A conversation with a retina specialist

Since the time that I penned my last post, I have had the opportunity to speak with a retina specialist on the subject of vision restoration. In today’s post I would like to share with you, his view on this subject. Please note that this is not a transcript of the conversation. Scattered here and there, are words and phrases in italics, and these are simply my thoughts and not part of the discussion.

First question: Would it be right to say that we would have to recreate the entire visual pathway - retina, optic nerve, visual parts of the brain - in order to restore vision in ND?

The answer: No. The eye undergoes histological changes and dysplasia. We cannot hope to restore vision in this manner.

Histology is the study of the microscopic anatomy of cells and tissues of plants and animals.
Dysplasia is an ambiguous term used in pathology to refer to an abnormality of development.

[Source: Wikipedia]

In his opinion, the best (and only) strategy for a person affected by Norrie disease, would be to opt for non-visual development of visual ability. 

What? 

He went on to explain that non-visual senses could be used to provide an ability similar to sight. Sensory substitution, in other words. He referred to a device which has been developed to provide visual information to the brain via electrodes placed on the tongue. He suggested I look it up on the internet, which I did, and it seems that such a device has even been approved by the FDA last year. 

I think we all know the drill by now – mount a camera on a pair of glasses, transmit the image (by a wire or wirelessly) to a processer which converts it to electrical pulses, then use these pulses to stimulate some part of the body or brain thereby giving the person some sense of what he/she is ‘seeing’. Here are a couple of links which provide all the details:

Device That Helps Blind People See With Their Tongues Just Won FDA Approval

Loser: Tongue Vision

The latter article is a critic’s point of view with the subheading “A fuzzy outlook for an unpalatable technology”!

This is really useless, I’m thinking. Is this the best that researchers can come up with? Because if it is, then I’m all for packing up my bags. 

Oh well, I’ll let you decide for yourself. 

So this ought to have been the end of my discussion, but since I had my list of questions before me, I decided to go ahead and ask them anyway.

So, I know that a retinal detachment implies separation of the neural retina from the RPE (retinal pigment epithelium) and I wondered if after several years of detachment - as in the case of ND – would the RPE be dead or alive? Alive. In other words, if one had to go down the retina transplant route – which route the specialist advised me not to go down – then it is the neural retina which would need to be transplanted and not the RPE and not the choroid.

The neural retina consists of several layers of cells including the photoreceptors and the ganglion cells. It sits on top of a single cell layer called the RPE, whose function is to nourish the neural retina cells amongst other things. The RPE, in turn, is firmly attached to the choroid, which is a vascular structure filled with blood vessels which bring nutrients and oxygen to the eye. So the sequence is as follows:

Cornea – Lens – Vitreous – Neural Retina – RPE – Choroid

On to the next question.

From what I have read on the subject these past few weeks, I believe that the biggest hurdle to a retina transplant is that the connection of the retina to the brain (the optic nerve in other words) is not going to happen on its own. Because the optic nerve is nothing but the axons of the retinal ganglion cells (RGCs) which extend all the way back to the brain, and we know that once there is damage to the optic nerve, as in the case of glaucoma, the nerve does not regenerate on its own. So something would need to be done, in order to persuade these axons to regrow. Not only that, they would have to make the right connections in the brain.

I also know that the NEI (National Eye Institute) has launched an initiative titled Audacious Goals Initiative (AGI). As part of this initiative, the NEI has funded six projects (a total of $12.4 million) to identify biological factors which influence optic nerve regeneration.
[Aside: MaryAnn, should you be reading this: remember the press release from the NEI you had sent me? This is it.]

My question to the specialist: Shouldn’t this, at some point of time, enable researchers to experiment with retina transplants in an animal model at least?

His answer: No. What researchers are exploring is the possibility of regenerating the optic nerve when the other components of the visual system – the eye and the brain – are intact. Even if they were to succeed, it does not imply success in a case like ND where the retina is detached and consequently the visual circuitry of the brain too is impacted. The two situations are not comparable as it were.

On the subject of a cortical implant, his view was that it won’t work for those who are blind from birth, and this is something I have read elsewhere too. Not sure what is the reasoning behind this.

End of story. Almost. I think I will try and speak to a few more people, in case they have a different perspective to offer. I need to finish what I have started.

Till next time then,

Meenu.

To see or not to see (Part II)

In my previous blog post I had written about what – in my opinion – could be done to restore vision for a person with Norrie disease. 

However, even if we had the perfect method to correct the optics in the eye,  functional vision would still be determined by the retina-brain interaction. This is because vision involves perception (which in turn involves the brain), and not just an optically perfect image. Unfortunately, normal development of the visual cortex (the part of the brain responsible for seeing) requires visual inputs to have been present during the early years of development. Scientists refer to this early period as the ‘critical period’ for vision.

How do we know all this? 

Well, from experiments conducted by dark rearing animals. The following links provide examples of some such experiments:

Visual acuity and visual responsiveness in dark-reared monkeys (Macaca nemestrina) [Regal et al., 1976]

Development of visual acuity in infant monkeys (Macaca nemestrina) during the early postnatal weeks [Teller et al., 1978]

Long-term effects of dark rearing on a visually guided reaching movement in cats [Fabre-Thorpe M et al., 1990]

Functional postnatal development of the rat primary visual cortex and the role of visual experience: Dark rearing and monocular deprivation [Fagiolini et al., 1994]

The role of visual experience in the development of columns in cat visual cortex [Crair et al., 1998]


We also see the effects of early sensory deprivation on vision in the case of amblyopia or ‘lazy eye’. In amblyopia, one eye fails to achieve normal visual acuity even with prescription glasses. The reason for this is reduced visual inputs to the affected eye during infancy or early childhood. This can happen for many reasons, cataracts for example. In the absence of visual inputs during the critical period, the visual circuitry of the deprived eye is permanently compromised. The result is normal vision for one eye and blurred vision for the other.

So would this compromised vision be better than no vision at all? I honestly do not know the answer to this. But I find myself veering towards ‘yes’.

Why?

Firstly, because even light perception has some benefit. Secondly, because there may come a time when scientists are able to rewire the brain to its childlike state, thus enabling visual circuits to develop as they normally would in childhood.

Oh wow, what’s this all about? 

Well, if you want details, I refer you to an article titled The Power of the Infant Brain. The article is authored by Prof. Takao Hensch of the Center for Brain Science at Harvard University and it was published in the journal Scientific American. Here are the salient points of the article:

• The child brain develops vision and other abilities during “critical periods,” when the brain is primed to undergo lasting change in response to sensory and social stimuli.
• Critical periods open at defined times during the course of childhood and adolescence to allow the molding and shaping of neural connections - a property known as brain plasticity.
• Growing understanding of the molecules that both start and stop critical periods has let scientists gain a measure of control over their timing, restoring plasticity even in adulthood.
• Regulating the biology of early development may one day allow drugs or medical procedures to restart critical periods later in life to correct early developmental problems.

So this is as far as my thinking goes on the subject of vision restoration. As I’m not an expert of any kind, I would like to speak with those who are (ophthalmologists, retina specialists…..), in order to have some clarity on this subject.

If you would like to suggest someone with whom I could speak about what can be done for restoring vision in the case of ND, do get in touch. You can either drop me an email or write a comment at the end of the post.

Till next time then,

Meenu.