Sensorineural hearing loss in Lassa fever: two case reports

Lassa fever is an acute arena viral haemorrhagic fever which was first identified in Lassa village, Borno State in the northeastern region of Nigeria in 1969 [1]. This rodent-borne virus is highly contagious through the urine of Mastomys natalensis (the multimammate rat) or via the body fluid of infected humans. The clinical diagnosis is often difficult because of the varied and non-specific modes of presentation of Lassa fever.

Neurological complications, notably sensorineural hearing loss, have been associated with Lassa fever. According to a fairly recent WHO report, deafness occurs in about 25% of Lassa fever patients [2]. However, no case of sensorineural hearing loss from patients confirmed with Lassa fever has been reported in our environment, according to a search we conducted on PubMed. Here, we present the occurence of sensorineural hearing loss in two confirmed cases of Lassa fever in Irrua, Nigeria.

Viral infections such as mumps, rubeolla, rubella, herpes zoster and cytomegalo viruses are known causes of hearing loss in human. These infections usually lead to the loss of hair and supporting cells of the cochlear during the active phase of the infection. The tectorial membranes are disrupted and the stria vascularis atrophy leading to end-vessel thrombosis and inner-ear fibrosis. Direct invasion of the spiral ganglion may also result in the loss of integrity of the vestibulocochlear nerve [3]. All these pathogenic processes are known to occur during the acute phase of the viral infections. Often they result in sudden SNHL and, because of limited diagnosis, most are classified as idiopathic. About 57% to 60% of patients are known to recover spontaneously during convalescence [4, 5].

In contrast, hearing loss associated with Lassa fever infections usually occurs at the convalescent stage of the infections [6, 7]. However, hearing loss following the illness can also occur during the active phase of the illness as illustrated in the above case series. Further deterioration in hearing over time may be recorded even after full recovery, as shown in Case 1. Cummins et al. [7] postulated that the viraemia is not responsible for the hearing loss in Lassa fever infections; instead, such hearing loss is due to immune response reactions against the elements of the inner ear. Similar observations were also made by Liao et al. [8]. This notion is further supported by the fact that early commencement of ribavirin therapy seems not to offer protection against development of SNHL. Recent research has shown that the strain of Lassa viruses found at the West African coast varies in its amino-acid sequence (genome) and tends to elaborate an already exaggerated immune responses, involving high titers of IgG and IgM [9, 10]. Consequently, inhabitants of endemic areas of Lassa fever with sub-clinical infections from exposures in the past stand a higher risk of developing SNHL during their lifetime.

One would have expected such an immunologically-induced systemic hearing loss to be bilateral as demonstrated clinically and by audiological assessment in the above case reports. However, reports of unilateral sudden SNHL associated with Lassa fever infection have been documented [11]. This raises further questions on the actual mechanism involved in Lassa fever-induced SNHL. Further research into this mechanism is necessary.

There is no active mode of management for Lassa fever-induced hearing loss. Conservative methods include hyperbaric labyrinthine vasodilators (e.g. nicotinic acid), hyperbaric oxygen and carbogen therapy [12] to enhance the oxygen pressure/perfusion to the inner ear. Steroids and low molecular weight dextran have also been used. However, there is no clear evidence on the efficacy of these methods of treatment. The effect appears to be similar to that seen in the management of idiopathic hearing loss where most improved cases are believed to be spontaneous rather than as a result of treatment [5, 13]. Hence, prevention of contact with the Lassa fever virus and its main vector remains the most efficient mode of control. In the endemic areas such as Edo State, Nigeria, where multimammate rats abound [14], total elimination of the vector of the virus is extremely difficult.

Innovative strides towards development of vaccines for Lassa fever infections are currently being made. It was recently reported that trials on primates have been successful [15]. It will be important to determine the possible long-term effects of the vaccines (immunological antibodies) on the inner ear before application to human. This poses further need for research on the mechanism of Lassa virus-induced hearing loss.

SNHL from Lassa fever infection can occur at both active and convalescent stages and is probably immune induced. However, the definitive pathogenesis is key to advancement into prophylactic and active treatment of the condition. Therefore, further research is urgently needed in this field.

Written informed consent was obtained from the patients for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.