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Dizziness

Information on Caloric Testing


Caloric nystagmus is the most important tool for assessing unilateral vestibular function. 

The most common diagnoses of the peripheral vestibular system include benign paroxysmal positioning vertigo and vestibular neuritis. Following the acute stage of vestibular neuritis most patients become largely symptom free through central nervous system adaptive processes. However, loss of inner ear function does not always recover. A proportion of patients have ongoing symptoms of dizziness and disequilibrium, reflecting a lack of CNS compensation. Other patients may have such symptoms without recall of an acute episode of a vestibular event.

The demonstration of a paresis or asymmetry of vestibular function on caloric testing can confirm the underlying cause and enable vestibular rehabilitation with physiotherapy to proceed with greater understanding and confidence. Cochrane reviews support vestibular rehabilitation as a recognised treatment for inner ear imbalance. Exercises are given to induce imbalance and dizziness to induce central nervous system compensation.

See http://www.dizziness-and-balance.com/treatment/cawthorne.html
Information on Caloric Testing
The caloric test involves a subject lying supine with the head inclined at 30° with cool (30°C) and warm (44°C) water irrigation into each of the four external auditory canals. (see Fig). In this position the horizontal semicircular canals are gravitationally vertical, enabling the maximum effect of convection. Being closest to the middle ear the horizontal semicircular canals also develop the largest temperature gradient. With warm water irrigation, endolymph within the adjacent horizontal semicircular canal rises because of decreased density, causing the cupula to deviate and produce nystagmus.  Cool water irrigation reverses the thermal gradient, endolymph flow, cupula deviation and direction of nystagmus.  Visual fixation suppresses vestibular nystagmus, so analysis of caloric slow phase velocity is made in darkness.
 



The results of the caloric test, in terms of the responses to four stimuli, are used to obtain a measure of canal paresis (also known as unilateral weakness) and directional preponderance. A canal paresis results from reduced horizontal canal excitability canal in the induced nystagmus following caloric stimulation from one ear as compared to those obtained from the opposite ear. A directional preponderance indicates that the nystagmus response is greater in one direction as compared to the other.

 

 
  (R44°C + R30°C) – (L44°C + L30°C)  
Canal Paresis  = slow phase velocity  -------------------------------------------  
  (R44°C + R30°C) +(L44°C + L30°C)
Normal < 20%
 

Directional preponderance indicates an asymmetry in the cumulative  right and left beating nystagmus from both labyrinths.
 
  (R44°C + L30°C) – (L44°C + R30°C)  
Directional prepondernace  = slow phase velocity -------------------------------------------  
  (R44°C + L30°C) +(L44°C + R30°C)
Normal < 20%
 

The caloric responses have a moderate range of asymmetry before being classed as abnormal.  This is because the slow phase velocity of nystagmus is variable depending on the aural irrigation, morphology of the ear canal and pneumatisation of the mastoid ear cells, as well as the patient’s alertness.

Example of caloric trace:
 
Right Ear 44 degrees temperature
Slow phase velocity of Right beating nystagmus
Left Ear 30 degrees temperature
Slow phase velocity of Right beating nystagmus
 
calorics_pic.jpg
Right Ear 30 degrees temperature
Slow phase velocity of Left beating nystagmus
Left Ear 44 degrees temperature
Slow phase velocity of Left beating nystagmus
 
This patient had a right base of skull fracture with right ear deafness and dizziness.  The results on show reduced caloric function of the right inner ear. The peak velocity of eye movement as a consequence of water irrigation to the right ear is less than that on the left. The caloric test is abnormal with 44% canal paresis to the right, and 27% directional asymmetry to the right. This is consistent with a moderate paresis of the right horizontal semicircular canal and confirms a likely inner ear origin of symptoms.