SHIMP is the latest test protocol for vHIT (suppression head impuse test). The SHIMP test allows you to determine the extent of vestibular function. E.g. in cases where vHIT is hard to interpret alone (low gain), the SHIMP can help show if the vestibular system is functioning.
You can also use the SHIMP test for corroborating the level of residual function to help realistic patient expectations before starting rehabilitation.
How to perform the test?
Place the goggle on the patient's head in exactly the same way as you would position it for vHIT. The eye should be centered in the viewing area to ensure that any reflections are beneath the pupil.
After adjusting the goggle, centering the laser fixed dots on the wall, and calibrating the head and eye movements, you are ready to perform the SHIMP test. Be sure the 'display laser during HIT' in the settings tab is set to always on.
SHIMPs are performed on the lateral canal by turning the head at least 7 times at high velocities to both the left and right sides. It is important to instruct the patient to fixate on the center dot generated by the head-fixed laser projected on the wall. The laser dot pattern is the same 5-dot pattern that is used for the calibration process.
The appearance of five dots instead of one is not a problem; just ask the patient to focus on the center dot.
- Have the patient relax his neck, open his eyes wide and fixate on the center dot in the 5-dot pattern.
- Turn the patient's head either to the right or the left. The 5-dot laser pattern will move with the head so the dots are now located in a new position.
- Instruct the patient to keep his eyes on the center dot, so when the head moves the eyes should be focused on the newly positioned center dot.How to interpret the test?
The VOR gains should be similar in vHIT and SHIMP tests. However, the pattern of saccades generated is different. vHIT rarely generates catch-up saccades in healthy patients, while in SHIMP testing, healthy subjects will make a large saccade at the end of the head turn (see figure below). This is referred to as a 'SHIMP saccade'.
This pattern of result is exactly opposite for impaired patients. An impaired VOR system will lead to a catch-up saccade on the vHIT but no (or very few) SHIMP saccades.
For further details on the SHIMP test, please refer to the Interacoustics Quick Guide on the subject.
If you are lucky to be present at the upcoming AAA conference in Nashville, you can learn much about the SHIMP test at the Interacoustics booth.