Severe isolated bilateral posterior semicircular canal hypofunction inducing perverted head shaking downbeat nystagmus

Downbeat nystagmus evoked by horizontal head shaking is most often a sign of central vestibular pathology. Rarely, however, such a sign may be present in peripheral pathologies that are well recognized clinically and semeiologically. Isolated hypofunction of the two posterior semicircular canals is also a very rarely described condition, often idiopathic or mainly due to deficiency, traumatic, autoimmune or ototoxic disorders. A 77-year-old man presented to our observation with a history of severe unsteadiness. The clinical and instrumental examination showed a downbeat nystagmus elicited by a horizontal head shaking test (perverted) and an impairment of both posterior semicircular canals. Findings suggesting other peripheral vestibular pathologies responsible for this sign or even a central vestibular lesion were absent. Clinical evaluation and blood testing ruled out the conditions reported as potentially responsible for bilateral posterior canal hypofunction. The case described should lead us to consider isolated idiopathic hypofunction of both posterior semicircular canals associated only with the nystagmus finding of a head shaking downbeat nystagmus, among the peripheral vestibular pathologies already described in the literature that could be responsible for perverted Head Shaking Down Beat Nystagmus (p-HSDBN). Therefore, when faced with the latter finding in a non-emergent outpatient setting, among the various etiopathogenetic hypotheses, this type of peripheral cause should be carefully excluded before sending patients for more invasive and costly tests that may be useless and may delay healing and recovery. The aim of our study is to present the unique case of a patient with downbeat nystagmus elicited by a horizontal head shaking test (perverted) due to an impairment of both posterior.