Tinnitus is a condition affecting one’s hearing and manifests as recurrent auditory hallucinations, often heard as a ringing or buzzing sound but sometimes described as humming, grinding, hissing, whistling or even singing or musical. One subtype of tinnitus, pulsatile tinnitus, is experienced as pulsatile and synchronous with the patient’s own pulse.
Tinnitus is a poorly understood phenomenon and is relatively difficult to study owing to its subjective nature. Studies suggest that it can originate from different physiological areas of the ear and neural circuitry; and while it is often combined with hearing loss it is sometimes experienced independently. Tinnitus is sometimes associated with hyperacusis, a hypersensitivity to everyday noises.
Why Tinnitus Happens
Two widely accepted theories regarding the etiology of tinnitus are damage to or loss of auditory hair cells and damage to the auditory nerve which relays sensory information from the ear to the brain. Auditory hair cells are sensors located on the organ of Corti in the cochlea of the inner ear. They are fragile and can be damaged by various factors from infection, trauma and loud noise to aging. Unfortunately for humans and other mammals, these never grow back under normal circumstances.
Both hair cell loss and auditory nerve damage usually result in some degree of hearing loss and evidence generally points to a connection between hearing loss and tinnitus. For example, people with otosclerosis, a correctible condition whereby the joints of the little bones in the middle ear occurs harden, will often experience tinnitus alongside their hearing loss. When they undergo stapedectomy both their hearing loss and the tinnitus often resolve. The link between hearing loss and tinnitus is also evidenced through the experiences of patients receiving cochlear implants. Implant recipients tend to report that their tinnitus resolves when the implant is turned on and that this improvement persists for a while after it has been turned off again. In addition, temporary hearing loss and accompanying tinnitus can be induced after an auditory shock, with both resolving over time (for example, the ‘ringing in the ears’ regularly experienced after a loud rock concert). Some studies suggest that a reduction in the number of hair cells sets up a permanent imbalance in the cochlea and that this may be responsible for the patient’s perception of phantom sounds while other studies suggest that the brain itself compensates for the missing input and that this may be the root cause of tinnitus. Lesions in the hair cells responsible for the more subtle pitches (above 8,000 hertz) could explain why some patients experience tinnitus without any significant hearing impairment.
There may be a further complication in the pathogenesis of certain manifestations of tinnitus with evidence that, over time, tinnitus-inducing neural networks can become independently established in the brain and that these can persist even if the original cause of the tinnitus is resolved (e.g. the Organ of Corti repaired). It is suspected that these neural networks will weaken over time, causing the tinnitus to resolve or become less intense, but testing this theory will only become possible with new reliable interventions.
In short, understanding tinnitus presents significant challenges to researchers and medical professionals and developing an effective treatment is not straightforward.
Possible Roles for Stem Cells and Genetic Engineering in Tinnitus Recovery
Stem cells carry the body’s own regenerative potential while genetic engineering modifies the expression of genes to control the production of various proteins, including those involved in cell formation. Both disciplines have much to contribute to tinnitus research. Various studies are focused on finding ways to stimulate the production of sensory hair cells using gene therapy while others are looking at how to cultivate stem cells and then introduce them into areas of damage via transplantation, either directly into the epithelium of the Organ of Corti (to form sensory hair cells), or into the auditory nerve itself, where this has been damaged, to form nerve tissue. There is much research still to be done in both of these areas but a lot of progress has been made. For example, in 2015 researchers from Rockefeller University identified two genes which seem to be responsible for the development of sensory hair cells in mice. Part of the experiment involved turning on these genes in mature mice which led to the growth of new hair cells. The researchers are now investigating which chemical processes are involved in the usual expression of these genes. That same year, in Kyoto, Japan, scientists identified a new method of auditory nerve stem cell transplantation which appeared to aid the movement of stem cells across the glial scar (this is naturally formed as a protective response to CNS damage).
The Hearing Restoration Project, supported by the long-established Hearing Health Foundation, has also brought together a consortium of 14 of the world’s leading hearing research scientists from the US, Canada and the UK, including experts in regenerative medicine, in an attempt to accelerate progress in this area.
The Benefits and Limitations of Animal Models
As with all drug trials (non-autologous stem cells are treated as drugs by the FDA), no human trials can take place until successful interventions have been proven in animals. However, the subjective nature of tinnitus presents many challenges to researchers who first need to design experiments able to identify the experience of tinnitus through the physiological and behavioral responses of animals. Some of the behaviors that have been associated with tinnitus in mice are indicative of anxiety and depression and include avoidant behaviors and attempts to locate the source of a sound while physiological signs include tremors, spasms in the respiratory and digestive systems, irregular heart rate, changes to sleep patterns (insomnia) and changes in blood pressure. Through monitoring these signs and manipulating behaviors (e.g. through fear conditioning) it has been possible for researchers to discriminate between when a mouse is experiencing silence and when they are experiencing auditory hallucination (i.e. tinnitus).
Studying birds and fish could prove another fruitful line of enquiry since, unlike mammals, these groups of animals are able to naturally regenerate the hair cells in their hearing organs.
Existing Stem Cell Treatments for Tinnitus
Due to the lengthy clinical trials process and the specific nature of the physiological and neurological mechanisms responsible for tinnitus, direct stem cell interventions are not yet available. Some patients claim to have experienced an improvement in their tinnitus symptoms when receiving injections of autologous stem cells. While it is not impossible that the overall systemic improvement that often results from autologous stem cell treatment might positively impact the patient’s experience of their tinnitus symptoms, such anecdotal evidence should be regarded with caution, particularly since many tinnitus sufferers experience variability in their symptoms anyway.
Nevertheless, autologous stem cell treatments / tinnitus stem cell therapy, whereby a patient’s own stem cells are harvested, cultured and then redeployed, are becoming routinely used in experimental studies here in the United States. These are perfectly safe procedures when administered by a competent medical team and there is a growing body of evidence to support their efficacy in symptom reduction and tissue regeneration across a wide range of health conditions.
While tinnitus sufferers should understand that researching and developing targeted stem cell treatments is a very time-consuming process, they can at least rest assured that their condition is not being ignored and that the field is moving in the right direction.
The Mississippi Stem Cell Treatment Center is actively treating patients living with a wide range of conditions including orthopedic, neurological and autoimmune disorders. To find out more about what we do, call us on (866) 885 4823. More information and latest updates can be found on our website at: http://www.gulfcoaststemcell.com