Generality
Dermatomers are the portions of skin on which the single spinal nerve fulfills its sensitive functions.
A dermatomer is a well-circumscribed cutaneous area, which rarely (and in any case always very lightly) overlaps with adjacent dermatomers.
In the human being, for each half of the body, there is a very precise dermatomere for each spinal nerve, except for the spinal nerve C1 and the spinal nerve Co1, which are not associated with any dermatome.
Apart from anatomical aspects, dermatomers also have a diagnostic utility; they allow, in fact, to establish the presence and extent of spinal cord injuries, and identify any spinal nerve infections sustained by shingles.
Brief Review of the Spinal Nerves
The spinal nerves are the nerves of the peripheral nervous system that originate from the ventral and dorsal roots of the spinal cord, and are distributed to the left and to the right of the latter.
Spinal nerves occur in pairs (or pairs ); in each pair of spinal nerves, the constituent elements are distributed, in a completely symmetrical manner, one to the right and one to the left of the human body.
In all, the pairs of spinal nerves are 31 and the name of each single pair corresponds to the segment of spinal cord from which it derives (the figure helps to understand the structure of the spinal nerves along the spinal cord).
Spinal nerves belong to the category of mixed nerves, therefore they contain both neurons with sensory functions and neurons with motor functions.
| Spinal nerves | Specific number and name |
| Cervical spinal nerves | There are a total of 8. They are indicated with the letter C and with the numbers from 1 to 8 (C1, C2, C3 etc.). |
| Thoracic spinal nerves | There are a total of 12. They are indicated with the letter T and with the numbers from 1 to 12 (L1, L2, L3 etc.). |
| Lumbar spinal nerves | They are in all 5. They are indicated with the letter L and with the numbers from 1 to 5 (T1, T2, T3 etc.). |
| Sacral spinal nerves | They are in all 5. They are identified with the letter S and with the numbers from 1 to 5 (S1, S2, S3 etc.). |
| Coccygeal spinal nerves | It is only one. It is identified with the acronym Co1. |
What are Dermatomers?
Dermatomers are the portions of skin innervated by the dorsal root of a single spinal nerve.
In light of this and since the dorsal root of the spinal nerves represents the sensory component of these, each dermatome reflects the skin area where the single spinal nerve carries out its sensory function.
For the avoidance of doubt, it should be pointed out that dermatomers are to be understood as ideal subdivisions of the skin, identified after extensive studies on the sensitive control of spinal nerves.
Important note
The symmetry that governs the distribution of the elements of each pair of spinal nerves, to the right and to the left of the human body, results in a symmetry of the dermatomers present on the right half and on the left half of the human body.
Although for some it may seem trivial and immediate, this concept is very important, because:
- It represents a confirmation of the fact that the elements of each pair of spinal nerves have the same distribution to the right and to the left of the human body;
- It allows you to talk about the dermatomers and associated spinal nerves without having to specify each time which half of the human body you are referring to (because, given the present symmetry, it would be superfluous).
Features
- A dermatomer is a very precise cutaneous region, which, on the right half and on the left half of the human body, is innervated by a specific pair of spinal nerves;
- The entire skin surface of the human body is ideally subdividable into dermatomes;
- On the surface of the human body, it is possible to identify only one specific dermatome for each spinal nerve, except for the spinal nerve C1 and for the coccygeal spinal nerve, to which no dermatomer can be associated;
- Usually, there is no overlap between adjacent dermatomers and, where it exists, it is something just mentioned. Therefore, it is possible to affirm that the various dermatomers correspond to well-defined areas;
- The name of each dermatome corresponds to the name of the associated spinal nerve. For example, the dermatomere associated with the I sacral spinal nerve (spinal nerve S1) is called dermatomer S1;
- In the chest and abdomen, the dermatomes appear as a stack of overlapping discs; at the level of the limbs, on the other hand, they have a longitudinal distribution, ie they are arranged along the length (eg: each dermatomer of the upper limbs passes through the arm, forearm and hand).
- Dermatomers are the skin areas where the spinal nerves perform their sensitive functions. This is the reason why the suffering of the spinal nerves (in particular, of their sensitive component) results in an alteration of the sensitivity at the level of the correlated dermatomes.
Development
The embryonic origin of the skin areas corresponding to the dermatomes has as protagonists the so-called somites ; arranged in pairs, the somites are the cellular groupings, derived from the organization of the paraxial mesoderm, which, starting from the 20th day of embryonic life, begin to generate the so-called axile skeleton (ie the vertebral column), the musculature and precisely the skin of dermatomers .
Each somita has a ventral portion and a dorsal portion.
The cells of the ventral portion constitute the so-called sclerotome, whose destiny is to give life to the axial skeleton; the cells of the dorsal portion, on the other hand, constitute the so-called dermomyotome, the purpose, after being divided into myotome and dermatome, is to generate the muscles (through the myotome) and the skin (through the dermatome) of the human body.
Mapping
The distribution of individual dermatomes on the skin surface of the human body is difficult to report in writing; this is why, in recent years, anatomists have developed a simplified subdivision of dermatomers, based on sections of the spinal cord, according to which:
- Cervical dermatomes ; are the dermatomes which reflect the cutaneous innervations of the cervical spinal nerves and which "cover" the skin areas of:
- Back of the head;
- Neck;
- Shoulders;
- Outside of upper limbs and hands.
- Thoracic dermatomes ; are the dermatomes which reflect the cutaneous innervations of the thoracic spinal nerves and which "cover" the skin areas of:
- Inner part of the arms;
- Chest;
- Abdomen;
- Middle part of the back.
- The lumbar dermatomers are the dermatomes that reflect the cutaneous innervations of the lumbar spinal nerves and which "cover" the skin areas of:
- Lower back;
- Front of the lower limbs;
- Exterior of thighs and calves;
- Upper surface and lower surface of the feet.
- Sacral dermatomers ; are the dermatomes which reflect the cutaneous innervations of the sacral spinal nerves and which "cover" the skin areas of:
- Genital region and anal region;
- Back of thighs and legs;
- Outside of the feet.
Is the distribution of the Dermatomers the same for everyone?
The areas of competence of dermatomes on the skin surface are inspired by a valid reference model for all human beings.
However, it is necessary to point out that the precise extension of each individual dermatome varies, even if only slightly, from individual to individual; in a certain sense, dermatomers are equivalent to fingerprints: these are inspired by a common model, but, analyzed in detail, they report different characteristics from person to person.
Dermatomer Maps
Currently, there are two "maps" of dermatomes accepted by the medical profession and used to illustrate their ideal distribution on the human body.
The maps in question are: the so-called " Foerster map ", whose formulation dates back to 1933, and the so-called " Keagan and Garrett map ", dated 1948; between the two, the most used is the first.
| Dermatomer areas of expertise: some special features | |||
| Spinal nerve - Dermatomer | What innervates significant? | Spinal nerve - Dermatomer | What innervates significant? |
| C2 | Occipital region (see occipital bone). | T9 | Point of intersection of the mid-clavicular line with the horizontal line originating three quarters of the distance that separates the sternal xiphoid process from the navel. |
| C3 | Above the supraclavicular fossa, in correspondence of the mid-clavicular line. | T10 | Intersection point of the mid-clavicular line with the horizontal line originating from the navel. |
| C4 | Above the acromioclavicular joint. | T11 | Point of intersection of the mid-clavicular line with the horizontal line originating halfway between the navel and the groin. |
| C5 | Radial side of the ante cubital fossa, just before the elbow. | T12 | Intersection point of the mid-clavicular line with the central area of the inguinal ligament. |
| C6 | Dorsal surface of the proximal phalanx of the thumb. | L1 | Halfway between the T12 and L2 dermatomes. |
| C7 | Dorsal surface of the proximal phalanx of the middle finger. | L2 | Medial portion (internal) anterior of the thigh, halfway between the inguinal ligament and the medial epicondyle of the femur. |
| C8 | Dorsal surface of the proximal phalanx of the little finger. | L3 | Medial epicondyle of the femur. |
| T1 | Ulnar (medial) side of the ante cubital fossa, just before the medial epicondyle of the humerus. | L4 | Above the medial malleolus. |
| T2 | Apex of the armpit. | L5 | Back of the foot, up to the third metatarsophalangeal joint. |
| T3 | Point of intersection of the mid-clavicular line with the third intercostal space. | S1 | Lateral aspect of the calcaneus. |
| T4 | Point of intersection of the mid-clavicular line with the fourth intercostal space (at the level of the nipples). | S2 | In the middle of the popliteal fossa (back of the knee). |
| T5 | Point of intersection of the mid-clavicular line with the fifth intercostal space. | S3 | Above the tuberosity of the ischium (or ischial tuberosity). |
| T6 | Point of intersection of the mid-clavicular line with the horizontal line originating from the xiphoid process of the sternum. | S4 and S5 | Perianal region, less than one centimeter away from the mucocutaneous area. |
| T7 | Intersection point of the mid-clavicular line with the horizontal line originating at a quarter of the distance that separates the sternal xiphoid process from the navel. | - | - |
| T8 | Point of intersection of the mid-clavicular line with the horizontal line originating halfway between the sternal xiphoid process and the navel. | ||
Clinical meaning
Dermatomers have a very useful diagnostic utility.
In fact, thanks to their connection with the spinal nerves, they allow doctors to establish the presence and extent of a spinal cord injury (or spinal cord injury ), and even determine spinal nerve infections sustained by shingles (or Sant'Antonio ).
Dermatomers and Herpes Zoster
To understand…
The spinal ganglia are the small swellings observable on the spinal roots, shortly after their origin, in which all the bodies and the relative cell nuclei of the neurons forming the sensory component of the deriving spinal nerve are contained.
Herpes zoster is a virus capable of nesting, entering a sort of latency state, in the spinal ganglia of the dorsal roots, and awakening from this state of latency, following some specific events (eg: psychophysical stress, excessive exposure to cold, excessive exposure to sunlight, decrease in immune defenses, etc.).
The dermatomers allow to immediately identify the awakening of herpes zoster and the precise spinal nerves involved, because on the dermatomes themselves a characteristic rash (red spots and vesicles filled with liquid) appears extremely painful.
Dermatomers and Spinal Cord Injuries
To assess the presence and extent of a spinal lesion through dermatomes, the diagnostic protocol includes:
- The patient's closing of the eyes in such a way that the latter has no way of seeing the subsequent operations of the doctor;
- The control of superficial cutaneous sensitivity, carried out through the light rubbing of a wad of cotton wool along the different areas of limbs and bust corresponding to the dermatomes.
During this check, the patient's task is only to communicate, by means of a gesture or word, whenever he feels the contact with the cotton wool pad;
- The test of skin reactivity to pain, carried out by pin punctures in correspondence of the various dermatomers.
Similar to the previous evaluation, even during this test, the patient's task is only to report to the doctor, with a gesture or by speech, when he feels the pinprick.
Spinal lesions interrupt the sensitive communication between the spinal nerves and the skin surface connected to them (ie the dermatomers); this means that where the patient does not feel anything, in spite of the sensory stimulation, the connection between spinal nerves and associated dermatomas has failed.
INTERPRETATION OF RESULTS: WHY APPLY FOR TWO TESTS?
Combining the control of superficial cutaneous sensitivity with the test of reactivity to pain is fundamental to understand the severity of the lesion. Indeed:
- In the presence of a mild lesion, the control of superficial skin sensitivity will give a positive result (ie the patient will not feel anything), while the test of pain reactivity will be negative (ie the patient will feel pain);
- In the presence of an important lesion, both the control of superficial skin sensitivity and the test of pain reactivity will give a positive result (ie the patient will not feel anything).
Therefore, the use of both tests is essential to understand, in the presence of a spinal lesion, the severity of the sensory alteration at the level of the dermatomes.
