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Interpretation of OCT : A Basic Approach

Optical Coherence Tomography (OCT) is a non invasive and well-tolerated technology which uses a beam of light to image the layers of retina resulting in 10-20 times higher resolution than ultrasound images. OCT performs micro scale and cross-sectional topographic imaging which provides quantitative idea of the macula. This has helped in diagnosing, monitoring and managing various posterior segment diseases. Furthermore, it has helped in the advancement in the research area.

To master any ocular diagnostics tool, a clinician must have command over the three key components which are:
  •                     Instrumentation,
  •                     Operational, and
  •                      Interpretation.
However, this writing is primarily focused about the basic interpretation of the OCT report aiming to ease the clinician but, at the same time, the instrumentation and operational parts are of equal importance too.Before knowing how the diseased retina or macula looks like, it’s very important to learn the normal macula on an OCT at first.
A projection of Macular image from the scan of the area of interest

Normal macular OCT scanPicture showing layers of retina
Usually it is characterized by two high reflective structures, the nerve fiber layer (NFL) and retinal pigment epithelium (RPE) choriocapillaries complex (RPE-CC). So, in between the two red (strong) layers, there are the various layers of the sensory retina represented with shades of green corresponding to particular layers of the retina.

The learning of this part can be acheived only if we are able to ask 4 set of standard questions to ourselves. The 4 set of questions are as:
  • How does the vitreo retinal interface appear?
  • What is the foveal contour like?
  • Is the retinal architecture altered (status of the neurosensory retina)?
  • Whether the uniformity of retinal epithelial choriocapillaries(RPE-CC) is disrupted?

1. How does the vitreo retinal interface appear? 

 A normal vitreo retinal interface is: Above: Optically clear Vitreous Below: A high reflective nerve fibre layer (NFL).

Normal vitreo retinal interface
  • Sometimes, the vitreo retinal interface abnormalities can be a membrane that may be single or double and they may have no attachment, partial attachment or total attachment.

2. What is the foveal contour like?

i) The normal foveal contour is a V-shaped depression
ii) The abnormality in the foveal contour can be in the form of obliteration of the foveal contour which is due to either pulling mechanism (from an overlying membrane) or by pushing mechanism (from underlying intraretinal fluid).
iii) Widening of the foveal contour is another abnormality which indicates thinning of the foveal area.
Foveal contour

iv) Macular Holes: may be full thickness or lamellar (inner lamellar or outer lamellar). Macular hole index (MHI) from various methods can be measured from the OCT.
Meanwhile, we need to rule out pseudo hole from the inverted mushroom shape of inner lamellar hole.
Macular hole

3.  Is the retinal architecture altered?

Always remember the normal retinal architecture and its orientation and reflectivity.
i) The alteration in the retinal architecture can be due to accumulation of fluid which is either intraretinal or sub retinal fluid. Further, the intra retinal fluid accumulation can be diffuse or with cystic spaces.
Retinal architecture
ii) The hard exudates appear as spots of increases hyper reflectivity with a trail of shadowing behind it.
iii) Schisis, split in macula, can be easily diagnosed.
exudates seen in OCT

4. Whether the uniformity of retinal epithelial choriocapillaries (RPE-CC) is disrupted?

i) The regular uniformity of the hyper reflectivity of RPE-CC complex can be disrupted by a bumpy appearance due to drusen or fusiform thickening (CNVM).
ii) There may be elevation noted due to RPE-detachments, commonly called as PEDs.
(a) Serous PED: Optically clear space with distinct reflectivity from the choroid.
(b) Hemorrhagic PED: Similar to serous PED but absence of reflectivity from the underlying choroid as because of shadowing effect of the blood beneath RPE.
(c) Fibrovascular PED: Multiple medium reflectivity echoes and absence of distinct reflectivity of the choroid.
uniformity of retinal epithelial choriocapillaries

This writing has been prepared during the internship period (2012/13) at LVPEI, Visakhapatnam, India. 

Acknowledgements:
Dr. Avinash Pathengay, Vitro-Retina Surgeon.
P. Anand and G. Rekha, Optometry Head.
LVPEI-Visakhapatnam Team

About the author: Birendra Mahat is Research Optometrist of Nepalese Association of Optometrists (NAO), Kathmandu, Nepal and Optometrist of Grande International hospital (GIH), Dhapasi, Kathmandu, Nepal.


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