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A new type of vertigo is characterized by recurrent spontaneous vertigo and marked interictal headshaking nystagmus, but has no known cause, according to researchers in South Korea.

However, the disorder may be treatable, reported Ji-Soo Kim, MD, PhD, of Seoul National University, and colleagues in .

"This can be difficult to diagnose and quite debilitating for people, so it's exciting to be able to discover this new diagnosis of a condition that may respond to treatment," Kim said in a statement.

Episodic vestibular syndromes represent the largest group of dizzy patients, usually stemming from benign positional paroxysmal vertigo, vestibular migraine, Meniere's disease, and extravestibular disorders like hemodynamic orthostatic vertigo and posterior fossa transient ischemic attacks. But even when rare diagnoses are considered, a group of unspecified cases remain, noted Alexandre Bisdorff, MD, PhD, of Centre Hospitalier Emile Mayrisch in Luxembourg, Belgium, and Jorge Kattah, MD of University of Illinois College of Medicine in Peoria in an .

This study has "identified a new entity, reducing the group of so-far undiagnosed patients," Bisdorff and Kattah wrote. By showing that in some patients, dysfunction of central structures leads to a benign, potentially treatable cause of recurrent vertigo, "this syndrome challenges traditional concepts when approaching a patient with vertigo, notably the dichotomy into peripheral (generally benign) and central (generally serious) conditions."

Kim's group first encountered this condition in 2004, in a young man who had recurrent spontaneous vertigo with vigorous interictal headshaking nystagmus (HSN) and severe motion sickness, but no auditory symptoms or history of migraine. From 2004 to 2015, they saw 3,990 patients with recurrent spontaneous vertigo (RSV) at the Dizziness Clinic of Seoul National University Bundang Hospital, and could not determine an etiology for 338 of them.

For this study, they divided the 338 patients into those with RSV-HSN (n=35) and those who had benign recurrent vertigo without headshaking nystagmus (n=303), randomly selecting 35 patients from the second group as controls.

They also compared the time constant (the time it took for responses to decrease by a factor of 0.632) of headshaking nystagmus of patients with RSV-HSN to patients with vestibular neuritis (n=30), vestibular migraine (n=30), and unilateral Meniere's disease (n=30), randomly selected from the dizziness registry of the same institution.

The researchers applied paced horizontal headshaking to patients, recording their eye movements until the nystagmus subsided or for at least 1 minute. Some patients also underwent rotary chair testing to evaluate their vestibulo-ocular reflex.

Patients with RSV-HSN showed more severe motion sickness than those with benign recurrent vertigo. The time constant of the primary headshaking nystagmus phase in RSV-HSN patients was 12 seconds, considerably larger than in patients with vestibular neuritis (5 seconds), vestibular migraine (5 seconds), or Meniere's disease (6 seconds). The duration and peak slow-phase velocities of the second phase of headshaking nystagmus also were larger than those of the patients with vestibular neuritis. In certain RSV-HSN patients, even a brief headshaking session of only 2 to 5 seconds elicited strong, long-lasting nystagmus.

Time constants of the horizontal vestibulo-ocular reflex also were larger during the rotatory chair test in RSV-HSN patients.

These features indicate that RSV-HSN patients may have a hyperactive, asymmetric velocity-storage mechanism that may be responsible for intermittent attacks of spontaneous vertigo, the researchers concluded.

"It's possible that the vertigo occurs when this unstable mechanism is disrupted by factors either within the person's body or in their environment," Kim said.

The researchers prescribed preventive medication -- including nimodipine, propranolol, baclofen, and several others not available in the U.S. -- for 20 patients who had the most frequent and severe symptoms. Six patients reported partial recovery of symptoms and one reported complete recovery after treatment.

In a long-term follow-up analysis of 31 RSV-HSN patients that averaged 12 years from symptom onset, five individuals reported no further attacks, 14 said symptoms had improved, and one said symptoms had grown worse. No patients developed vestibular migraine, Meniere's disease, or cerebellar dysfunction.