There aren’t many of us who won’t have read the heart-warming story about Joanne Milne, an Usher syndrome sufferer who, after being profoundly deaf her whole life, was able to hear for the first time in nearly 40 years thanks to a cochlear implant.
The condition which caused her deafblindness, as Ms Milne spent half her life with deteriorating sight, is a rare autosomal recessive disorder called Usher syndrome which harbours a 1 in 4 chance of the defective genes being passed on from unaffected “carrier” parents.
Usher syndrome constitutes the major cause of deafblindness, with hearing being detrimentally affected by sensori-neural deafness and blindness commonly being caused by the degenerative eye disease retinitis pigmentosa (RP). The first of these afflictions is as a result of the disruption and dysfunction of stereocilia (hair cells) within the inner ear which houses the cochlea. These cells are critically important in the maintenance of hearing and balance, both of which deteriorate in Usher syndrome. The vestibularcochlear nerve, which transmits information from the ear to the brain regarding sound and balance, is also affected, which causes alterations in spatial awareness. On the other hand, RP is caused by changes in the light sensing photoreceptors within the retina in the eye which eventually gives rise to the lack of night vision, and subsequent lack of tunnel vision, seen in Usher syndrome sufferers.
Interestingly, both stereocilia and photoreceptors have the ability to transmit neurological signals via specialised synapses called ribbon synapses. These connections allow for the generation and amplification of signals from the cells which initially receive the sensory input.
Three types of Usher syndrome exist – type 1, 2 and 3 – which are categorised due to their differences in symptomatic onset and duration, amongst other factors.
- Most common form
- Causes profound deafness from birth
- Balance commonly affected from birth
- Vision deficits caused by RP occur before teenage years
- Night vision affected before peripheral
- Gene affected: USH1G (one of many) stereocilia and photoreceptor development
- Second most common form
- Causes varying degrees of deafness from birth but rarely profound
- Vision deteriorates later in life
- Balance usually normal
- Gene affected: USH2A, encodes usherin protein involved with tissue maintenance of inner ear and retinal tissue
- Hearing usually normal at birth but may progressively deteriorate
- Vision difficulties vary from case to case but is often affected in teens
- Balance sometimes affected
- Gene affected: CLRN1, encodes clarin 1 a protein which may be important in synaptic function
It is thought that the many genes which are implicated in Usher syndrome encode for a wide variety of proteins, ranging from actin-binding proteins, adhesion molecules and calcium binding proteins found in both stereocilia and photoreceptors.
With disorganised retinal cell layers and cochlear hair bundles, it is not surprising that these sensory tissues lose some of their function. Despite there being no cure for Usher syndrome, there is hope; as was the case with Joanne Milne.
Cochlear implants can prove greatly beneficial for those suffering from Usher syndrome. Rather than making sound louder like hearing aids, they directly stimulate the auditory nerve through signal transduction. The implants are inserted just above and behind the ear beneath the skin and consist of external and internal parts.
The external part houses a microphone, where speech and sound are recognised; a speech processor, which converts sound waved into an electrical signal; and a transmitter coil, which receives the electrical signal and sends it via radio waves to the internal portion. Once the receiver has received the signal, it is passed down a series of electrodes to the cochlea where the auditory nerve is stimulated and sound is perceived.
Here’s the moment when Joanne’s hearing was restored for the first time: http://www.bbc.co.uk/news/uk-england-tyne-26779079
Want to know what it might sound like hearing through a cochlear implant? Check this out… http://www.actiononhearingloss.org.uk/your-hearing/about-deafness-and-hearing-loss/cochlear-implants/sound-through-a-cochlear-implant.aspx
And finally, an amazing video from an amazing woman- Molly Watt explains what it’s like living with Usher Syndrome https://www.youtube.com/watch?v=YMQK1uKghcg&desktop_uri=%2Fwatch%3Fv%3DYMQK1uKghcg&app=desktop
Thanks for reading!
Cosgrove, D., & Zallocchi, M. (2014). Usher protein functions in hair cells and photoreceptors The International Journal of Biochemistry & Cell Biology, 46, 80-89 DOI: 10.1016/j.biocel.2013.11.001
Loundon, N., Marlin, S., Busquet, D., Denoyelle, F., Roger, G., Renaud, F., & Garabedian, E. (2003). Usher Syndrome and Cochlear Implantation Otology & Neurotology, 24 (2), 216-221 DOI: 10.1097/00129492-200303000-00015
Petit C (2001). Usher syndrome: from genetics to pathogenesis Annu Rev Genomics Hum Genet