History: 50 year old male with mouth pain and swelling.
This is a submandibular space abscess. Head and neck anatomy is some of the most complex anatomy in the human body. The submandibular space is superior the hyoid bone, lateral or superficial to the mylohyoid muscle sling, and deep to the platysma muscle. It is sometimes confused with the sublingual space, which is deep and medial to the mylohyoid muscle and lateral to the genihyoid/genioglossus muscles (labeled with red arrows in the image above). The sublingual space communicates with the posterior superior submandibular space and inferior pararpharyngeal space.
Submandibular space abscesses are most commonly odontogenic in nature (as shown in the image below), but can also be caused by a submandibular duct calculus. Read about deep neck infections here.
Tooth abscesses can also cause sinusitis.