A dermatome is a specific area of skin that receives sensory innervation from a single spinal nerve root. Sensation (touch, pain, or temperature) from this region travels through one nerve pathway back to the spinal cord. Assessing sensation across these mapped areas is a standard method used by healthcare practitioners to evaluate the integrity of the nervous system. This assessment helps localize the exact level of the spine where a nerve root may be affected by damage or compression.
Understanding the Sensory Map
The body’s dermatomes form an organized sensory map corresponding to the spinal column’s nerve roots. Spinal nerves are grouped into four main regions: cervical (C1-C8), thoracic (T1-T12), lumbar (L1-L5), and sacral (S1-S5). Each pair of spinal nerves, except for the first cervical nerve, supplies a precise band of skin. This consistent pattern allows a practitioner to correlate a change in skin sensation with a specific level of the spinal cord.
In the torso, dermatomes are arranged in simple horizontal segments, like stacked rings around the body. For example, the T4 dermatome corresponds to the skin around the nipple line, and T10 aligns with the area of the umbilicus. The organization becomes more complex in the limbs due to the rotation of the embryonic limb buds during development, resulting in a longitudinal arrangement. For instance, the C6 nerve root provides sensation to the thumb, while the L4 nerve root covers the medial side of the knee and the great toe.
The Step-by-Step Testing Procedure
Checking dermatomes for sensory loss requires a systematic approach to ensure reliable and comparable results. The patient should be positioned comfortably, typically sitting or lying down, with the skin areas to be tested fully exposed. Before the examination, the patient is instructed to close their eyes during the testing to prevent visual cues from influencing their perception of the stimulus.
The examiner should first establish a reference point by testing a patch of skin known to have normal sensation, such as the upper arm or sternum. Sensory testing evaluates different types of sensation, including light touch, pain (sharpness), and temperature. Light touch is tested using a clean cotton wisp, while a disposable neurotip is used for a pinprick stimulus. Pinprick and light touch tests are often prioritized for efficiency over temperature assessment.
Testing is performed by applying the stimulus to corresponding dermatomes on both the left and right sides of the body for comparison. The patient is asked to state whether the sensation feels the same on both sides, or if it feels duller, sharper, or absent on one side. To accurately define the border of any sensory change, the stimulus should be moved from an area of altered sensation toward an area of normal sensation. The examiner must carefully document the location and nature of any difference.
Interpreting Sensory Changes
After completing the sensory examination, the findings must be analyzed to determine the nature and location of any neurological compromise. Sensation is typically graded on a simple scale:
- Grade 2 indicates normal sensation.
- Grade 1 signifies altered or impaired sensation.
- Grade 0 means the sensation is absent.
A decreased appreciation of sensation is termed hypoesthesia, while a complete lack of feeling is known as anesthesia. Conversely, an increased or exaggerated sensitivity to a stimulus is called hyperesthesia. When sensory loss is strictly confined to the area of a single dermatome, it suggests a problem with the corresponding spinal nerve root, such as pressure from a herniated disc, a condition known as radiculopathy.
The pattern of sensory loss helps distinguish a nerve root issue from a peripheral nerve issue. A loss that follows a specific dermatomal map points toward a spinal problem. Conversely, a loss that follows the path of an extremity’s main nerve trunk indicates a peripheral neuropathy. Conditions like sciatica, involving the L4-S3 nerve roots, often cause pain and tingling that follow these lower-limb dermatomes.