Final answer:
Benign Paroxysmal Positional Vertigo (BPPV) is the most likely diagnosis for a patient presenting with vertigo upon head movement, as evidenced by the positive Dix-Hallpike test showing rotary nystagmus. It is caused by dislodged otoliths in the semicircular canals leading to abnormal stimulation of hair cells. BPPV presents with brief episodes of vertigo related to changes in head position, and symptoms often lessen with repeated testing.
Step-by-step explanation:
The most likely diagnosis for a patient experiencing vertigo with Dix-Hallpike testing indicating rotary nystagmus is Benign Paroxysmal Positional Vertigo (BPPV). BPPV occurs when calcium carbonate crystals, also known as otoliths or canaliths, migrate from the utricle into one of the semicircular canals of the inner ear.
As the semicircular canals are responsible for detecting rotational movements through hair cells embedded in a gelatinous structure called the cupula, the presence of these crystals affects normal fluid movement within the canals and leads to inappropriate stimulation of the hair cells when the head is moved. This aberrant stimulation is interpreted by the brain as movement, causing the sensation of vertigo.
The semicircular canals are oriented in three spatial planes and sense angular acceleration or deceleration. During sudden head movements, the fluid contained within the canals, known as endolymph, exerts force on the cupula and bends the hair cells, sending information about movement to the brain.
In the case of BPPV, the displaced otoliths disrupt this process and continue to move due to gravity, even after the head has stopped moving, leading to continued stimulation of the hair cells and a delayed sense of rotation, or vertigo.
This diagnosis is supported by the typically short duration of vertigo that is associated with positional changes and is often triggered by lying down or turning over in bed. The nystagmus observed during Dix-Hallpike testing is a characteristic sign of BPPV, resolving with time as the otoliths resettle or as the brain adapts to the aberrant signals.
Repeat testing leading to a diminishing response suggests habituation, another indication of BPPV. Other conditions such as CNS lesions, labyrinthitis, and Meniere's disease tend to have additional symptoms or different presentations.