OCTeye test results

Most MS patients experience damage to their anterior visual system at some point in their disease course [1]. Therefore, OCT is an effective diagnostic tool for evaluating disease activity related to MS. Damage from MS can impact the retina in multiple different ways. Optic nerve demyelination, due to clinical or subclinical optic neuropathy in MS, results in retrograde degeneration of optic nerve axons. Since these axons originate from the retinal nerve fibers, this process results in RNFL thinning which can be detected on OCT [2]. RNFL thickness has been shown to correlate with disability, brain atrophy, and visual function in MS patients [3, 4]. Retinal nerve fiber degeneration can also lead to the death of ganglion cells and contribute to reductions in decreased GCLIPL and total macular volume as measured by OCT [5].

Compared to MS, optic neuritis related to neuromyelitis optica (NMO) results in more severe RNFL atrophy and greater inter-eye RNFL differences [12]. Optic neuritis in NMO is more likely to be bilateral [18] and as mentioned above, OCT is more likely to demonstrate microcytic macular edema in NMO [15].

Optical CoherenceTomography ppt

OCT results consistent with acute optic neuritis or remote optic neuropathy increase the likelihood of underlying MS. Clinical optic neuritis occurs in roughly 50% of patients with MS and is found pathologically in nearly all MS patients at autopsy [12, 1]. Clinical studies have demonstrated that OCT evidence of a 5 μm intereye difference in RNFL thickness and a 4 μm intereye difference in GCIPL thickness was highly accurate for detecting prior unilateral optic neuritis [13].

Optical coherencetomography machine

Optic neuritis demonstrates distinct changes in both the acute and chronic phases detectable on OCT (10). Acutely, optic neuritis results in increased RNFL thickness due to inflammation within the retinal nerve fibers. This swelling can persist for weeks or months. As it resolves, “pseudonormalization” occurs in which RNFL thickness decreases but stays within a normal range due to the counterbalancing effects of resolving inflammation and progressive retinal nerve fiber atrophy. After the inflammation resolves, optic nerve atrophy remains detectable as RNFL thinning. GCLIPL thinning occurs within weeks of optic neuritis and typically precedes RNFL thinning. Clinical studies have demonstrated GCLIPL thinning as early as four weeks after optic neuritis onset [11]. Given that optic neuritis is typically unilateral, these chronic changes result in asymmetric RNFL and GCLIPL thinning in the affected eye.

OCT is a rapid, non-invasive, office-based imaging technique that provides objective quantification of retinal structures with high resolution. The OCT measurements that are clinically meaningful for MS are peri-papillary retinal nerve fiber layer (RNFL) thickness, ganglion cell layer and inner plexiform layer (GCLIPL) thickness, and macular cube volume. These measurements are calculated for each eye and for individual quadrants within each eye. Cross-sectional images are created across the optic nerve head and the macula to allow for qualitative analysis of these areas. OCT processing software compares individual measurements to age-matched controls to provide normative readouts.

Obtaining and interpreting OCT testing within a clinical center requires numerous specialized staff members and procedures to ensure high quality data capture, accurate interpretation of the results, and expedient reporting of emergent findings. The technical staff performing the OCT assessments must be adequately trained to use the technology. They must be able to assess the quality of a scan to determine if immediate retesting is required. Clinicians interpreting OCT tests must be trained to systematically assess the generated data. They need to be able to detect technical artifacts and incidental ophthalmological findings within the study. Numerous resources exist to aid with this assessment (8, 9). A system must be in place for the detection of non-MS related pathology. Patients with incidental findings should be referred to a retinal specialist for further evaluation.

At our center, OTC is used as an ancillary test in specific clinical situations that arise during the work up and management of MS. These clinical scenarios and the interpretation of the OCT results within each context are summarized below. In these scenarios, OCT is used as a para-clinical test to complement the neurological exam, lab studies, and MR imaging.

Optical coherencetomography angiography

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There are no absolute contraindications to OCT testing. However, OCT testing requires patients to focus their gaze on a target for at least 2 seconds. In patients with impaired cognition, encephalopathy, severely decreased visual acuity, or profound nystagmus, OCT testing may not be feasible.

Microcystic macular edema is an OCT finding occurring in roughly 4.7% of MS patients and  25% of NMO patients [14 ,15]. It is defined as cystic, lacunar areas of hyporeflectivity with clear boundaries seen within the retina on spectral domain OCT macular volume images, after excluding lesions due to speckling artifact [16]. The presence of microcystic macular edema is correlated with a higher EDSS and higher MS severity scores [14]. Emerging research indicates that thinning of the retinal inner nuclear layer (INL) and outer nuclear layer (ONL) is associated with, and occurs more rapidly in, primary and secondary progressive MS compared to relapsing remitting MS [17]. Further clinical research is necessary to determine if INL and ONL thinning can be used as a biomarker for progressive MS.

Numerous factors decrease the accuracy of OCT for MS.  In certain individuals, RNFL and GCLIPL levels can vary beyond normative levels due to physiologic variation. These variations can occur between eyes in the same individual causing inter-eye asymmetry in RNFL and GCLIPL thickness. Additionally, baseline RNFL thickness levels may vary between different ethnic groups (6). There are limited OCT data for healthy pediatric controls which limits the ability to normalize OCT testing in the pediatric population (7). Retinal structures can also be impacted by other ocular pathologies including high myopia, diabetic macular edema, ischemic optic neuropathy, glaucoma, and optic disc drusen. These factors may confound the results of OCT testing and decrease the specificity for MS.

There are several OCT machines available for clinical use. Studies have demonstrated significant variability between OCT machines in RNFL thickness and GCLIPL [19] and, therefore, we do not recommend comparing OCT results obtained on different machines.  Therefore, patients should be evaluated longitudinally using the same type of OCT machine.