Monday, October 8, 2018

Question: Caloric test


Q) Caloric test was done on right side with cold water and eyes were moved to opposite side. Which of the following correspond to interpretation of nystagmus in this test?
1) Eyes moves slowly to right
2) Eyes moves slowly to left
3) ‎Eyes moves rapidly to left
4) ‎Eyes moves rapidly to right
Answer in 24 hours with explanation of Caloric test.

So no doubt this question is super easy and only slight confusion that would be created in such types of question are options. Here it's about eye movement being slow or rapid. I personally don't think that even that would be an issue. Its obviously going to be rapid movements. 

Correct option is 3- Eyes moves rapidly to left.

The very famous mnemonic for Caloric test, as you all know is COWS.

C=Cold water 
O=Opposite side
W=Warm water
S=Same side

When we put Cold water in ear, eyes moves to opposite side of that ear. For example If I put cold water in left ear then eyes will move to right side.

When we put Warm water in ear, eyes moves to same side of the ear. For example If I put warm water in right ear then eyes will to right side only.

So, here is the mechanism for the test
 (Always consider Lateral semicircular canal!)

For proceeding further you need to remember a small diagram.

As you can see cold water inhibits the potassium channels / hair follicles and warm water stimulates the potassium channels/ hair follicles.
( Stimulus from lower hair to higher hair-Activates 
Stimulus from higher hair to lower hair-Inhibits) 

In above question, we used right ear and cold water. As we know that cold water will inhibit the potassium channel so ultimately we can say that right ear is overactive or stimulated. This leads to stimulation of the right vestibular nucleus. Now, fibers from right vestibular nucleus crosses and stimulate left 6th nerve nucleus and this leads to stimulation of the left LR muscle. Leading to abduction of the left eye.

Now, fibers from left 6th nerve nucleus also crosses with the help of medial longitudinal fibers and stimulate right 3rd nerve nucleus which stimulates right MR. This leads to adduction of the right eyeball. 

So we have now,
Left eyeball abduction+ Right eyeball adduction = Both eyes looking towards left.

Mechanism sounds confusing so here is a chart.

Demotional bloke

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