A sharp and dull test
The sharp and dull test, or pinprick test, is a simple point of care neurological exam that tests the quality of sensation of different dermatomes. The test makes use of a dull object and a sharp object in order to test the patient’s ability to discriminate between the two within each dermatome. There are three dermatomes that are typically tested on the forefoot: L4, L5, and S1, as seen in figure 1. The medial aspect of the hallux is used to test L4, the lateral aspect of the hallux, all aspects of digits 2-4 and medial aspect of the 5th digit are used to test L5, and the lateral side of the 5th digit is used to test S1. As depicted above (Fig 2), we can use a cotton tip probe, broken into two pieces, in order to create a sharp edge where the break is. Step 1: demonstrate what each side of the probe feels like on a more proximal area on the patient, such as the arm, in order to give the patient understanding of what each side should feel like. Step 2: have the patient close their eyes and instruct them to verbalize which side they feel on their skin. Step 2: apply one of the two sides of the probe at a time throughout each dermatome and take note of the patient’s ability to accurately define which side it is they feel.
Boulton AJM et.al.: Comprehensive foot examination and risk assessment: a report of the task force of the foot care interest group of the American Diabetes Association, with endorsement by the American Association of Clinical Endocrinologists. Diabetes Care 31(8), 2008.