Scratch Collapse Test
The scratch collapse test is a provocative physical examination maneuver where light scratching over a suspected nerve compression site causes momentary loss of resisted shoulder external rotation on the affected side, indicating peripheral nerve entrapment.
How the Test is Performed
The test follows a specific sequence 1:
- Patient positioning: The patient stands or sits with elbows flexed at 90 degrees
- Baseline resistance testing: The patient resists bilateral shoulder external rotation while the examiner applies opposing force
- Nerve stimulation: The examiner lightly "scratches" the skin over the area of suspected nerve compression (e.g., carpal tunnel, cubital tunnel, peroneal nerve)
- Immediate re-testing: Resisted shoulder external rotation is immediately repeated
- Interpretation: A positive test shows momentary loss or collapse of shoulder external rotation resistance on the affected side
Clinical Applications
The scratch collapse test has been studied for multiple compression neuropathies 1, 2, 3:
- Carpal tunnel syndrome: Scratch over the volar wrist
- Cubital tunnel syndrome: Scratch along the medial elbow, particularly at Osborne's band
- Common peroneal nerve compression: Scratch over the fibular head
- Neurogenic thoracic outlet syndrome: Scratch over supraclavicular or infraclavicular regions 4
- Long thoracic nerve entrapment 5
Test Characteristics
The scratch collapse test functions as a confirmatory test rather than a screening test due to its low sensitivity but high specificity 6:
- Overall pooled data: Sensitivity 38%, Specificity 94% 6
- Carpal tunnel syndrome: Sensitivity 64%, highest negative predictive value 73% 1
- Cubital tunnel syndrome: Sensitivity 69% 1, can localize maximal compression at Osborne's band 3
- Peroneal nerve compression: Sensitivity 77%, Specificity 99% 2
The test demonstrates higher sensitivity than traditional Tinel's sign and flexion/compression tests for both carpal and cubital tunnel syndromes 1.
Physiologic Basis
The test represents a protective reflex pathway 7: Light stimulation of an injured nerve travels through afferent pathways in the fasciculus cuneatus of the spinal cord, directly connecting to the lateral reticulospinal tract. This causes inhibition of extensor muscles (shoulder external rotators) and activation of flexors, creating a withdrawal response that teleologically protects the injured nerve by pulling the limb toward the trunk.
Clinical Caveats
- Use as a confirmatory test: Given low sensitivity, a negative test does not rule out nerve compression 6
- Objective measurement controversy: One study using digital dynamometry found no clinically significant strength difference (only 0.08 kg) in severe carpal tunnel syndrome, questioning the test's validity when measured objectively 8
- Subjective interpretation: The test relies on examiner perception of resistance loss, which may introduce variability
- Always confirm with electrodiagnostic studies: The scratch collapse test is an adjunct to, not a replacement for, nerve conduction studies and EMG 1
In cubital tunnel syndrome specifically, a positive scratch collapse test a few centimeters distal to the medial epicondyle reliably localizes maximal compression at Osborne's band, which can guide surgical planning 3.