An exostosis is a benign, slowly growing, dome shaped area of localized bony hypertrophy arising from the medial surface of the tympanic bone (the bone of the deep external ear canal).
Exostoses appear to arise in susceptible individuals in response to the repeated stimulation of the bony external canal by cold water. The initial cold induced vasoconstriction of the deep canal is followed by a reactive hyperaemia and a stimulation of the periosteum lining the medial surface of the tympanic bone which lays down consecutive layers of subperiosteal bone.
A history of frequent aquatic activities (swimming, diving, surfing etc.) will often be obtained in these patients.
Exostoses are usually multiple, occur bilaterally and are more common in males than females.
Two exostoses are present a large sessile anterior exostosis and a smaller pedunculated posterior exostosis.
The tympanic membrane, malleus handle and fibrous annulus are visible behind the exostoses.