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Biomedical subjects

K W Mitchell

Publications and source records attributed to K W Mitchell.

At least 19 recordsLinked to original sources
PubMed2001

Clinical comparison of ocular blood flow tonometer, Tonopen, and Goldmann applanation tonometer for measuring intraocular pressure in postkeratoplasty eyes.

PURPOSE: Intraocular pressure (IOP) measurements vary with different tonometers in irregular corneas. The purpose of our study was to determine the comparability of the ocular blood flow tonometer (OBF) and Tonopen with the Goldmann applanation tonometer (GAT) in measuring IOP in post-keratoplasty eyes. METHODS: The IOP was measured using the OBF tonometer, GAT, and Tonopen in 69 post-keratoplasty eyes by two observers. The same instruments were used, and the sequence of measurements was randomized. Pachymetry to measure corneal thickness was done in all patients. RESULTS: Interobserver variation for IOP measured with GAT was statistically significant ( p = 0.039). Two-way analysis of variance (ANOVA), however, did not demonstrate any statistical difference between measurements with the three tonometers and two observers. Corneal thickness did not appear to have an effect on measurements with any of the tonometers. CONCLUSION: There was no significant difference in the overall accuracy of the OBF tonometer over the Tonopen versus the GAT. The OBF tonometer can be a useful alternative in measuring IOP in post-keratoplasty eyes.

Adolescent↗
PubMed2001

Clinical features of a novel TIMP-3 mutation causing Sorsby's fundus dystrophy: implications for disease mechanism.

AIMS: To describe the phenotype in three family members affected by a novel mutation in the gene coding for the enzyme tissue inhibitor of metalloproteinase-3 (TIMP-3). METHODS: Three members of the same family were seen with a history of nyctalopia and visual loss due to maculopathy. Clinical features were consistent with Sorsby's fundus dystrophy. Exon 5 of the gene coding for TIMP-3 was amplified by the polymerase chain reaction, single strand conformation polymorphism analysis undertaken and exon 5 amplicons were directly sequenced. RESULTS: Onset of symptoms was in the third to fourth decade. Five of six eyes had geographic macular atrophy rather than neovascularisation as a cause for central visual loss. Peripheral retinal pigmentary disturbances were present. Scotopic ERGs were abnormal in all three. Mutation analysis showed a G-->T transversion in all three resulting in a premature termination codon, E139X, deleting most of the carboxy terminal domain of TIMP-3. CONCLUSIONS: The patients described had a form of Sorsby's fundus dystrophy which fell at the severe end of the spectrum of this disease. Postulated disease mechanisms include deposition of dimerised TIMP-3 protein.

Adult↗
PubMed1999

The effect of long-term use of benzodiazepines on the eye and retina.

Thirty patients on long-term benzodiazepine medication were subjected to full ophthalmoscopic examination. Of these 19 (63.3%) complained of symptoms of irritation, blurred vision or difficulty in reading. None, however, had reduced visual acuities apart from two where the cause was longstanding amblyopia. Thirteen patients had some form of retinal finding, 9 macular and 4 non-macular. Of the 14 who presented for flash and pattern electroretinography (ERG), none showed any abnormality which could be ascribed to the medication, its total dose, or duration. We conclude, on the basis of the evidence from this small cohort, that long-term benzodiazepine medication has little effect upon retinal function as signalled by the ERG.

Adolescent↗
PubMed1999

Reversible retinal toxicity associated with retained intravitreal copper foreign body in the absence of intraocular inflammation.

PURPOSE: To report on a patient who developed retinal toxicity from an intraocular copper foreign body in the absence of any signs of chalcosis or inflammation. METHOD: Case report. RESULT: Removal of the copper foreign body resulted in electroretinographic improvement and the patient has retained good vision. CONCLUSION: Retinal toxicity from intraocular copper can be subclinical and such foreign bodies should be removed. Removal of the foreign body can be associated with at least partial reversal of retinal toxicity.

Adolescent↗
PubMed1997

The effectiveness of topical diclofenac in relieving discomfort following traumatic corneal abrasions.

Diclofenac is a non-steroidal anti-inflammatory drug available in an ophthalmic preparation. We present a prospective randomised double-masked placebo-controlled trial involving 40 patients that assessed the effectiveness of topical diclofenac in relieving pain from traumatic corneal abrasions. Statistical analysis of visual analogue and categorical pain scores revealed a significant reduction in pain experienced by subjects in the diclofenac group (p < 0.02).

Administration, Topical↗
PubMed1997

The prognostic value of flash visual evoked potentials in the assessment of non-ocular visual impairment in infancy.

The results of flash visual evoked potentials (VEPs) in 44 infants blind or severely visually impaired from non-ocular causes are presented, and related to the subsequent visual outcome. Ocular causes of visual impairment were excluded by clinical examination and electroretinography. Using a 2 x 2 contingency table, a significant association between VEP and outcome was demonstrated (chi 2 = 3.51, 1 d.f., p = 0.05). Of 13 infants with normal VEPs, 11 demonstrated substantial visual improvement (negative predictive value = 84.6%). However, of the 31 with abnormal VEPs, only 14 remained severely impaired/blind; the other 17 demonstrating visual improvement (positive predictive value = 45.1%). The sensitivity of the method was high in that 14 of 16 (87.5%) infants who remained impaired/blind had abnormal VEPs, but specificity was low as only 11 of 28 (39.3%) who showed visual improvement had normal VEPs. The accuracy of the technique was therefore low, 25 of 44 (56.8%) being true positive/ negative. With regard to visual outcome when faced with an apparently blind infant, it is important not to be too pessimistic for, as is shown in this study, 28 of 44 demonstrated substantial improvement. There are no absolute indicators of prognosis, but the presence of structural cerebral lesions and a history of either neonatal meningitis or encephalopathy are relatively bad prognostic signs. The flash VEP, despite its limitations, is a useful prognostic tool, particularly in those apparently blind infants whose normal ocular examination/electroretinogram is accompanied by normal VEPs. Those with abnormal VEPs, however, do not necessarily have a poor prognosis, but should be followed-up as maturational changes and/or improvements in function of the sensory pathway will be reflected in the evoked potentials.

Blindness, Cortical↗
PubMed1993

Autosomal recessive 'optic atrophy' with late onset and evidence of ganglion cell dysfunction: a sibship of two females.

Two sisters aged about 40 years presented with a recent moderate reduction in visual acuity and pale optic discs. The nonconsanguineous parents, 7 other siblings and all other family members have normal vision. This strongly indicates a hereditary, autosomal recessive origin. A primary ganglion cell pathology is strongly suspected because of the significant attenuation of the early component of the onset/offset visual-evoked potential in the presence of little change in the potential to a pattern reversal stimulus.

Adult↗
PubMed1992

Electrophysiologically determined contrast sensitivity in patients with ocular hypertension and chronic glaucoma.

Subjectively assessed contrast sensitivity has been found to be abnormal in many patients with glaucoma. We previously reported the use of onset-offset visual evoked potential measurements to determine contrast threshold objectively. We now studied 216 patients (79 with ocular hypertension and 137 with chronic simple glaucoma) with this technique. In comparison with an age-matched control group (68 subjects), mean contrast threshold was found to be significantly different in both patient groups, the degree of significance being greater in the patients with chronic simple glaucoma. Additionally, the slope of the CI-CII amplitude versus log contrast plot was shown to be depressed in the majority of affected eyes in patients with unilateral chronic simple glaucoma. This measure appears to give an indication of suprathreshold contrast processing and is related to the difference in luminance between pattern elements, rather than the quality of the border or "edge" between them. The data support not only an increase in contrast threshold (reduced sensitivity) in early glaucoma and some patients with ocular hypertension but also a suppression in suprathreshold function that is not readily measurable with standard psychophysical methods. The findings are consistent with recent theories concerning the effect of early chronic simple glaucoma on the function of Y-type units of the M (magnocellular)-type pathways.

Aged↗
PubMed1991

Prognostic significance of the pattern visual evoked potential in ocular hypertension.

This paper reports a prospective study on 49 ocular hypertensive patients to evaluate the prognostic significance of transient abnormalities in the pattern visual evoked potential (VEP) in the development of glaucoma. Seven of 24 patients with VEP abnormalities at diagnosis of ocular hypertension developed glaucomatous field defects in the follow-up period as compared with none of 25 patients with normal VEPs at diagnosis. We conclude that appropriately designed pattern VEP testing is a valuable complement to careful (preferably computerised, static) perimetry. In addition, our findings support the contention that, in glaucomatous disease of the optic nerve, rudimentary pattern processing mechanisms--that is 'Y'-type units of the magnocellular pathways--may be affected earlier than luminance processing mechanisms.

Evoked Potentials, Visual↗
PubMed1989

The visual evoked potential in acute primary angle closure glaucoma.

Visual evoked potentials (VEPs) were elicited from 29 patients who had experienced a previous attack of acute primary angle closure glaucoma. The VEPs were shown to be abnormal in at least one of the measures (latency, amplitude, contrast threshold, or slope) in 72.4% of affected eyes, whereas only 41.4% indicated obvious optic nerve damage. It is notable that 48.1% of fellow eyes with no (known) history of acute pressure rise also showed some form of VEP abnormality. The possible pathophysiological mechanisms operating in both affected and fellow eyes are discussed. It is concluded that, despite the presence of possible artefactual influences, the results probably reflect the presence of primary angle closure glaucoma.

Adult↗
PubMed1988

Pattern visual evoked potentials in hyperthyroidism.

Pattern reversal visual evoked potentials (VEPs) have been elicited in 16 female hyperthyroid patients before and after treatment and compared with those from a similar group of age and sex matched control subjects. No effect on latency was seen, and although larger amplitude values were noted in the thyrotoxic group these too were not significant. We would conclude that hyperthyroidism per se has little effect on the pattern reversal VEP, and any observed effect on these potentials is probably due to other factors.

Adult↗
PubMed1987

Somatosensory evoked potentials to electrical stimulation of the mental nerve.

A clinical test to elicit somatosensory evoked potentials (SEP's) to electrical (current) stimulation of the mental nerve, is described. Control data from 22 normal individuals were obtained which showed that in the initial 100 ms following the stimulus, SEPs with four positive components and five negative components could consistently be recorded. SEPs from three patients with trigeminal sensory symptoms were also elicited and these showed abnormal potentials in two cases with traumatised inferior dental and mental nerves, and normal potentials in the third case with subjective evidence of mental anaesthesia.

Adult↗
PubMed1987

Visual evoked potentials in the older population: age and gender effects.

Pattern reversal Visual Evoked Potentials (VEPs) were elicited from 68 normal subjects (31 males, 37 females) in the age range 40-80 years. With VEP latency, a significant age effect, increasing values with age, was demonstrated, but none in relation to gender. The converse held true for VEP amplitude however, no age effect being observed, but some gender effect in that lower values were elicited from males as compared to females.

Adult↗
PubMed1986

Visual evoked cortical potential to paracentral retinal stimulation in chronic glaucoma, ocular hypertension, and an age-matched group of normals.

Visual evoked cortical potentials (VECPs) to midperipheral stimulation of the visual field were elicited in age-matched groups of patients with unilateral chronic simple glaucoma or ocular hypertension and normal subjects. With a multimodal stimulus approach comprising transient and steady-state pattern reversal and transient onset-offset VECPs, 82.1% of the glaucoma group and 50.9% of the ocular hypertensives revealed significant abnormalities (p less than 0.01) in one or more of the electrophysiological features measured, i.e., latency, phase, amplitude, and contrast threshold.

Adult↗