What is the difference between peripheral nerves and dermatomes

A group of muscles that is innervated by the motor fibers that stem from a specific nerve root is called a myotome. An area of the skin that is innervated by the sensory fibers that stem from a specific nerve root is called a dermatome. These patterns of myotome and dermatome are almost always identical from person to person.

There are occasionally variances, but that is rare. This consistency allows doctors to treat nerve pain in patients. If a certain area is hurting, they know that it is attributed to a certain myotome or dermatome, whichever the case may be, and its corresponding nerve root. Problems with nerve damage is often the result of stretching the nerve or compressing it. When the nerves are injured in specific areas like the lumbosacral or brachial plexus, it presents as sensory and motor deficits in the limbs that correspond to them.

Myotomes and dermatomes are used to assess the extent of damage. When a doctor tests for nerve root damage in a patient, he or she will often test the myotomes or dermatomes for the nerves assigned to that location. A dermatome is tested for abnormal sensation, such as hypersensitivity or lack of sensitivity. This is done by using stimulus inducing tools such as a pen, paper clip, pinwheel, fingernails, cotton ball, or pads of the fingers. Thus, each spinal nerve relays sensation from a particular region of the skin to the brain.

Most importantly, dermatome innervation is unique to each individual, similar to a fingerprint. The disease symptoms occurring in dermatome indicate pathology related in the nerve root.

Therefore, dermatomes are useful in determining whether the sensory loss on a body part is corresponding to a single spinal segment. Moreover, dermatomes are helpful to find out the presence and the extent of a spinal cord lesion. Cutaneous innervation refers to the area of the skin which has a nerve supply by a specific cutaneous nerve. Thus, cutaneous nerves are primarily responsible for providing sensation to the skin.

Cutaneous nerves can be mainly sympathetic and autonomic afferent sensory fibres. There are many different cutaneous nerves present in our body. A dermatome is an area of the skin which is supplied by a single spinal nerve. Meanwhile, the cutaneous innervation refers to an area of the skin innervated by a specific cutaneous nerve. So, this is the key difference between dermatome and cutaneous innervation.

Cutaneous nerves are the nerves that provide nerve supply to the skin. There are many cutaneous nerves in our skin. Cutaneous innervation refers to an area of the skin supplied by a specific cutaneous nerve. Meanwhile, the dermatome is a specific area of the skin which receives nerve supply by a spinal nerve. It is a kind of cutaneous innervation. With regards to peripheral nerve lesions this blog will discuss only the sensory deficits and how we might be able to distinguish the pattern of sensory loss related to a spinal nerve level or peripheral nerve lesion.

As clinicians it is pertinent that we can differentiate between cutaneous nerve distributions and spinal dermatomes in order to understand whether the lesion is affecting a spinal nerve or a peripheral nerve. Bilateral simultaneous stimulation is not required for peripheral testing - unless you are unsure if the patient is a reliable source of information.

Other aspects of sensory assessment such as proprioception, stereognosis, graphaesthesia etc. What is important is to test light touch discrimination between areas of the affected limb. This is most commonly done in a dermatomal pattern. What you may notice from the two images below is how the dermatomal pattern of a spinal level differs from a peripheral cutaneous nerve.

This is something we all learnt in our training but it is easy to forget. You can also keep a picture on your desk at work in case you forget. Dermatome map of the body. Image source: Google Images.

Clinically, we can often differentiate between upper and lower motor neurone lesions from the subjective history.

For example;. The history is crucial in understanding the mechanism of injury, nature, severity of symptoms, and contributing medical conditions. It is generally recommended that you begin your assessment with light touch. This is a fine discriminatory sense and if this sense is impaired then deeper sensations will often too be impaired as they belong to the same sensory tract dorsal medial lemniscus tract.

For example, if you cannot localise light touch, then the ability to localise joint position is often unlikely proprioception.

As you may have read from the previous blog on sensation , it is normally conducted in a distal-to-proximal fashion as this saves time and allows for multiple dermatomes to be tested simultaneously without testing the entire dermatome distribution. If a deficit is noted, then further examination is required to specifically map out the impairment. Essentially the first aim is to identify is sensation is intact or impaired, and the second aim is to map out the distribution of sensory loss on the given limb and determine if this is in the distribution of a cutaneous nerve, spinal nerve or multiple nerves.

Just as it is important to know the mapping of dermatomes and cutaneous nerves in the detection of light touch, it is also important to know the expected distances for two-point discrimination in the body to allow for accurate interpretation of the information we gain from the assessment. The final aspect of sensory testing that I wanted to cover with this blog is terminology O'Sullivan et al.

Here are a few words When I think back to my university days, I remember being quite overwhelmed with the number of sensory tests we learnt in neurology and musculoskeletal subjects. Always remember that when put in the context of a full neurological screen deep tendon reflexes, strength and sensation the clinical patterns are much easier to recognise. The focus initially is to rule out any sensory impairment during a neurological exam and once sensory impairment has been detected, you can then be more specific in your mapping of the distribution and description of the symptoms.

New York: McGraw Hill.