Audiological and radiological study of eight polish patients with alpha-mannosidosis

Alpha-mannosidase catalyze lysosomal cleaving of mannose residues from glycoproteins. The enzyme is encoded by the MAN2B1 gene. Biallelic pathogenic variants cause enzymatic deficiency, which clinically results in alpha-mannosidosis (AM), an autosomal recessively inherited condition. Typical features observed in AM patients include intellectual disability, loss of speech, dysmorphic features, progressive motor problems, ataxia, hearing impairment and recurrent otitis. The cause of the latter is mainly attributed to immunodeficiency. The aim of our study was to demonstrate the otolaryngologic and hearing outcomes in patients with AM. The study group consisted of 8 AM patients: 6 males and 2 females, aged 2.5-37 yrs. The clinical course, dysmorphic ENT features, hearing status and the HRCT scans of the temporal bones were analyzed. MS Excel for Windows and Statistica software package were used for the comparison of interaural audiometric loss, mean hearing loss and mean hearing threshold for each patient's audiometric frequency tested. We identified ENT dysmorphic features in all of our AM patients, while the hearing loss was detected in 6 out of our 8 patients. For those cases, the onset of deafness was noted in the first decade of life, this impairment was sensorineural, of cochlear origin, bilateral, of a moderate degree (mean loss 62.76 dB; median 60 dB, standard deviation 12.5 dB), symmetrical and stable. The shape of the audiometric curves of our patients can be described as slightly sloping towards the higher tested frequencies, with a marked improvement at 4 kHz. The radiological examination revealed normal structures of the ears, with the exception of one case where a persistent otitis generated a cochlear gap. We therefore concluded that the hearing loss in our AM patients derived from cochlear impairment unrelated with recurrent otitis.