Investigation of Vestibular Function in Adult Patients with Gitelman Syndrome: Results of an Observational Study

Gitelman syndrome (GS) is a rare salt-losing tubulopathy caused by an inactivating mutation in the SLC12A3 gene, encoding the thiazide-sensitive sodium chloride cotransporter (NCC). Patients with GS frequently complain of vertigo, usually attributed to hypovolemia. Because NCC is also located in the endolymphatic sac, we hypothesized that patients with GS might have vestibular dysfunction. Between April 2013 and September 2016, 20 (22%) out of 90 patients followed at the reference center complained of vertigo in the absence of orthostatic hypotension. Sixteen of them were referred to an otology department for investigation of vestibular function. The vertigo was of short duration and triggered in half of them by head rotation. Seven patients (44%) had a vestibular syndrome. Vestibular syndrome was defined: (1) clinically, as nystagmus triggered by the head shaking test (n = 5); and/or (2) paraclinically, as an abnormal video head impulse test (n = 0), abnormal kinetic test (n = 4) and/or abnormal bithermal caloric test (n = 3). Five patients had associated auditory signs (tinnitus, aural fullness or hearing loss). In conclusion, we found a high frequency of vestibular disorder in GS patients suffering from vertigo, suggesting a role of NCC in the inner ear. Referent physicians of these patients should be aware of this extrarenal manifestation that requires specific investigations and treatment.


SUPPLEMENTAL MATERIAL TABLE OF CONTENTS
. Responses of the Gitelman patients to the survey on vertigo clinical presentation.
- Table S2. Variants detected in Gitelman syndrome patients in the SLC12A3 gene and their classification.
- Table S3. Baseline characteristics and treatment of the sixteen Gitelman syndrome patients with vertigo.
- Table S4. Comparative biological and clinical results between patients with normal and abnormal vestibular function.
- Figure S1. Ion exchanges in the endolymphatic sac.
- Figure S2. Clinical set up and principle of the Video Head Impulse Test (VHIT).
- Figure S4. Clinical set up and principle of kinetic tests.
- Figure S5. Clinical set up and principle of caloric tests.
-Supplemental material references.     Figure S1. Ion exchanges in the endolymphatic sac. The endolymphatic sac regulates endolymph homeostasis through control of ion and water transport from epithelial cells of the endolymphatic sac. The molecules involved include ion channels such as the epithelial sodium channel ENaC, localized at the apical membrane, and K + and Clchannels, localized at the basolateral membrane; cotransporters such as thiazide-sensitive Na + -Clcotransporter NCC and bumetanide-sensitive Na + -K + -2Clcotransporter 2 (NKCC2). An Na + ,K + -ATPase at the basolateral membrane is also involved. The camera tracks eye movements and registers the head movement simultaneously. The system focuses on the ability to keep eyes fixed on the target, which is detected by an absence of movement of the eyes while the head is moving. If there is a vestibular abnormality, the eyes do not remain focused but instead move with the head and then make a saccade (quick backward movement) to return to the target [16]. showing that all responses were normal during the test and that the vestibulo-ocular reflex gain was normal (between 0.8 and 1.2). If the test was abnormal, the dots would be colored in red and would be located outside the light green portion.  Figure S4. Clinical set up and principle of kinetic tests. The patient is seated on a rotatory chair wearing a mask. The chair moves in a sinusoidal way for the sinusoidal harmonic acceleration test, whereas it makes a quick acceleration followed by a quick deceleration in the impulsive rotational test [17]. Normally, the head stays fixed with the rest of the body during the test. In other words, the head and body move with the chair as one single element. These movements of the chair generate a vestibular stimulation that is manifested by a nystagmus. We used videonystagmoscopy (an infrared camera located in the mask, which films eye movements) to observe and quantify the intensity and the frequency of the nystagmus. If the result is normal, the velocity of the eyes are symmetrical [18]. The test result is considered abnormal if there is a directional preponderance (left or right eye) of 2°/second or more. Figure S5. Clinical set up and principle of caloric tests. The patient wears a mask and is seated with the chair back at an angle 30° from the vertical plan. The lateral semi-circular canal is the only canal tested. The ears are alternatively irrigated for 30 seconds with 200 mL of cold (30 °C) then hot (44 °C) water. This irrigation creates a nystagmus. A hot-water irrigation induces an excitation of the vestibule generating a nystagmus beating toward the stimulated ear, whereas a cold-water irrigation induces an inhibition of the vestibule generating a nystagmus beating toward the non-stimulated ear. The induced nystagmus is recorded with videonystagmoscopy until it stops. Lateral canal dysfunction is defined as a difference of 15% or more between the responses of the two ears [18] .