This case report describes a patient presenting with painful diplopia as a consequence of root canal dental treatment. Following root canal treatment a non-resolving tooth pain led to a decision to extract the tooth, leading to onset of severe periorbital pain, diplopia, headache, nausea and vomiting the next day. Magnetic resonance imaging demonstrated a mass lesion in the cavernous sinus. Corticosteroid therapy was started and marked pain relief occurred within 72 hours. Following five months of treatment there was complete resolution of diplopia and a normal appearance of the affected cavernous sinus. Cavernous sinus thrombosis is usually caused by infection but this was not the case here, as the problematic tooth was not infected. The authors question that the cause of ophthalmoplegia could be the intraorbital local anaesthetic injection used. Neuroimaging is suggested in cases of non-resolving cavernous sinus syndromes.