Asymmetric FDG Uptake in the Extraocular Muscles Resulting From Oculomotor Nerve Paralysis

Unilateral Lateral Rectus Paralysis Showing Incomplete "V" Sign' on 18F-FDG PET/CT

Extraocular muscles show physiologic radiotracer uptake on F-FDG PET/CT as is encountered in most routine scans. Certain pathologies can cause palsy of one or more of the extraocular muscles, leading to hypometabolism on F-FDG PET/CT. We present a case with a classical finding of unilateral hypometabolism of the lateral rectus muscle, disrupting the "V" sign formed due to physiologic uptake. The patient had presented with the complaint of diplopia and was being evaluated further to rule out the possibility of inflammatory/granulomatous infectious etiopathogenesis leading to compression of the abducens nerve resulting in lateral rectus palsy.

The Monocle Sign in FDG-PET: A Sign of Contralateral Facial Nerve Palsy

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.

Compensatory asymmetric hemidiaphragmatic uptake secondary to contralateral hemidiaphragmatic paresis

A 59-year-old man with diffuse large B-cell lymphoma presenting as an anterior mediastinal mass underwent PET/CT after 2 cycles of chemotherapy. PET/CT shows moderately increased FDG uptake in a predominantly left prevascular mass, consistent with partially treated lymphoma. The left hemidiaphragm is elevated. PET shows asymmetric uptake along the right hemidiaphragm, which is hypothesized to represent compensatory increased workload related to contralateral left diaphragmatic paresis. The left diaphragmatic paresis is likely caused by phrenic nerve involvement by left prevascular mass.