Discussion

After entering the middle ear via the canaliculus of the chorda, the chorda tympani travels between the malleus and long process of incus before exiting through the petrotympanic fissure. Its close proximity to the tympanic membrane means that cholesteatoma arising from retraction pockets commonly involves the chorda tympani, resulting in chronic inflammation and causing dysfunction associated with the vascular degeneration of Schwann cells and disorganisation of axons.^{Reference Gedikli, Doğru, Aydin, Tüz, Uygur and Sari4}

The sense of taste is mediated by a complex neural network. The chorda tympani and lingual branch of the glossopharyngeal nerve supply taste to the tongue, whereas the greater superficial petrosal nerve and pharyngeal plexus of the vagus nerve supply taste to the palate and epiglottis. Some have suggested that sectioning of the chorda tympani would cause disinhibition of the glossopharyngeal and greater petrosal nerves.^{Reference Kveton and Bartoshuk5} This theory is also supported by neurophysiological data in animal studies where a central disinhibition was seen from the nucleus of the solitary tract after chorda tympani anaesthesia.^{Reference Dinkins and Travers6}

After chorda tympani sectioning, the selective attenuation of taste response from the intact side of the tongue is known to release local diffusible factors that contribute to an enhancement of taste sensation on the contralateral side of the tongue.^{Reference Hendricks, Sollars and Hill7} Therefore, unilateral chorda tympani damage causing a localised taste loss in one area can trigger a heightened taste response from other areas. It is possible that once the diseased chorda tympani was transected in our patient, there was a compensation from other parts of the neural network and the contralateral chorda tympani. Similarly, such a mechanism could also explain why patients are less likely to develop dysgeusia after middle-ear surgery on ears that are chronically inflamed,^{Reference Gopalan, Kumar, Gupta and Phillipps8} as there is compensation for the gradually deteriorating chorda tympani function in these cases.

It is controversial whether an injured chorda tympani should be preserved or divided intra-operatively. Some have found that transection of the chorda tympani is more likely to lead to post-operative gustatory symptoms than a preserved chorda tympani.^{Reference Mahendran, Hogg and Robinson9} Others have suggested that incomplete injury to the chorda tympani is more likely to cause taste disturbance than total division.^{Reference Lloyd, Meerton and Graham10}

In one study of 140 subjects, dysgeusia was observed in 57 per cent of patients whose chorda tympani nerve incurred a stretch injury, compared to 10 per cent whose nerve had been transected.^{Reference Michael and Raut11} It is postulated that a stretched nerve will suffer neuropraxia, and the subsequent taste disturbances from the neuropraxic injury will not be compensated for by other parts of the taste network because of a lack of disinhibition.

Dysgeusia is a rare, perhaps underreported feature in cholesteatoma. In this report, a patient with cholesteatoma who presented with dysgeusia experienced symptom resolution after the diseased chorda tympani was sacrificed during mastoidectomy. It is possible that sectioning of the chorda tympani could lead to a beneficial outcome in patients.