The effects of deferoxamine mesylate and hypoxia on the cochlea

Early Enzyme Replacement Therapy Improves Hearing and Immune Defects in Adenosine Deaminase Deficient-Mice

Background: Inherited defects in adenosine deaminase (ADA) cause severe immune deficiency, which can be corrected by ADA enzyme replacement therapy (ERT). Additionally, ADA-deficient patients suffer from hearing impairment. We hypothesized that ADA-deficient (-/-) mice also exhibit hearing abnormalities and that ERT from an early age will improve the hearing and immune defects in these mice. Methods: Auditory brainstem evoked responses, organ weights, thymocytes numbers, and subpopulations, lymphocytes in peripheral blood as well as T lymphocytes in spleen were analyzed in ADA-/- and ADA-proficient littermate post-partum (pp). The cochlea was visualized by scanning electron microscopy (SEM). The effects of polyethylene glycol conjugated ADA (PEG-ADA) ERT or 40% oxygen initiated at 7 days pp on the hearing and immune abnormalities were assessed. Results: Markedly abnormal hearing thresholds responses were found in ADA-/- mice at low and medium tone frequencies. SEM demonstrated extensive damage to the cochlear hair cells of ADA-/- mice, which were splayed, short or missing, correlating with the hearing deficits. The hearing defects were not reversed when hypoxia in ADA-/- mice was corrected. Progressive immune abnormalities were detected in ADA-/- mice from 4 days pp, initially affecting the thymus followed by peripheral lymphocytes and T cells in the spleen. ERT initiated at 7 days pp significantly improved the hearing of ADA-/- mice as well as the number of thymocytes and T lymphocytes, although not all normalized. Conclusions: ADA deficiency is associated with hearing deficits and damage to cochlear hair cells. Early initiation of ERT improves the hearing and immune abnormalities.

Bilateral auditory ossicular expansions in a child with beta-thalassemia major: Case report and literature review

Marrow proliferation of the ossicular chain is a rare phenomenon. To date, only two other cases have described this rarity. We report a third paediatric case from Australia. A seven-year-old with thalassemia major demonstrated conductive impairment during surveillance for Deferasirox ototoxicity. Otitis media was assumed, however, CT scan of the petrous temporal bone revealed extramedullary haematopoiesis causing bilateral ossicular expansions and fixed conductive deficit. Reports of hearing loss in the thalassemia population focus on sensorineural impairment from iron chelation therapies. Clinicians should suspect ossicular deformation where treatment has been delayed, poorly controlled or conductive deficit persists without effusion.

Auditory neuropathy spectrum disorder: a review

PURPOSE

Auditory neuropathy spectrum disorder, or ANSD, can be a confusing diagnosis to physicians, clinicians, those diagnosed, and parents of children diagnosed with the condition. The purpose of this review is to provide the reader with an understanding of the disorder, the limitations in current tools to determine site(s) of lesion, and management techniques.

METHOD

This article is a review of what is known about ANSD. It includes descriptions of assessment tools, causes of ANSD, and patient management techniques.

CONCLUSIONS

This review is a guide to audiologists, speech-language pathologists, and early interventionists who work with individuals diagnosed with ANSD and/or their families. It highlights the need for more precise tools to describe the disorder in order to facilitate decisions about interventions and lead to better predictions of outcome.

Desferioximine induced Ototoxicity in Thalassemic patients

BACKGROUND

Thalassemia is a hereditary disorder requiring regular blood transfusion.

PURPOSE

To determine hearing sensitivity in transfusion-dependent thalassemics who were receiving desferioximine and on oral iron chelation (desferioximine) therapy.

METHODS

26 patients with B-thelassemia in the age range of 5-22 years were enrolled in the thalassemia transfusion unit in the Advanced Pediatric Center of Postgraduate Institute of Medical Education and Research (PGIMER), Chandigarh, India. The patients to be enrolled in this study were randomly selected.

RESULTS

It was observed that the hearing deteriorated in patients who were given injected desferioximine whereas it fell within normal limits in case of those who were given oral iron chelation therapy.

CONCLUSON

This study confirms the necessity of regular audiological monitoring during the course of the disease.

Hearing screening outcomes in infants of pregestational diabetic mothers

PURPOSE

Hearing screening results for newborns of diabetic mothers were compared with those of nondiabetic controls.

METHOD

This study was a retrospective chart review of mothers with pregestational diabetes mellitus and their neonates (n=73) who received newborn hearing screening between January 1, 2000, and May 1, 2002. A group of nondiabetic mothers and their infants (n=73), with birth dates that matched the diabetic group, served as controls. A 2-tiered hearing screening protocol, employing distortion product otoacoustic emission (DPOAE) and automated auditory brainstem response (A-ABR) screening techniques, was used.

RESULTS

The DPOAE screening failure rate was 5.5% (4/73) for babies in the nondiabetic control group and 11.0% (8/73) for infants of diabetic mothers; this difference was not statistically significant. The A-ABR failure rate was 9.1% (1/11) for the diabetic group compared with 0% (0/4) for the controls, but the A-ABR was measured for only a small number of participants in each group. The frequency of premature birth and abnormal birth weight was significantly greater for the infants of diabetic mothers compared with controls.

CONCLUSIONS

Given the greater frequency of prematurity and abnormal birth weight in the population of neonates born to diabetics, additional research using A-ABR is recommended.

An animal model of auditory neuropathy

OBJECTIVE

We describe an animal model of auditory neuropathy in which subjects have extensive, scattered inner haircell loss but with a relatively intact outer haircell population.

DESIGN

Such a pattern of cochlear haircell damage can be produced in the chinchilla by treatment with the anticancer agent carboplatin.

RESULTS

In these subjects, otoacoustic emissions (OAEs) and cochlear microphonics remain normal while auditory brain stem evoked potential (ABR) thresholds are significantly elevated. However, in the same subjects, central auditory neurons (in the inferior colliculus) have response thresholds that are considerably lower (by up to 50 dB) than ABR thresholds. These findings parallel the characteristics of auditory neuropathy in humans, in which absent or abnormal ABRs are recorded in patients with only mild to moderate audiometric thresholds and preserved OAEs.

CONCLUSIONS

We suggest that scattered inner haircell lesions also can result from long-term cochlear hypoxia, and we propose that this is a likely candidate for the etiology of many types of auditory neuropathy in human subjects.

Attenuation of neomycin ototoxicity by iron chelation

Increasing evidence suggests that aminoglycoside ototoxicity is mediated by the formation of an aminoglycoside-iron complex and that the creation of this complex is a preliminary step in generation of free radical species and subsequent hair cell death. In this study we have assessed the ability of the iron chelator deferoxamine to attenuate the hearing loss induced by an ototoxic dose of the aminoglycoside neomycin (100 mg/kg per day for 14 days). Experiments were carried out on pigmented guinea pigs weighing 250 to 300 g. Changes in auditory sensitivity were characterized by monitoring shifts in compound action potential (CAP) thresholds, recorded through indwelling electrodes implanted at the round window, vertex, and contralateral mastoid. Results show that animals receiving neomycin alone suffered a mean threshold shift exceeding 35 dB at all test frequencies (2.0, 4.0, and 8.0 kHz) 30 days after initiation of treatment. In comparison, all animals receiving cotherapy of neomycin and deferoxamine (150 mg/kg twice daily for 14 days) maintained their CAP threshold, suggesting significant protection from neomycin ototoxicity. A statistical comparison of treatment groups showed that in the animals receiving cotherapy with neomycin and deferoxamine, deferoxamine produced a significant protective effect against neomycin-induced ototoxicity (P < 0.001). These results provide further evidence of the intrinsic role of iron in aminoglycoside ototoxicity and suggest that deferoxamine may have a therapeutic role in attenuating the cytotoxic action of aminoglycoside antibiotics.

Audiologic and impedancemetric findings within thalassaemic patients

The objective of this study is to investigate hearing losses in thalassaemic patients. This study was conducted on 34 thalassemic patients of which 27 of them had thalassaemia major and the remaining 7 had thalassaemia intermedia. Six (11.11%) of the 54 ears with thalassaemia major were found to have normal hearing. Thirty-two ears (59.26%) had conductive hearing loss, 8 ears (14.81%) had mixed type of hearing loss. Moreover, in this group, there was no ear which had pure sensorineural hearing loss. Of the majority of patients having thalassaemia major and thalassaemia intermedia, an air bone gap was found even though there was no negative pressure in the middle ear. In these patients a high degree of static compliance and normal shaped, stiff amplitude, normal pressure tympanograms were observed. Moreover, in most of these patients acoustic reflexes were not obtained. These findings are rather interesting since they show the pathological changes which may cause stiffness in the middle ear sound transmission system.

Assessment of ultrastructure in isolated cochlear hair cells using a procedure for rapid freezing before freeze-fracture and deep-etching

Separated cochlear outer hair cells and isolated strips of organ of Corti containing hair cells and supporting cells have been rapidly frozen before freeze-fracture and deep-etching by immersion of samples sandwiched between two copper plates into liquid nitrogen-cooled propane: isopentane. Assessment of this procedure has shown that no significant freezing damage occurs. The ultrastructure of the hair cells revealed by freeze-fracture of these non-chemically fixed preparations was generally very similar to that seen in fixed material. This indicates that the processing of cochlear tissue normally used for electron microscopy produces few obvious structural artefacts. It also demonstrated that procedures for isolating cochlear hair cells generally do not affect cell structure significantly. However, some isolated hair cells did show abnormalities within the membranes of the lateral cisternae. Such membrane alterations, which would not be identified by light microscopy, occurred to a variable extent but were more commonly present after prolonged periods in maintenance medium. Deep-etching of the preparations to examine extracellular features around stereocilia revealed clearly lateral cross-links between stereocilia. However, tip-links could not be positively identified in either unfixed or prefixed preparations.

Cytoarchitecture of cochlear nucleus in the chinchilla

The morphology of the cochlear nucleus in the normal, adult chinchilla, as demonstrated by Nissl staining, was examined. The cytoarchitecture was determined from sections viewed at the light microscope level. The chinchilla cochlear nucleus was found to possess most of the features reported in other mammalian cochlear nuclei. It could easily be divided into dorsal and ventral components due to an intervening layer of granule cells, and most cell types previously reported in mammals were also found in the chinchilla cochlear nucleus. A distinct distribution pattern of cell types exists within each part.

The importance of early fixation in preservation of human cochlear and vestibular sensory hair bundles

Human labyrinths were fixed with glutaraldehyde as soon as possible and certainly within 15 min of clinically confirmed death. By following this protocol, we have obtained excellent preservation of human material comparable to the best that we were able to obtain with animal tissues. Structural features that are indicative of good fixation in animal hair bundles, namely well-defined lateral cross-links, upward pointing tip links and the 'granular' appearance of the surface membrane of stereocilia are confirmed for human hair cells. Our results show that excellent preservation of human hair bundles is critically dependent upon early fixation.

Desferrioxamine ototoxicity: evaluation of risk factors in thalassaemic patients and guidelines for safe dosage

Forty-seven patients with thalassaemia have been studied to define risk factors for development of sensorineural hearing loss, and to establish guidelines for safe chelation. Sensorineural hearing loss was only present in patients who had previously received desferrioxamine (DFO). The two most significant risk factors were the maximum dose of DFO previously received (P less than 0.01), and a serum ferritin of less than 2000 micrograms/l at that time (P less than 0.001). A therapeutic index obtained from the ratio of the mean daily dose of DFO mg/kg divided by the serum ferritin identifies patients with a ratio of greater than 0.025 as at risk of sensorineural hearing loss (P less than 0.001) and can be used as a guideline for safe DFO dosage. Follow-up audiometry of the affected patients over a 2-year period indicated that adjustment of the dose to a therapeutic index of less than 0.025 resulted in the stabilization of hearing loss in seven patients and improvement in two.

Post-mortem changes in hair bundles of the guinea pig and human cochlea studied by high-resolution scanning microscopy

High-resolution scanning electron microscopic studies have been made on the guinea pig cochlea on material fixed from 15 min to 4 h post-mortem. Changes in the surface texture and cross-links of stereocilia were detected after only 15 min. Such changes included an increase in granularity of the surface membrane, thickening, stretching and fracturing of all types of cross-links accompanied by splaying apart and loss of rigidity of stereocilia. These changes were more pronounced in the basal turns of the cochlea and in general increased in severity and spread more apically with increasing times post-mortem. By 4 h, many hair bundles consisted of a fused amorphous mass in which individual stereocilia were not discernible. Remarkably at this time, some hair bundles appeared to have suffered little damage. These results will facilitate better discrimination between effects solely due to post-mortem necrotic changes and those due to specific actions of ototoxic drugs and other insults.