The Monocle Sign in FDG-PET

Abstract

We report three cases of unilateral F-FDG uptake in the orbicularis oculi muscle in subjects with contralateral peripheral facial nerve palsy. We argue that this asymmetric uptake pattern in fact reflects lack of metabolism on the side affected by facial nerve palsy, owing to denervation. Since the unilateral periorbital uptake resembles a monocle, we chose to call this finding the monocle sign. The monocle sign should not be confused with inflammation or tumor, but should prompt a neurological assessment for facial nerve palsy and a potential underlying disease.

The Monocle Sign on 18F-FDG PET Indicates Contralateral Peripheral Facial Nerve Palsy

“…This finding has been termed the "monocle sign." 12 The nonparetic OOMs (but not the paretic OOM) in patients with peripheral FNP also have significantly higher FDG uptake than the OOM of patients without FNP. In contrast, in patients with central FNP, no significant difference in FDG uptake between their OOMs was found.…”

“…Our study validates the hypothesis that the nonparetic OOM of patients with peripheral FNP exhibits abnormally high FDG uptake and rejects the hypothesis that the paretic OOM displays abnormally low FDG uptake. 12 The elevated FDG uptake of the nonparetic OOM in peripheral FNP patients may be attributed to various factors. It is known that OOM paralysis leads to incomplete eyelid closure (lagophthalmos), which can serve as a constant stimulus for forced eyelid closure due to continuous light input, with afferent signal transmission to the bilateral visual cortex, and hence the efferent signal overstimulates the nonparetic OOM.…”

“…Much like the recognizable vocal cord sign observed in individuals with recurrent laryngeal nerve palsy and incomplete "V" sign for lateral rectus muscle paralysis due to abducens nerve palsy, the monocle sign (unilateral high periorbital muscle uptake) has been documented as an indicator of FNP in 4 reported cases. [11][12][13][14] These cases include patients with parotid gland cancer, bladder cancer with meningeal metastases, lung cancer with bone metastasis in the clivus, and squamous cell carcinoma of the parotid gland. [11][12][13][14] The monocle sign is believed to result from an asymmetrically increased metabolism in the contralateral and unaffected orbicularis oculi muscle (OOM).…”

“…[11][12][13][14] These cases include patients with parotid gland cancer, bladder cancer with meningeal metastases, lung cancer with bone metastasis in the clivus, and squamous cell carcinoma of the parotid gland. [11][12][13][14] The monocle sign is believed to result from an asymmetrically increased metabolism in the contralateral and unaffected orbicularis oculi muscle (OOM). 12 Hence, the aim of our study was to conduct a retrospective analysis of FDG PET/CT data to determine the presence of the monocle sign in both patients with FNP and those without, aiming to confirm its diagnostic significance in identifying contralateral FNP.…”

“…[11][12][13][14] The monocle sign is believed to result from an asymmetrically increased metabolism in the contralateral and unaffected orbicularis oculi muscle (OOM). 12 Hence, the aim of our study was to conduct a retrospective analysis of FDG PET/CT data to determine the presence of the monocle sign in both patients with FNP and those without, aiming to confirm its diagnostic significance in identifying contralateral FNP.…”