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Chiasmal Calculator

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Chiasmal Calculator

Posted on Friday December 9, 2022

The Chiasmal Calculator is a tool that can calculate optic nerve misrouting with the use of a 3-channel Onset Pattern VEP or Flash VEP.  It calculates a coefficient and plots a graph, both of which can be used to draw conclusions about the presence of chiasmal misrouting.

 

For whom can this be useful?

This chiasmal coefficient calculator may be used as a diagnostic tool for albinism and other misrouting disorders. This method may be used for both children and adults. Children under 6 years of age are better suited for the Flash VEP technique, while adults and children over 6 years qualify for the pattern onset VEP. A reminder that a flash exam does not require patient fixation to the same extent as pattern test. However, a pattern VEP is the preferred for best results.

 

How to interpret the results?

There are two ways to interpret the results of this chiasmal calculator; graphically or with the coefficient.

Graphically, the spreadsheet plots the difference between right and left hemispheric VEP data. In cases of misrouting, we find an inversion of the waveforms recorded from right and left eyes.

In these graphs, we see RE (PO7 – PO8) and LE (PO7 – PO8) plotted in yellow and blue, respectively. A normal patient response shows RE and LE data as symmetric. However, a patient with misrouting will show an inversion of RE and LE VEPs.

Chiasmal Coefficient

In these graphs, the coefficient for a normal patient calculates to 0.86 and –0.87 for someone with misrouting due to albinism.

The coefficient values will fall between –1 and +1, while 0 indicates pure noise. A coefficient in the negatives, closer to –1 represents an inverse relationship, and therefore misrouting. Whereas, a symmetric response generates a coefficient closer to +1, indicating symmetrical optic nerve routing.

 

How to use the tool?

The chiasmal calculator is a multi-tabular spreadsheet into which exported 3-channel VEP data can be pasted.

First, set the patient up for a 3-channel VEP, using either the O1 and O2, or PO7 and PO8 electrode locations; both are effective, although PO7 and PO8 are recommended.

Next, run a Diagnosys-provided 3-channel VEP protocol which calculates the difference between P07-PO8 (or O1 and O2). Ask a Diagnosys representative for this if you do not presently have it available.

Lastly, export your patient’s VEP data and copy/paste it into the spreadsheet. More detailed instructions are provided within the spreadsheet itself; however, the spreadsheet then calculates everything for you, creating a final graph with coefficient.

 

References:   

Kruijt CC, de Wit GC, Talsma HE, Schalij-Delfos NE, van Genderen MM. The detection of misrouting in albinism: evaluation of different VEP procedures in a heterogeneous co- hort. Invest Ophthalmol Vis Sci. 2019;60:3963–3969.

Pott JWR, Jansonius NM, Kooijan AC. Chiasmal coefficient of flash and pattern visual evoked potentials for detection of chiasmal misrouting in albinism. Documenta Ophthalmologica 2003; 106: 137-143.

Soong F, Levin AV, FRCSC, Westall CA. Comparison of Techniques of Detecting Visually Evoked Potential Asymmetry in Albinism. J AAPOS 2000;4:302-10. 

van Genderen MM, Riemslag FCC, Schuil J, Hoeben FP, Stilma JS, Meire FM. Chiasmal misrouting and foveal hypoplasia without albinism. Br J Ophthalmol 2006;90:1098–1102. doi: 10.1136/bjo.2006.091702  

 

Please indicate below if you are interested in using this tool; it comes at no cost to current