The scratch collapse test is a provocative test for nerve
entrapment or compression. This is becoming a popular test, and it is one of
many examination techniques used in the diagnosis of nerve compression,
entrapment, or injuries. The scratch test is a simple examination test that is
similar in sensitivity to other examination tests in the diagnosis of cubital
tunnel syndrome and other entrapment areas of the different nerves, such as
radial tunnel syndrome, pronator teres syndrome, and other nerve entrapment
areas. This test supplements, but does not replace, other information that we
collect during obtaining the history and physical examination of the patient. It
is really an added, helpful test that will precisely localize the site of nerve
compression. Do this test if you need to. Not only can this test add or provide
confirmation where entrapment of the nerve is located, but it can also
precisely localize the area of the entrapment of the nerve that `is known to
have different sites of entrapment, such as the ulnar nerve. If the patient has
a nerve entrapment at a specific site, after the scratch, the patient will
temporarily lose the ability to resist the internal rotation force to their
arm. The arm will collapse in the direction of internal rotation. The mechanism
is unknown, and it could be a reflex response. Because after you scratch or
stroke the skin above the nerve, the arm seems to have no power, and it
collapses as we test the resistance and internally rotate the arm. There might
be bias from the examiner due to the subjective evaluation of the brief,
temporary loss of resistance or loss of power after the scratch. To perform the
test, have the patient standing or sitting with the arms at the sides and the
elbows flexed to 90 degrees. Have the fingers and the wrist extended, then the
examiner applies force against the patient’s forearm to internally rotate the arm
and ask the patient to resist this force. The examiner and the patient will
both assess the baseline resistance of the patient. The skin over the potential
nerve entrapment area is scratched by the examiner, and then the examiner
immediately repeats the test. The change in resistance is assessed. Positive
scratch collapse test occurs when the patient has no resistance to the
examiners force and the arm collapses in internal rotation. There should be no
delays in retesting the patient because it may produce a false negative result.
Adding ethyl chloride (the cold spray) will temporarily numb or anesthetize the
skin superficial to the nerve of interest. It will freeze out a response to
scratching. It also may show secondary areas of compression of the same nerve
or different nerves. It also may show secondary areas of compression of the
same nerve or different nerves. After you apply the cold freezing spray to the
area of interest, the test is repeated. The cold spray should freeze out the
response to scratching. If you suspect multiple sites of entrapment, use the
freezing spray to numb the area then scratch it, and usually the patient will
have strength return after scratching the area. The freezing spray can make the
examiner eliminate sites or add sites of entrapment to the differential
diagnosis. It could be helpful in identifying multiple areas of compression for
the same nerve.
