The Spark ImageWise 28 – Characteristics of Myelinated Retinal Nerve Fiber Layer on SDOCT

Dr. Swati Mishra, Dr. Sarang Lambat, Dr. Prabhat Nangia, Dr. Vinay Nangia

Case description

A male, 73 years of age, presented to SEI for glaucoma follow up. He had a history of angle closure and was using Latanoprost +Timolol maleate combination eye drops for the same. On examination the visual acuity was 6/9, N6 in both eyes (BE) with a correction of +1.75 DS in RE and + 2.50 DS with +0.50 DC in LE. The anterior segment showed BE shallow anterior chamber with Von Hericks grading I and immature cataract. BE had Yag-PI which was patent. Intraocular pressure (IOP) measured with Goldman applanation tonometer was 24 mmHg in right eye (RE) and 20 mmHg in left eye (LE) on combination eye drops. Gonioscopy in BE showed angles opening upto anterior trabecular meshwork in all quadrant. 

Fig 1: RE colour fundus photo suggestive of gray-white well-demarcated patches (blue arrow) along the super and inferior vascular arcades with frayed borders on the anterior surface of the neurosensory retina suggestive of myelinated retinal nerve fibre layer. 
Fig 2: Left eye colour fundus photo suggestive of healthy disc. A shiny Epiretinal membrane can be seen in the inferior quadrant (yellow arrow)
Fig 3: RE circumpapillary RNFL OCT shows abnormal appearance of RNFL (yellow arrow) in the area of myelinated RNFL. This appearance is associated with segmentation error and increased RNFL thickness readings. There’s an artifactual peak and trough of the outer retinal layers. (Blue arrow)
Fig 4: Follow up of  RE circumpapillary RNFL OCT done after 6 months shows a different pattern of the artefact. Here the artifactual peak is not seen  and the trough is broadened with loss of peak along the temporoinferior  segment. (Green arrow) 
Fig 5: RE circumpapillary RNFL OCT scan in display mode shows healthy RNFL and no artefact.
Fig 6: LE circumpapillary RNFL OCT showing RNFL loss in supero-temporal sector. There is a nasal shift of the superotemporal RNFL giving an  appearance of RNFL loss in the supero-temporal segment. 

Discussion

In our patient,  the myelinated retinal nerve fibers were seen only in the Right eye clinically and in the  parapapillary area. There was a hint of myelination in the left eye also inferior and superior to the disc.   The appearance of the RNFL in the retinal thickness scan  gives the appearance of hyper reflective peaks in the area of  presence of myelination of RNFL. These peaks  are unnatural  and often  trick the OCT into a false segmentation and a giving a relatively higher RNFL thickness measurement.  In such a situation it is best left to our  clinical judgement specially by looking at the cup disc ratio.  Often the only thing seen clearly is the cup and  sometimes parts of the neuroretinal rim are also seen. In our patient  fortunately only a small part of the superior neuroretinal rim of the RE is not visible clearly in the RE.   On  a different scan protocol  as seen in figure 5,  the segmentation will not take place, but the clinical appearance of the RNFL is relatively normal.  This  can be used in the absence of segmentation and RNFL thickness values, based on the clinical appearance specially of the superior and inferior  humps and the macular RNFL to help make a decision if the optic nerve is healthy.  A manual measurement of the Retinal nerve fibre  at  the humps,  and at the macular may also be done  to help  with the  follow up. Glaucoma is perhaps the most important  diagnosis that one needs to consider. The other important option is to  do a ganglion cell scan and a  perimetry in such patient and not rely completely on the RNFL thickness values.  it is of interest that the hyper reflectivity of the myelinated RNFL in the scan does not allow the outer retinal layers and the choroid to be seen clearly.  Retinal myelin is known to be lost in the area of the glaucomatous damage. This would need serial colour fundus optic disc  photography follow up in patients suspected or diagnosed to have glaucoma. 

 It has also been shown that multicolour photography and infrared images may be better able to delineate the optic nerve margins and therefore the neuroretinal rim may be assessed better using these modalities in such patients. 

Our patient may be an important example since he has been diagnosed to have primary angle closure disease and is on treatment. While the eye with the obvious myelinated nerve fiber does not show any definite loss, there is  an apparent loss in the LE, largely due to the nasal shifting of the RNFL. However  the message is  that this patient would need to be on   long term follow up to detect the first signs of evolving glaucomatous damage. 

ReadWise

  1. Shelton JB, Digre KB, Gilman J, Warner JE, Katz BJ. Characteristics of myelinated retinal nerve fiber layer in ophthalmic imaging: findings on autofluorescence, fluorescein angiographic, infrared, optical coherence tomographic, and red-free images. JAMA Ophthalmol. 2013 Jan;131(1):107-9. doi: 10.1001/jamaophthalmol.2013.560
  2. Bass SJ, Westcott J, Sherman J. OCT in a Myelinated Retinal Nerve Fiber Syndrome with Reduced Vision. Optom Vis Sci. 2016 Oct;93(10):1285-91. doi: 10.1097/OPX.0000000000000951
  3. Agarwal A, Arora V. Spectral domain optical coherence tomographic characteristics of unilateral peripapillary myelinated retinal nerve fibers involving the macula. J AAPOS. 2010 Oct;14(5):432-4. doi: 10.1016/j.jaapos.2010.05.010
  4. Katz SE, Weber PA.  Photographic documentation of the loss of medullated nerve fibers of the  retina in uncontrolled primary open angle glaucoma.  Journal of Glaucoma, 01 Dec. 1996, 5(6) 406-409. 

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