A dermatome is defined as ‘a strip of skin that is innervated by a single spinal nerve’. They are of great diagnostic importance, as they allow a clinician to determine whether there is damage to the spinal cord and to estimate the extent of a spinal injury if one is present. In this article, we shall look at the embryonic origins of dermatomes and explore their clinical uses.
We can trace the origins of dermatomes back to the 3rd week of embryogenesis. At around day 20, the tri-laminar disc has been established and the middle layer (mesoderm) has differentiated into its different types. The portion of the mesoderm that is directly adjacent to the neural tube is called the paraxial mesoderm.
From day 20 onwards, the paraxial mesoderm differentiates into segments called somites. Initially, there are 44 pairs of somites, however, 13 of these eventually break down leaving 31 somites. This corresponds to the 31 sets of spinal nerves in the body.
Each somite consists of a ventral and a dorsal portion. The ventral portion is made up of the sclerotome, the precursor to the ribs and vertebral column. The dorsal portion is known as the dermomyotome and, over time, the myotome proliferates and the dermatome disperses to form the dermis. During limb growth, the dermis associated with the precursor of the limbs is stretched and moved down the limb, resulting in the segmental innervation associated with the Keegan and Garrett dermatome map of 1948.
There are two main maps that are accepted by the medical profession. The first is the Keegan and Garret map of 1948. This depicts dermatomes in a way that correlates with the segmental progression of limb development. The second is the Foerster map of 1933, which shows that the medial part of the upper limb is innervated by T1-T3, following the distribution of pain from angina or a myocardial infarction (MI). This is the most commonly used dermatome map and is featured on the ASIA scale of assessing spinal injury.
Both maps depict progression of limb growth around an axial line. Across this line there is no overlap between dermatomes, however, those next to each other may have some slight overlap.
Following a traumatic injury resulting in a potential spinal cord lesion, a clinician can use dermatomes to assess the presence and extent of the lesion. Firstly, light touch sensation is tested along the limbs and torso, with the clinician touching areas that correspond to the different dermatomes. Secondly, a small pin is used to test for pain responsiveness. The patient is instructed to close their eyes and say when they feel contact with their skin (to differentiate light touch and pain, as their fibres travel in different parts of the spinal cord – see here).
By using their knowledge of dermatomal and peripheral cutaneous innervation, and noting any areas of paresthesia, the clinician is able to ascertain whether there is any nerve involvement, as well as whether this is at the spinal root or peripheral nerve level.
The overlapping lines of dermatomes, which are responsible for sensory distribution in the body, demonstrate the importance of assessing spinal cord lesions. When determining the presence and extent of a spinal cord lesion, clinicians may use a process of testing dermatomes to understand the impact of an injury.
Firstly, clinicians will use cotton wool to test for light touch sensation along the limbs and torso. Areas corresponding to different dermatomes are touched in this process. Secondly, a small pin is used to assess the patient’s responsiveness to pain. It is important that the patient has their eyes closed during this procedure, and they must inform the clinician when they feel contact with their skin. It is because light touch and pain typically travel through different parts of the spinal cord.
By utilizing their knowledge of dermatomal and peripheral cutaneous innervation, as well as any present paresthesia, clinicians can determine whether any nerve involvement exists. Furthermore, they can figure out if the incident has caused damage to a spinal root or peripheral nerve.
Assessing spinal cord lesions is a complicated process for medical professionals. It requires a detailed knowledge of the anatomy of the human body, as well as skill in the application of certain procedures and tests. Clinicians must take note of any sensory deficits, as well as measure the reflex responses of the patient. All these factors must be considered to accurately assess the extent of a spinal cord injury.
Although the process of assessing a spinal cord lesion can be difficult, it is an essential part of the diagnosis and treatment of the patient. By understanding the anatomy behind the injury, clinicians can make an accurate prognosis as well as determine the best course of treatment.
The ability to assess spinal cord lesions is a valuable tool that clinicians can use to understand and treat these injuries. By taking into account the sensory distribution of dermatomes, clinicians can gain a valuable insight into the nerve damage sustained by the patient.
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