Prevalence of rapidly progressive hearing loss

The exact prevalence of patients presenting with rapidly progressive hearing loss is unknown. The limited search result reflects the uncommon nature of the presentation (Figure 3). Kishimoto et al. retrospectively reviewed cases of bilateral progressive SNHL over a five-year period, and found that 1.32 per cent of cases presented with rapidly progressive hearing loss (12 out of 908).^{Reference Kishimoto, Yamazaki, Naito, Shinohara, Fujiwara and Kikuchi3}

Patient presentation

Rapidly progressive hearing loss can present as either unilateral or bilateral hearing loss. Both sequential and simultaneous onsets of bilateral rapidly progressive hearing loss have been reported in the literature. All reported cases of rapidly progressive hearing loss were sensorineural in nature (Table 2).^{Reference Kishimoto, Yamazaki, Naito, Shinohara, Fujiwara and Kikuchi3,Reference Ishikawa, Kawamata, Kawashima, Yamaguchi, Kubo and Hori5–Reference Santucci, Garber, Ivory, Kuhn, Stephen and Aizenberg26}

Rapidly progressive hearing loss can present as an isolated symptom or as part of a constellation of symptoms, either as an initial presenting feature, or preceded by other manifestations of systemic disease (Table 2). When rapidly progressive hearing loss is the first presenting symptom of systemic disease, often further symptoms subsequently develop within weeks (Table 2).

Causes of rapidly progressive hearing loss

The underlying causes identified for rapidly progressive hearing loss are outlined in Table 2. They can be broadly categorised into: iatrogenic (medication-induced), paraneoplastic, inflammatory or autoimmune, infective, and intracranial or skull base lesion related. Inflammatory and autoimmune aetiologies were the most common causes associated with rapidly progressive hearing loss. All intracranial and skull base lesions, malignant or benign in nature, were associated with high patient mortality and morbidity.^{Reference Kishimoto, Yamazaki, Naito, Shinohara, Fujiwara and Kikuchi3,Reference Ishikawa, Kawamata, Kawashima, Yamaguchi, Kubo and Hori5–Reference Hausmann, Hausmann, Probst and Gratzl9}

Pathophysiology

The pathophysiologies of rapidly progressive hearing loss are summarised in Table 3.

The pathophysiology of autoimmune inner-ear disease is thought to have both humoral- and cell-mediated mechanisms.^{Reference Mijovic, Zeitouni and Colmegna19} In autoimmune inner-ear disease, damage to the cochlea and its nerve has been associated with unilateral or bilateral loss, which can be either sequential or simultaneous. Often the hearing loss will fluctuate.^{Reference Mijovic, Zeitouni and Colmegna19} The vestibular organ and vestibular nerves are commonly affected; therefore, vestibular symptoms are commonly associated with autoimmune inner-ear disease.^{Reference Mijovic, Zeitouni and Colmegna19} It has been reported that up to one-third of autoimmune inner-ear disease cases occur in the context of a systemic autoimmune disease.^{Reference Mijovic, Zeitouni and Colmegna19}

When considering the pathophysiology of rapidly progressive hearing loss as a result of an intracranial pathology, there are reports of both benign and malignant skull base lesions as the underlying cause (Table 2). Malignant lesions can be either primary tumours or distant metastases.^{Reference Kishimoto, Yamazaki, Naito, Shinohara, Fujiwara and Kikuchi3,Reference Ishikawa, Kawamata, Kawashima, Yamaguchi, Kubo and Hori5–Reference Zhang, Zhao, Sa, Qin, Qu and Wang8} Rapidly progressive hearing loss is more commonly associated with malignant neoplasms.^{Reference Kishimoto, Yamazaki, Naito, Shinohara, Fujiwara and Kikuchi3,Reference Ishikawa, Kawamata, Kawashima, Yamaguchi, Kubo and Hori5–Reference Zhang, Zhao, Sa, Qin, Qu and Wang8}

The rapid onset of hearing loss as a result of a malignant neoplasm contrasts with the onset of hearing loss due to a benign tumour. The most common benign neoplasms that cause hearing loss are vestibular schwannomas.^{Reference Sunwoo, Jeon, Park, Song, Song and Choi27} Evidence suggests that the majority of vestibular schwannoma cases present with a chronic progressive hearing loss occurring over months to years, and 3–8 per cent present as sudden SNHL (onset of less than 72 hours).^{Reference Samii and Matthies28,Reference Foley, Shirazi, Maweni, Walsh, Walsh and Javadpour29} This may be explained by malignant lesions having a higher propensity to progress at a faster rate, resulting in a more rapid onset of hearing loss.^{Reference Ishikawa, Kawamata, Kawashima, Yamaguchi, Kubo and Hori5}

Mangham et al. suggest that rapidly progressive hearing loss is a characteristic feature of cavernous haemangioma of the internal acoustic meatus (IAM).^{Reference Mangham, Carberry and Brackmann30} A more recent literature review presents contrasting evidence that these vascular malformations can present with hearing loss that is sudden, rapidly progressive or insidious in onset.^{Reference Mastronardi, Carpineta, Cacciotti, Di Scipio and Roperto31}

Infectious causes of rapidly progressive hearing loss include cryptococcal and chronic herpes meningitis. These conditions are thought to cause rapidly progressive hearing loss through direct inflammation of the vestibulocochlear nerve, and through the formation of meningeal granulomas.^{Reference Chrétien, Lortholary, Kansau, Neuville, Gray and Dromer32,Reference Tyler33} Both forms of meningitis are associated with high patient mortality and long-term morbidity if not diagnosed and treated early.^{Reference Tyler33,Reference Pasquier, Kunda, De Beaudrap, Loyse, Temfack and Molloy34}

Superficial siderosis is a rare pathology caused by chronic or intermittent extravasations of blood into the subarachnoid space, with a subsequent haemolysis and dissemination of haem by the circulating cerebrospinal fluid.^{Reference Koeppen, Michael, Li, Chen, Cusack and Gibson35} The deposited haem undergoes conversion into free iron, ferritin and haemosiderin, which in turn causes damage to the surrounding nerve tissues.^{Reference Koeppen, Michael, Li, Chen, Cusack and Gibson35} The vestibulocochlear nerve has been found to be particularly vulnerable.^{Reference Koeppen, Michael, Li, Chen, Cusack and Gibson35} The most common clinical presentation is slowly progressive cerebellar ataxia associated with hearing impairment. However, rapidly progressive hearing loss with or without vestibular symptoms can be the initial presenting symptom.^{Reference Kumar36} Early diagnosis and appropriate management has been shown to prevent further deterioration.^{Reference Kumar36}

Medication-induced rapidly progressive hearing loss is the result of ototoxic side effects. This is believed to be dose-dependent and cumulative in nature.^{Reference Schacht, Talaska and Rybak25,Reference Santucci, Garber, Ivory, Kuhn, Stephen and Aizenberg26,Reference Selimoglu37} Symptoms can develop days to weeks after the commencement of drug therapy. Cisplatin and aminoglycoside antibiotics are the two most widely implicated ototoxic medications.^{Reference Schacht, Talaska and Rybak25,Reference Selimoglu37} Hearing loss as a result of their administration is reported to begin at high frequencies, extending to lower frequencies with prolonged treatment.^{Reference Schacht, Talaska and Rybak25} Santucci et al. suggest that patients undergoing platinum-based chemotherapy should have their hearing monitored pre-, peri- and post-treatment.^{Reference Santucci, Garber, Ivory, Kuhn, Stephen and Aizenberg26}