The sensory distribution of each nerve root is called a dermatome. A dermatome may also be defined as the area of skin supplied by a single nerve root. The area innervated by a nerve root is larger than that innervated by a peripheral nerve. Descriptions of dermatomes given in most texts should only be considered as examples because slight differences and variables occur with each patient. In addition, dermatomes also exhibit considerable overlap (For example, the loss of one dermatome in the thoracic spine can often go unnoticed because of the overlap of the adjacent dermatomes).
The S3 nerve root dermatome supplies the area of skin of the groin and medial thigh up to the knee. Furthermore, the S3 nerve root dermatome does not have any myotomal component nor does it affect any specific reflex test. It also is not implicated in any specific paresthesias. [1] Paresthesia is an unpleasant sensation that occurs without an apparent stimulus or cause to the patient. [1, pg 10]
By way of digression, myotomes are defined as groups of muscles supplied by a single nerve root while a sclerotome is an area of bone or fascia supplied by a single nerve root. A lesion of a single nerve root is usually associated with paresis, or an incomplete paralysis of the myotome (muscles) supplied by that nerve root (isometric testing of myotomes is usually held for at least 5 seconds because muscle weakness may take some time to become apparent). Peripheral nerve lesions on the other hand, especially axonotmesis or neurotmesis injuries, lead to complete paralysis of the muscles supplied by the nerve and the weakness is evident immediately. [1, pg 26]
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