Reply: Both mitochondrial DNA and mitonuclear gene mutations cause hearing loss through cochlear dysfunction

Beyond the cochlea: exploring the multifaceted nature of hearing loss in primary mitochondrial diseases

Primary mitochondrial diseases, with diverse systemic manifestations, often present with auditory impairments due to mitochondrial dysfunction. This study provides an in-depth exploration of auditory deficits in primary mitochondrial diseases, highlighting the impact of various pathogenic variants on both cochlea and neural/central auditory functions. An observational study involving 72 adults with primary mitochondrial diseases was conducted. Participants underwent extensive audiological evaluations including pure-tone audiometry, tympanometry, acoustic reflex thresholds, quick speech-in-noise test, listening in spatialized noise-sentences test, auditory-evoked brainstem responses and distortion product otoacoustic emissions. Multivariate analysis of covariance and logistic regression analyses assessed the influence of various pathogenic DNA variants, accounting for age, cognitive status via the Montreal Cognitive Assessment and disease severity through the Newcastle Mitochondrial Disease Adult Scale. Participants with the pathogenic m.3243A>G/T variants (m.3243A>G n = 40; m.3243A>T n = 1) exhibited significant elevations in pure-tone audiometry thresholds, especially at high frequencies, suggesting cochlea involvement. Notably, the listening in spatialized noise-sentences test showed significant spatial processing deficits in the m.3243A>G/T group, possibly indicating a unique mutation-specific impact on central auditory processing. Auditory-evoked brainstem response results highlighted a higher likelihood of auditory brainstem response abnormalities in this group, further substantiating neural/central auditory pathway involvement. This study emphasizes the heterogeneous nature of hearing impairment in primary mitochondrial diseases, with a genotype-phenotype correlation, particularly in the m.3243A>G/T group. These insights advocate for personalized, genotype-specific auditory assessments and targeted management strategies. Conventional hearing aids and cochlear implants are ineffective for those with central auditory dysfunctions related to mitochondrial mutations. There is an urgent need for innovative rehabilitation strategies catering for both cochlear and neural/central auditory pathways.

The Importance of Mitochondrial Disease Testing in Young Adults With New Onset Sensorineural Hearing Loss

OBJECTIVES

Sensorineural hearing loss (SNHL) occurs commonly as part of mitochondriopathies and varies in severity and onset. In this study, we characterized hearing with specific consideration for hearing loss as a potential early indicator of mitochondrial disease (MD). We hypothesize that genetic testing at the earliest detection of SNHL may lead to an earlier MD diagnosis.

DESIGN

We reviewed the clinical and audiometric data of 49 patients undergoing genetic testing for MD.

RESULTS

One-third of individuals with molecularly confirmed MD presented with SNHL. On average, patients had hearing loss at least 10 years before genetic testing. The collective audiometric profile includes mild to moderate SNHL at lower frequencies and moderate SNHL at 2 kHz and higher frequencies.

CONCLUSIONS

This study suggests that screening for SNHL could be an early indicator of MD. We propose that the audiometric profile for those with a MD diagnosis may have clinical triage utility.

Hearing Dysfunction in a Large Family Affected by Dominant Optic Atrophy (OPA8-Related DOA): A Human Model of Hidden Auditory Neuropathy

Hidden auditory neuropathy is characterized by reduced performances in challenging auditory tasks with the preservation of hearing thresholds, resulting from diffuse loss of cochlear inner hair cell (IHC) synapses following primary degeneration of unmyelinated terminals of auditory fibers. We report the audiological and electrophysiological findings collected from 10 members (4 males, 6 females) of a large Italian family affected by dominant optic atrophy, associated with the OPA8 locus, who complained of difficulties in understanding speech in the presence of noise. The patients were pooled into two groups, one consisting of 4 young adults (19-50 years) with normal hearing thresholds, and the other made up of 6 patients of an older age (55-72 years) showing mild hearing loss. Speech perception scores were normal in the first group and decreased in the second. Otoacoustic emissions (OAEs) and cochlear microphonics (CMs) recordings were consistent with preservation of outer hair cell (OHC) function in all patients, whereas auditory brainstem responses (ABRs) showed attenuated amplitudes in the first group and severe abnormalities in the second. Middle ear acoustic reflexes had delayed peak latencies in all patients in comparison with normally hearing individuals. Transtympanic electrocochleography (ECochG) recordings in response to 0.1 ms clicks at intensities from 120 to 60 dB peak equivalent SPL showed a reduction in amplitude of both summating potential (SP) and compound action potential (CAP) together with delayed CAP peak latencies and prolonged CAP duration in all patients in comparison with a control group of 20 normally hearing individuals. These findings indicate that underlying the hearing impairment in OPA8 patients is hidden AN resulting from diffuse loss of IHCs synapses. At an early stage the functional alterations only consist of abnormalities of ABR and ECochG potentials with increased latencies of acoustic reflexes, whereas reduction in speech perception scores become apparent with progression of the disease. Central mechanisms increasing the cortical gain are likely to compensate for the reduction of cochlear input.

MicroRNAs are appropriate in mitochondrial related hearing loss? Answer to the skepticism

AIM

We aim to clarify some points that have been criticized about our previous paper "Hearing Impairment in MELAS: new prospective in clinical use of microRNA, a systematic review".

MATERIAL AND METHOD

We answered to the criticism of Dr. Finsterer point by point, by citing all literature in support of our previous paper.

CONCLUSION

The point by point answering allows us to clarify doubts and to support the proposal that we exposed in our review, the possibility to use microRNA for detecting the hearing damage in patients affected from mitochondrial disease.

Hearing impairment in MELAS: new prospective in clinical use of microRNA, a systematic review

AIM

To evaluate the feasibility of microRNAs (miR) in clinical use to fill in the gap of current methodology commonly used to test hearing impairment in MELAS patients.

MATERIAL AND METHOD

A literature review was performed using the following keywords, i.e., MELAS, Hearing Loss, Hearing Impairment, Temporal Bone, Otoacustic Emission (OTOAE), Auditory Brain Response (ABR), and microRNA. We reviewed the literature and focused on the aspect of the temporal bone, the results of electrophysiological tests in human clinical studies, and the use of miR for detecting lesions in the cochlea in patients with MELAS.

RESULTS

In patients with MELAS, Spiral Ganglions (SG), stria vascularis (SV), and hair cells are damaged, and these damages affect in different ways various structures of the temporal bone. The function of these cells is typically investigated using OTOAE and ABR, but in patients with MELAS these tests provide inconsistent results, since OTOAE response is absent and ABR is normal. The normal ABR responses are unexpected given the SG loss in the temporal bone. Recent studies in humans and animals have shown that miRs, and in particular miRs 34a, 29b, 76, 96, and 431, can detect damage in the cells of the cochlea with high sensitivity. Studies that focus on the temporal bone aspects have reported that miRs increase is correlated with the death of specific cells of the inner ear. MiR - 9/9* was identified as a biomarker of human brain damage, miRs levels increase might be related to damage in the central auditory pathways and these increased levels could identify the damage with higher sensitivity and several months before than electrophysiological testing.

CONCLUSION

We suggest that due to their accuracy and sensitivity, miRs might help monitor the progression of SNHL in patients with MELAS.

Patient care standards for primary mitochondrial disease: a consensus statement from the Mitochondrial Medicine Society

The purpose of this statement is to provide consensus-based recommendations for optimal management and care for patients with primary mitochondrial disease. This statement is intended for physicians who are engaged in the diagnosis and management of these patients. Working group members were appointed by the Mitochondrial Medicine Society. The panel included members with several different areas of expertise. The panel members utilized surveys and the Delphi method to reach consensus. We anticipate that this statement will need to be updated as the field continues to evolve. Consensus-based recommendations are provided for the routine care and management of patients with primary genetic mitochondrial disease.