At this moment, very little is known about the progress and the fastness with which eyesight and hearing will deteriorate. There are large individual differences and even within a family the progression sometimes turns out to be different than in siblings with the same mutation. More research is needed to chart the course in detail and to find an explanation for these individual differences.
However, a number of studies are currently being conducted into the natural course for USH 1 and USH 2.
As was always thought, the loss of hearing of people suffering from Usher Syndrome type 2A appears not to remain stable. The average deterioration is 5 dB per 10 years, but there are major differences. In the doctoral research of Bas Hartel, 110 Dutch and Swedish patients with two mutations on the USH 2A gene are involved.
The detailed mapping out of the natural history of the functioning of eyesight and hearing in the different types of Usher Syndrome is essential in order to be able to determine the effectiveness of future therapy. Only after studies have demonstrated the effectiveness and safety of certain therapy types, these will be made available to patients on a large scale after a long process of market implementation.
The doctoral research of Bas Hartel (2018) studies differences in the loss of field of vision and sharpness of vision between 152 people suffering from Usher Syndrome type 2A and 73 people suffering from a non-syndromic type of retinitis pigmentosa.
This last group of people does have a mutation in the USH 2A gene, but these people are not born hard of hearing. With all people involved in the study at least one mutation in the USH 2A gene.
Here there are clear differences as well:
- people suffering from Usher Syndrome will sooner develop visual problems: in average at the age of about 15 years against 25 years.
- they also are more often blind at a younger age with an eyesight of less than 30% (54 years against 72 years).
However, age-related hearing impairment appears to be more often found in people suffering from non-syndromic retinitis pigmentosa than on average in a normal population.
Further research will be required to find an explanation for these differences.
RESULTS OF THE USH 2A STUDY IN LONDON
The Moorfields Eye Institute in London selected 57 USH 2A patients for a natural development study. This group of patients was equally divided based on the various types of mutations.
This study demonstrated, among other things, that measuring the eyesight is not the best indicator for analysing the effects of a treatment. The eyesight of most Usher Syndrome patients remains quite stable for many years. By means of an OCT scan (Optical Coherence Tomography), which can determine the number of intact light-sensitive photoreceptors, an annual decrease of photoreceptors of 7% has been observed. This decrease is larger with younger patients (below 30 years old) than it is with older patients. This argues in favour of particularly including this group of young patients in natural development studies and clinical trials.
People suffering from Usher Syndrome experience a narrowing down of the field of vision within which sharp observation is possible. It is like looking through a tunnel (= tunnel vision). By means of funduscopy the retina can be made visible and a ring-shaped structure can be observed in the perimeter of the field of vision. This examination also makes accumulation of waste products produced by the degeneration of the retinal pigment epithelium (RPE) visible. The natural development study in London demonstrated that the diameter of the ring-shaped structure in the retina of Usher Syndrome type 2A patients annually decreases by an average of 11%. Therefore this is a good parameter for determining whether a therapeutic treatment has the desired result.