Issue 2, August 2013

The Value of Vertical Sections

Although horizontal OCT scans are the norm, vertical sections (Figure 1) have several clinical applications as well. For example, in hemi-retinal artery and vein occlusions, vertical scans allow direct comparison of normal vs. abnormal zones of involvement. In addition, in a high myope with a posterior staphyloma, vertical scans virtually always reveal more detail than horizontal scans. Exemplary cases will be presented in a future issue of this newsletter.

Figure 1. Click here to view larger.

The labeled OCT section in Figure 2 is a vertical scan, viewed horizontally. Large retinal blood vessels in the inner retina (or towards the vitreous) are present in most scans. It is important for the clinician to recognize the shadows cast by the retinal blood vessels as normal shadows, not as the absence of outer retinal tissue. Blood vessels are relatively symmetric above and below the horizontal raphe (or dividing line through the fovea) as demonstrated in the fundus image. Sometimes, a scan corresponds closely to a major retinal blood vessel; in such a case, a rather large shadow will result. These are normal findings.

In a vertical scan such as this one, the retinal nerve fiber layer (RNFL) is most often equally thick above and below. If the RNFL is not symmetric in vertical scans, glaucoma or one of the non-glaucomatous optic neuropathies needs to be considered.

The foveal pit is an important landmark. If the photoreceptor integrity line (PIL) is not present under the fovea, visual acuity will be quite reduced. With some experience, one can estimate the VA based upon the appearance of the PIL under the fovea.

Figure 2. Click here to view larger.

The subtle "blip" of the PIL in Figure 2 directly under the foveal pit is not a cyst but is a normal finding. It likely occurs because the outer segments of the cones under the foveal pit are longer and more narrow than cones (and rods) in any other zone of the retina. Note that the PIL is a bright band with a dark band above and below it. The dark band on the inner side (above it) is the inner segment (IS) of the photoreceptors while the outer dark band (below it) is the outer segment (OS). The PIL is often called the IS/OS junction, but recent anatomical evidence has demonstrated that this is imprecise. The most recent term for the PIL is ISe (or IS ellipsoid) but clinicians dislike this term because it fails to convey the importance of this line; hence PIL is preferred.

Occasionally we encounter mild reflections off the internal limiting membrane (ILM), as seen in Figure 2. These can be normal and are not necessarily indicative of pathology. In several future installments, abnormal findings such as epiretinal membranes and early separation of the posterior hyaloid will be presented and contrasted to the normal ILM reflections shown above. In the next issue of The ABCs of OCT, we will begin with the first of many abnormal OCTs!

Stay tuned for additional installments of this newsletter on a regular basis! For comments or suggestions, write to jpersico@jobson.com.