Caloric Vestibular Stimulation

Caloric Vestibular Stimulation is an experimental treatment for central pain syndromes such as post stroke pain syndrome (including thalamic stroke), Fibromyalgia, CRPS, and phantom limb pain. It is a very easy to administer and inexpensive treatment that patients can learn to do themselves at home.
==Pathophysiology==
One hypothesis for central pain is the "thermosensory disinhibition hypothesis." In this hypothesis central pain occurs due to the loss of central inhibition by cooling. With temperatures below 25°C there is activation of both cold thermoreceptors (A-delta fibres) and C-nociceptors. These two types of fibres pass through to the thalamus via the spinothalamic tract. From the thalamus, the C-fibres reach the anterior cingulate cortex (ACC), and the A-delta fibres reach the dorsal posterior insula (dpIns). In a normal state, the A-delta fibres activate the dpIns to reduce pain at the ACC. When the temperature falls below 15°C there is relatively greater C fibre activity, hence the ACC is not suppressed by the dpIns and the sensation is perceived as painful. With damage to the spinothalamocortical pathway that terminates in the dpIns there can be disinhibition of the ACC causing pain. 
The mechanism by which caloric vestibular activation relieves central pain is through to be through activation of the parieto-insular vestibular cortex (PIVC), a structure adjacent to the dpIns. There are two hypothesised mechanisms of action for activation of the PIVC.McGeoch PD, Williams LE, Lee RR, Ramachandran VS. Behavioural evidence for vestibular stimulation as a treatment for central post-stroke pain. J Neurol Neurosurg Psychiatry. 2008 Nov;79(11):1298-301. doi: 10.1136/jnnp.2008.146738. Epub 2008 Jun 11. PMID: 18550629.
* Activation of the adjacent thermosensory cortex in the dorsal posterior insula (dpIns)
* Rebalancing integration of thermosensory information by suppressing thermal pain at the anterior cingulate cortex (ACC)
There is generally greater and more sustained pain relief in the head and upper limbs than the lower limbs. Repeated irrigation may show greater relief in lower limb pain. The best results are seen in those where it is possible to activate the dominant PIVC, located in the non-dominant hemisphere. The vestibular outflow from the thalamus to the PIVC is located in the posterolateral thalamus.
== Procedure ==
Patients should be warned that the procedure causes temporary vertigo (along with observable nystagmus). Most will find the procedure temporarily unpleasant and some will be unable to tolerate the sensation. Out of 130 (general) patients, 50% experience nausea, 5% vomiting, and 12% headachesNgo TT, Barsdell WN, Law PCF, Arnold CA, Chou MJ, Nunn AK, Brown DJ, Fitzgerald PB, Gibson SJ, Miller SM. Tolerability of caloric vestibular stimulation in a persistent pain cohort. Brain Stimul. 2020 Sep-Oct;13(5):1446-1448. doi: 10.1016/j.brs.2020.07.003. Epub 2020 Jul 18. PMID: 32693181.. In a study of 25 patients being treated for pain, over 80% of responders were generally willing to repeat the procedure if it reduced pain by at least 50% for a week.. There is also a small risk of otitis external and tympanic perforation.
Equipment
* Catch basin
* Vomit bag
* Stopwatch
* Dry towel
* 50mL syringe.
The patient lies supine with the head flexed by 30 degrees. Place the catch basin under the ear being irrigated. Irrigate the ear with ice cold water (e.g. 4°C) for 30 seconds at a rate of 1-2mL per second. The nystagmus will begin approximately 30 seconds after the onset of irrigation and will increase in intensity over the subsequent 30 to 45 seconds. The procedure can be repeated in the other ear after five minutes.
If successful it may be repeated. Patients can be taught to do it at home as needed. Although no long term studies have been published, it is likely that the effect wears off. Responders appear to have a sustained effect for approximately one week. The effect is occasionally longer with some reports of effect up to a month.
==Resources==
Category:Miscellaneous
==References==