in branch retinal vein occlusion.

OBJECTIVE: Branch retinal vein occlusions (BRVOs) are known to occur most commonly in the vicinity of arteriovenous (A/V) crossings. The authors aimed to identify types of venous wall abnormalities in BRVO and document their position in relation to the A/V crossing. DESIGN: A retrospective review of the color photographs and fluorescein angiograms from the most recent 110 patients with first- or second-order BRVO was performed. MAIN OUTCOME MEASURES: The films were examined for the presence of angioarchitectural changes of specified type within one-quarter disk diameter of the A/V crossing involved in the BRVO. The specific changes noted were fluorescein leakage, presumed thrombi, and flow abnormalities, which were recorded along with their position in relation to the A/V crossing. RESULTS: Of the 110 patients diagnosed with BRVO, 59 had photography of satisfactory quality. Forty-one (70%) of these 59 patients had venous lesions, of which significantly more (chi-square -5.74, P < 0.02) were downstream (56%) than upstream (12%) from the A/V crossing. Thirty-two percent were upstream and downstream. Of the hemodynamic changes seen, 49% had late venous phase leakage of fluorescein, 85% had abnormal flow, and 7% had presumed thrombi. All thrombi seen were downstream. CONCLUSIONS: Venous lesions in the vicinity of the A/V crossing commonly are seen in BRVO, most of which occur downstream. This suggests that the venous narrowing at the crossing may induce downstream hemodynamic changes predisposing to endothelial damage and thrombus generation.

Arteriovenous Malformations↗
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

Culture and characterization of smooth muscle cells from porcine retinal microvessels.

PURPOSE: To establish long-term cultures of retinal vascular smooth muscle cells for future studies of their functions under normal and diseased conditions. METHODS: Smooth muscle cells (SMC) from porcine retinal microvessels were isolated and cultured. RESULTS: Cells in culture retained the common biochemical characteristics of SMC propagated from other sources. The cells were large, polygonal, spindle shaped and demonstrated the presence of smooth muscle-specific alpha-actin. Post-confluent cultures showed the 'hill and valley' multilayer growth characteristics. However, the growth rate was lower and the population doubling time was significantly larger than those reported for SMC propagated from big vessels. CONCLUSION: Retinal vascular SMC could be cultured separately from other cell types. The availability of this culture provides a unique model for functional and metabolic studies of the retinal vessels.

Actin Cytoskeleton↗
PubMed1998

Relation between pressure determined by ophthalmodynamometry and aortic pressure in the dog.

AIMS: Ophthalmodynamometry has been used extensively since the last century; however, controversy surrounds what it actually measures. This study was set up to determine the relation between ophthalmodynamometric (ODP) and systemic blood pressures. METHODS: Aortic pressure was continuously monitored and altered by phlebotomy in six anaesthetised dogs, while ophthalmodynamometry was performed, by directly altering intraocular pressure. Maxillary artery pressure was monitored in two animals. All pressure transducers were zeroed at eye level. RESULTS: Mean ODP was 96.6% (1.6%) (95% confidence interval, n = 49) of aortic pressure. Mean maxillary artery pressure was 95.7% (5.5%) (95% CI, n = 16) of aortic pressure. ODP was 1.9 (0.6) mm Hg (95% CI, n = 33) higher than maxillary artery pressures. CONCLUSION: ODP was only slightly below aortic pressure and not significantly different from maxillary artery pressure, the analogue of the internal carotid artery in humans. These results also suggest a retinal artery collapse pressure of at least 1.9 mm Hg.

Animals↗
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↗
PubMed1997

Retinal artery and vein pressures in the dog and their relationship to aortic, intraocular, and cerebrospinal fluid pressures.

The relationship between retinal arterial (Pra) and aortic (Pa) pressures is unknown, and the relationship between retinal vein (Prv) pressure and intraocular pressure (IOP) is not clear. Also unclear is the effect of cerebrospinal fluid pressure (CSFp) upon retinal venous pressure. We aimed to measure the relationships among Pra, Prv, Pa, IOP, and CSFp. Dogs were anesthetized while IOP, CSFp, and Pa were monitored. Pipettes with 2.5-micron diameter tips, connected to a servonulling pressure transducer, were used to record pressures from the retinal arteries and veins. Across a range of IOP (16-22 mmHg), CSFp (0-21 mmHg), and Pa (23-195 mmHg) the Pra = 0.72 Pa + 4.3 (r = 0.99, n = 61, P < 0.01), which suggests that the relationship between Pra and Pa is linear over a broad range of systemic blood pressures. The correlation coefficient between Prv and IOP was greater than 0.96 (P < 0.01) at all venous sites and whether IOP was greater than or less than CSFp. The transmural pressure varied along the retinal vein from 1.3 +/- 0.3 mmHg (+/-95% CI, n = 30) at 1 disk diameter from the optic disk rim to 0.3 +/- 0.2 mmHg (n = 66) at the optic disk, with a 0.9-mmHg/mm pressure gradient. These are the first measurements demonstrating a retinal vein transmural pressure close to zero.

Animals↗
PubMed1995

The influence of cerebrospinal fluid pressure on the lamina cribrosa tissue pressure gradient.

PURPOSE: To measure the tissue pressure gradient through the optic disk and to determine the relationship between intraocular, cerebrospinal fluid, and retrolaminar tissue pressures. The relationship of optic nerve subarachnoid space pressure to intracranial cerebrospinal fluid pressure also was explored. METHODS: Micropipettes coupled to a pressure transducer were passed through pars plana and vitreous to enter the optic disk in the anesthetized dog. Using a micromanipulator, pipettes penetrated the optic disk in steps while pressure measurements were taken. In some animals, pipettes also were passed into the optic nerve subarachnoid space. Lateral ventricle cerebrospinal fluid pressure, intraocular pressure, and arterial blood pressure were measured concurrently, and the effect of raising CSF pressure was explored. RESULTS: Retrolaminar tissue pressure was largely dependent on the surrounding cerebrospinal fluid pressure, which was on average 8.6 +/- 3.5 mm Hg (SD, n = 8) higher, and was independent of intraocular pressure. Most (85% +/- 15% [SD, n = 8]) of the pressure drop between intraocular pressure and retrolaminar pressure occurred across the anterior 400 microns of disk tissue. When the intraocular pressure was 21 mm Hg and the cerebrospinal fluid pressure was zero, retrolaminar tissue pressure averaged 7 mm Hg and the translaminar pressure gradient was 3.08 +/- 0.29 mm Hg/100 microns tissue (SD, n = 3). Optic nerve subarachnoid space pressure was equivalent to lateral ventricular pressure. CONCLUSIONS: These results show that cerebrospinal fluid pressure largely determines retrolaminar tissue pressure; hence, along with intraocular pressure, it is of major importance in setting the translaminar tissue pressure gradient. Results also demonstrate hydrostatic continuity between the optic nerve subarachnoid space and the lateral ventricle. That the translaminar pressure gradient can vary independently of intraocular pressure may be of importance in understanding the pathophysiology of glaucoma.

Animals↗
PubMed1994

Automated extraction and quantification of macular drusen from fundal photographs.

The objective quantification of drusen (and other macular lesions) should have applications epidemiologically, in the study of the natural history of drusen, and with such instruments as the scanning laser ophthalmoscope. The automated extraction of drusen from photographs is technically difficult because of uneven macular reflectance, and the confusing pattern of darker vessels. We have developed a method using an IBM personal computer, an image digitising board and specially written software. Once the image is digitised, no further input from the operator is necessary. We present the results of manual counting versus automated counting on a small series of patients with drusen. The automated technique is highly reproducible, and will calculate the retinal area occupied by drusen. The area and numbers of drusen can be compared over time, giving an index of progression. Hard drusen are fairly well detected, but the detection of soft drusen with their lower contrast remains a problem. The technique cannot distinguish between drusen and other pale lesions (e.g., atrophic retinal changes).

False Positive Reactions↗
PubMed1992

Agonist response of human isolated posterior ciliary artery.

The isometric responses of isolated human posterior ciliary artery to adrenergic agonists, histamine (HIS), and 5-hydroxytryptamine (5-HT) were studied in passively stretched ring segments mounted in a myograph bath. Cumulative dose response curves were measured for nine agonists: HIS, 5-HT, dopamine (DOPA), epinephrine (A), norepinephrine (NA), tyramine (TYR), phenylephrine (PHE), isoproterenol (ISOP), and xylazine (XYL), and the log(molar concentration) at which one half of the maximum active tension was developed (EC50) was estimated. The ring segments were unresponsive to DOPA and XYL; HIS and ISOP produced biphasic responses with a mild relaxation for low concentrations and small contractions for high concentrations of the agonist. The remaining agonists caused contractile responses of magnitude listed in the rank order following compared with the maximum active tension in response to 0.124 M K(+)-Krebs: Kmax much greater than A greater than 5-HT = PHE greater than NA greater than TYR It was concluded that functional HIS, alpha 1-adrenergic, and 5-HT receptors were present on human posterior ciliary artery but that there are no alpha 2-adrenergic receptors.

Adrenergic Agonists↗