Surgical Risks to the Olfactory System

Endoscopic Sinus/Transnasal Surgery

Chronic rhinosinusitis is the most common chronic inflammatory disease and is frequently associated with impaired sense of smell [198, 199]. When symptomatic patients do not improve on medical treatment, endoscopic sinus surgery (ESS) may be proposed. Nasal polyposis is considered as the ultimate stage of chronic rhinosinusitis for which the mainstay of treatment is medical, but in which ESS plays a part in the majority of cases resistant to medication. Assessment of preoperative olfactory function is important since patients suffering from chronic rhinosinusitis are not always aware of their olfactory dysfunction, and occurrence of olfactory loss or disorders after endonasal surgery has been reported to be as high as 1% [183, 200, 201]. Nevertheless, this may be an overestimation, as recent studies suggested [184, 185]. Regarding bilateral choanal atresia, surgical repair at relatively advanced ages (8-10 years) was not associated with olfactory improvement [202]. This observation suggests that early sensory exposure could be important for the normal development of olfactory function.

In most cases, ESS is associated with significant improvement of rhino-sinusitis symptoms and olfactory function [184, 185]. However, absence or deterioration of olfactory detection thresholds in patients with chronic rhinosinusitis after ESS have been reported [203, 204]. Post-ESS olfactory dysfunction could be due to several mechanisms with persistent mucosal inflammation/edema in the region of the olfactory epithelium being one possible explanation [205]. In addition to post-operative edema, local polyp recurrence, scar tissue, or granulation could also contribute to the absence of improvement in the sense of smell [206].

The olfactory mucosa of patients suffering from long lasting chronic rhinosinusitis could be altered by a variety of toxic inflammatory mediators. In parallel, repetitive URTIs probably alter the neuroepithelium even further [131]. Furthermore, the olfactory epithelium can degenerate in chronic rhinosinusitis and may be replaced by the respiratory epithelium [207]. Furthermore, all surgeons performing ESS should be aware of the risk of iatrogenic injuries of the olfactory epithelium associated with extensive ethmoidectomy [208].

Craniotomy

This paragraph focuses only on the interventions with access to the anterior fossa, since these are most likely to affect olfaction. As stated by Pas-sagia [209], the olfactory structures constitute a natural obstacle to the exploration of the anterior fossa. Therefore, anosmia is a frequent complication of surgical approaches to this region [209]. Nevertheless, techniques have been described which potentially preserve olfaction [210, 211]. One crucial point in preservation of olfactory structures is to respect the blood supply to the olfactory bulb [209]. Whereas leaks of cerebrospinal fluid can be treated without destruction of olfactory structures, oncologic surgery for ethmoidal adenocarcinoma or esthesioneuroblastoma usually leads to anosmia [212, 213]. Meningiomas, which preferably grow in midline structures and especially within the olfactory groove region, are potentially dis-sectible with preservation of olfaction [209]. However, most reports on olfactory impairment after surgery of the anterior fossa have been conducted on small samples [214] and olfactory function has rarely been measured properly [209]. Welge-Luessen et al. [215] have recently published a study focusing on the olfactory outcome after meningeoma surgery. They pointed out that preservation of olfaction ipsilateral to the tumor is extremely difficult. They also showed a correlation between preserved postoperative olfactory function and tumor size. Overall, it seems that preservation of olfactorily eloquent structures might be possible when the tumor size is small. Nevertheless, olfactory function seems to be very vulnerable and seems sometimes altered even though the surgeon did not touch the olfactory structures. This corroborates findings by Delank [146] on posttrau-

matic cadavers, that olfactory tracts and bulbs in certain people are severed even after minor tearing.

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