The following section will provide you with a systematic framework and background for interpreting retinal nerve fibre layer (RNFL) and ganglion cell complex (GCC) OCT images.

Systematic Approach to Interpreting RNFL OCT Images

The vitreoretinal anatomy on OCT can be divided into five unique zones with possible significant abnormalities in each zone:

1. Pre-retinal

2. Epi-retinal

3. Intra-retinal

4. Sub-retinal

5. Choroid

For RNFL OCT images, the focus is primarily the more inner retinal layers including the RNFL and retinal ganglion cell layer.

 

The macular OCT is labelled below, showing the position of a single retinal ganglion cell and its axon:

If we were to zoom into the retinal ganglion cells through a fundus photograph, you will see the following:

Crossed retinal ganglion cell axons from the nasal RNFL representing the temporal visual field are shown in blue.

Uncrossed retinal ganglion cell axons in the temporal RNFL representing the nasal visual field are shown in red.

These crossed and uncrossed retinal ganglion cell axons, and the fields they correspond to, are demonstrated below:

A normal Cirrus OCT of the RNFL from a healthy 30-year-old man is shown below. The "optic disc cube" scan protocol is used to image the RNFL over a 6 x 6mm peripapillary region using 200 x 200 pixel axial measurements.

Included with the RNFL OCT is an approach to interpreting the quality of image and recognizing any pathological states.

Ensure Date of Birth is Correct

- thickness measurements are compared against age-matched controls

RNFL Thickness Map

- this is a topographical display of the RNFL

- Normally, there is an "hourglass" shape of red and orange colours since the superior and inferior RNFL are thickest

Review the Signal Strength

- Once signal strength drops below 7 in the Cirrus platform, the segmentation algorithm may fail and produce errors in the RNFL thickness measurements

Review Key Parameters

- Key parameters are compared to normative data

Green: measurement is between 5% and 95% of that seen in the normative population

- Yellow: measurement is less than 5% of that in the normative population

- Red: measurement is less. than 1% of that in the normative population

- White: Greater than 95%

RNFL Deviation Map

- this is an en face fundus image that shows the boundaries of the cup, disc, RNFL calculation circle and the deviation of the RNFL thickness from normal

Horizontal and Vertical B-Scans

- these are extracted from the data cube through the center of the disc

- ILM and cup boundaries are shown in red and RPE and disc boundaries are shown in black 

RNFL Calculation Circle 

- shows boundaries of the RNFL segmentation with red and purple lines 

Neuro-retinal Rim and RNFL Thickness Profile 

- thickness of the neuro-retinal rim and the RNFL are compared to normative data

RNFL Quadrant and Clock Hour

- average thickness is matched to normative data

Potential Pitfalls in Interpreting OCT of the RNFL Include:

1. Age of the patient: Ensure correct DOB so thickness measurements are compared to age-matched controls.

2. Check signal strength: Reduction in signal strength can result in loss of retinal features and artifacts in segmentation and interpretation. Signal strength of at least 7/10 is preferable on Cirrus machines.

3. Check for errors in segmentation: An automated algorithm is used to identify the RNFL and may not be correctly identified in patients with poor signal strength or anomalous optic discs.

Systematic Approach to Interpreting GCC OCT Images

This section will walk you through how to interpret a Ganglion Cell Complex (GCC) Analysis by Optical Coherence Tomography. For a summary:

 

- Retinal Nerve Fibre Layer is formed by the axons of the retinal ganglion cells

- Ganglion Cell Layer is formed by the cell bodies of the retinal ganglion cells

- Inner Plexiform Layer consists of axons of bipolar and amacrine cells and dendrites of ganglion cells

On the Cirrus SD-OCT, the GCL and the IPL are measured together as the ganglion cell complex

If we were to zoom into a single retinal ganglion cell, the path is described below:

Cirrus GCC OCT of a healthy 31-year-old man is shown below. From the macular cube analysis shown on the left, the macular ganglion cell analysis can be derived as shown on the right.

Colour Thickness Map

- shows thickness measurement of GCL + IPL

Sector Map

- shows GCL + IPL thickness in sextants, which are colour coded in comparison to normative data

Deviation Map

- shows GCL + IPL thickness topographically compared to normative data

Horizontal B-Scan

- The purple line represents boundary between RNFL and GCL and the yellow lines represent the boundary between the IPL and INL

Summary:

1. OCT of the ganglion cell complex and RNFL provides valuable information when evaluating patients with optic neuropathies, chiasmal or retrochiasmal visual field defects.

2. The ganglion cell analysis has advantages over OCT of the retinal nerve fiber layer since it can be easily correlated with visual field defects and usually shows changes earlier. Be weary of segmentation issues with GCC OCT.

3. OCT of the RNFL provides an objective way to document the optic disc and to follow patients with a number of pathologies. Be weary of segmentation issues with RNFL OCT.

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