Tag: optic neuritis

#T4TD colour vision

When your neurologist looks in your eyes with an ophthalmoscope he/she is looking for the telltale signs of previous damage to the optic nerve. The sign we look for is optic disc pallor. The optic disc is made up of nerve fibres from the retina, which then pass out of the back of the eye to form the optic nerve. If you have had optic neuritis in the past and have lost nerve fibres this can be detected with an ophthalmoscope, OCT (optical coherence tomography) or with retinal photography. Nerve fibre loss from optic neuritis makes the optic disc look pale (see figure below).

The optic disc receives its blood supply from small arteries from the back of the eye; the amount of blood is proportional to the number of nerve fibres in the optic disc. The lower the number of nerve fibres the fewer blood vessels there are the paler the disc looks. Please remember red blood cells are red and give a health optic disc a pinkish colour (see top images above).

Did you know that with a typical attack of optic neuritis you lose about 20% of the nerve fibres in the eye? If you lose so many nerve fibres why isn’t your vision so badly affected in that eye? That is simply because your visual system is able to compensate for the damage; it has spare capacity. Despite this most pwMS who have optic neuritis will know that although their visual acuity, or gross vision, may have recovered they have subtle deficits that we don’t routinely test for. For example, colour vision is often abnormal; colours appear washed out. Contrast sensitivity is abnormal; you may have difficulty distinguishing between shades of grey. Depth perception is all over the place; you need binocular (both eyes) vision for accurate depth perception. If you have poor depth perception you may see things in 3D when they should be in 2D and you may have difficulty judging distances. You may also find that you are hypersensitive to bright lights or lights with certain wavelengths; I find a lot of pwMS become intolerant of fluorescent lights after an attack of optic neuritis. 

The problem with the COVID-19 induced changes in our MS service is that with remote consultations I can do this aspect of the neurological examination. Is it important? Yes, firstly it allows one to determine what neuronal systems have been affected by MS, which is required for diagnosis, i.e. dissemination of disease in space, and secondly for assessing your EDSS or Expanded Disability Status Scale. 

I am telling you all this as we developed a web-EDSS that requires you to know if your neurological examination is normal or abnormal. Having optic disc pallor is one clinical sign that may affect the EDSS. If you can’t get this information from your neurologist you can get a reasonable idea if your optic nerve has been affected by MS by downloading and using one of the many colour vision applications on your smartphone; we recommend using ‘eye handbook’ as it is free. So before completing your webEDSS you will need to know if you have abnormal colour vision in your left or right eye; this could be used as a proxy for optic nerve involvement. Please note if you have congenital colour blindness, which is more common in males, you can’t use this sign as a proxy for optic nerve involvement.

#T4TD = Thought for the Day

CoI: nil in relation to this post

#T4TD Driving at night

Did you know that more than half of people with established multiple sclerosis (pwMS) have had an episode of optic neuritis (an MS lesion in the optic nerve)? Not all episodes of optic neuritis cause symptoms and can be subclinical. We detect the latter using visual evoked potentials that show slowed conduction in a particular optic nerve. 

When optic neuritis recovers the conduction speed of nerve impulses travelling through the recovered nerve rarely gets back to normal and is slower than the conduction speed in the unaffected eye. In addition, the conduction speed can change depending on fatigue or ambient temperature changes. The conduction in the affected, by remyelinated, nerves can even fail and this is can result in intermittent blurring or loss of vision called Uhthoff’s phenomenon. 

The brain is usually quite good at compensating for the signals coming from the eyes at different speeds, but the one thing it is poor at doing is adjusting for depth perception. The reason we have two eyes with overlapping visual fields or binocular vision is for depth perception. This is why pwMS with previous optic neuritis may have poor depth perception, which affects their ability to judge where for example a cup or glass is on the table and as a result of this they often spill drinks. PwMS will also have difficulty playing ball sports such as tennis, table tennis or any sport that requires accurate depth perception. 

One function that seems to very sensitive to this phenomenon is judging distances whilst driving at night; for example, estimating how far you are from the traffic intersection or a stop sign. Many of my patients don’t like driving at night because of this. Why? Well, the brain uses a process called parallax to compensate for the loss of depth perception; i.e. the brain uses the relative size of familiar objects to help make a judgment on how far something is away from you in the distance. By using parallax somebody who is blind in one eye can judge depth and distance relatively well. However, at night in the dark when images are not well illuminated and use of colour vision drops judgement by parallax fails and depth perception deteriorates. 

I would be interested to know if any of you have any of these problems with depth perception and driving at night. If you do the chances are that you have had optic neuritis in the past. 

Unfortunately, there is little we can do to treat this phenomenon, but knowing about and understanding why it occurs may help you compensate for this impairment, disability or handicap. 

Parallax: Have you ever wondered why the setting-sun and full-moon on the horizon look so big compared to it being smaller when it above us in the sky? It is actually not bigger; simply having a reference on the horizon (trees, mountains, buildings, the sea, etc.) makes your brain perceive it as being larger, i.e. closer. This is how parallax affects how the brain works and judges distance from you to an object in the distance. 

#T4TD = Thought for the Day

CoI: nil in relation to this post

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