OCT linked to errors in retinal thickness measurements

Errors in the measurement of retinal thickness are a frequent occurrence with optical coherence tomography (OCT) segmentation and analysis algorithms, according to a study published in the February 2006 issue of Ophthalmology.

A team of researchers, led by Srinivas Sadda, MD of the Doheny Eye Institute, Los Angeles, USA, selected for review by two graders, one eye from each of 200 consecutive patients undergoing Stratus OCT imaging (Carl Zeiss Meditec, Dublin, California, USA) with radial lines or fast macular thickness based acquisition protocols. On each of the line scans, the graders evaluated the position of the automated retinal boundary lines, used by the OCT machine to calculate thickness, and ranked the positioning on a six point subjective, categorical scale to create an error score.

The presence of thickness errors were connected to various parameters including the confidence assessment report from the OCT software, diagnosis of disease and the foveal centre thickness standard deviation (FCTSD) to foveal centre thickness (FCT) ratio.

The authors of the study believe that the high rate of measurement errors should be taken into account by those carrying out clinical studies where OCT imaging is used.

Contamination risk for eye drops

There is a risk of microbial contamination from preservative free eye drops housed in multiple application containers, according to data published in the February 2006 edition of the British Journal of Ophthalmology.

Of the 95 bottles, significant bacterial growth was discovered in eight (8.4%). No contamination occurred in the antibiotic group but in the non-antibiotic bottles the incidence of contamination was measured at 19%; indicating a significant statistical difference.

Seven varieties of organism were found in total; staphylococcus aureus, coagulate negative staphylococcus, bacillus species, seratia species, klebsiella ocytoca, enterobacter cloacae and alpha streptococcus. Although staphylococcus aureus contamination was the most common, some bottles did contain more than one variety.

The authors believe that the design of the bottle may be a factor in this contamination; a pipette attached to the bottle cap is removed completely whilst the drop is being administered, exposing the contents to possible contamination. They suggested an alternative risk factor may be poor technique in applying the drops.

The study concluded that caution must be taken when prescribing the drops to patients with compromised ocular surface defences.

Membrane peeling enhances surgical outcomes

Removal of the epiretinal membrane (ERM) is required during vitrectomy in order to relieve the tangential forces involved in the development of macular holes (MH), according to a report published in the February 2006 issue of Eye.

David Snead, MD and his colleagues from the University Hospitals of Coventry and Warwickshire, UK and Addenbrooke's NHS trust, Cambridge, UK, enrolled 13 patients with stage four MH undergoing vitrectomy and membrane peeling to participate in the study. They inspected the eyes, preoperatively for ERM formation over the macula and to gauge how complete the separation of posterior hyaloid membrane (PHM) was from the retina. Specimens of ERM peel were obtained during surgery and sent for immunocytochemical and histological studies. These were compared to PHM specimens taken from a previous postmortem study of eyes with physiological posterior vitreous detachment but without macular holes.

All the eyes, when studied preoperatively, had an ERM over the macula and incomplete separation of the PHM. Histologically, the ERM specimens displayed similar morphological characteristics to PHM, consisting of an eosinophilic membrane of varying thickness with scattered spindle shaped cells. The membranes stained positive for type IV collagen while the cells were glial fibrillary acidic protein positive. Postoperatively, successful closure of MH was accomplished in each case.