ENT

Study find that the identifying unilateral optic nerve lesions in patients with multiple sclerosis; therefore according to a study published in the May issue of the Annals of Neurology. Optical Coherence Tomography is a non-invasive diagnostic technique that renders an in vivo cross sectional view of the retina. Because OCT utilizes a concept known as inferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns.

Cross-sectional map

OCT was first introduced in 1991 and has found many uses outside of ophthalmology; where it is use to image certain non-transparent tissues. Due to the transparency of the eye (i.e. the retina can be view through the pupil), OCT has gain wide popularity as an ophthalmic diagnostic tool.

Rachel C. Nolan-Kenney, from the New York University School of Medicine in New York City; and colleagues measured optical coherence tomography for 1,530 patients with multiple sclerosis and healthy controls seen at 11 international sites as part of the International Multiple Sclerosis Visual System Consortium. For this study; presence of an optic nerve lesion is define as history of acute unilateral optic neuritis.

Sclerosis and healthy controls

The researchers found that for identifying unilateral optic neuritis, receiver operating characteristic curve analysis showed an optimal peripapillary retinal nerve fiber layer intereye difference threshold of 5 µm and ganglion cell inner plexiform layer threshold of 4 µm (477 patients). An association was noted between greater intereye differences in acuities and greater intereye retinal layer thickness differences.

“These thresholds may be useful in establishing the presence of asymptomatic and symptomatic optic nerve lesions in multiple sclerosis and could be useful in a new version of the diagnostic criteria;” the authors write. From its inception, OCT images were acquired in a time domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Because the slices are 30 degrees apart; care must be taken to avoid missing pathology between the slices.

Symptomatic optic nerve

Spectral domain technology, on the other hand, scans approximately 20,000-40,000 scans per second. This increased scan rate and number diminishes the likelihood of motion artifact, enhances the resolution and decreases the chance of missing lesions. Whereas most time domain OCTs are accurate to 10-15 microns, newer spectral domain machines may approach 3 micron resolution. Whereas most time domain OCTs image 6 radial slices, spectral domain systems continuously image a 6mm area. This diminishes the chance of inadvertently missing pathology.

Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media opacities can interfere with optimal imaging. As a result, the OCT  limit the setting of vitreous hemorrhage, dense cataract or corneal opacities. As with most diagnostic tests, patient cooperation is a necessity. Patient movement can diminish the quality of the image. With newer machines (i.e. spectral domain ), acquisition time is shorter which may result in fewer motion related artifacts.