This animation shows the path of sound transit over the superior canal ampulla and through the dehiscence at the apex of the superior canal. This creates a stimulation of the superior canal ampulla with eye movements upward and rotating away from the stimulated ear. The patient will usually describe a tonic tilting of the horizon away from the stimulated ear. The vibratory stimulus of the ampulla by low frequency sounds (+ - + - …) creates a net excitatory stimulus (Ewalds’ Second Law)

Many patients with confirmed SCD syndrome have no complaint of or inducible Tullio. Sound pressure may travel partly up the common crus to the dehiscence in these patients.

A dehiscence at the apex of the superior canal may render the labyrinth vulnerable to fluid displacements from changes in intracranial pressure. Valsalva will cause a tonic inhibition of superior ampulla tone. Release of Valsalva will cause a momentary exitation of superior canal output. Because excitatory stimuli of the ampulla have greater magnitude than inhibitory stimuli patients are generally more symptomatic with release of Valsalva than during Valsalva.

Many patients with SCD syndrome present with low frequency conductive hearing loss. Low frequency sounds arriving through the middle ear preferentially exit the labyrinth through the dehiscence and will reach the cochlea only at elevated thresholds. The audiogram may look like otosclerosis but tympanometry will not show decreased compliance and acoustic reflexes will be intact. Since the intracranial space has the same acoustic impedance as the labyrinthine fluids there is no resistance to sound passage as there is at the round window membrane where the impedance of fluid in the labyrinth and the air in the middle ear are vastly different.

Many SCD syndrome patients experience an unusual ability to hear internally generated or conducted sounds loudly in their ears. Voice, heartbeat, footfalls, and even eye movements can be heard. The dehiscence may decrease impedance for entry of even bone-conducted sounds into the labyrinth. This animation shows how the dehiscence, acting as a low impedance window, may allow fluid conducted sounds from remote sites in the body directly into the labyrinth.

Sound pressure normally enters at the oval window and is released at the round window. Between the two windows is the proximal end of the basilar membrane.