Dermatomes: An Overview
Dermatomes are areas of skin that are primarily supplied by sensory nerves from a specific spinal nerve root. Each spinal nerve (except the first cervical nerve) corresponds to a specific dermatome, which is a region of skin that can be tested for sensory function. Dermatomes help map the body's nerve supply and are used to assess the neurological function of the spinal cord and nerves.
There are 31 pairs of spinal nerves, and each pair is associated with a specific dermatome. The skin areas they cover often overlap slightly, meaning that sensations from multiple nerves can contribute to feeling in any given region of skin.
How to Test Dermatomes
Testing dermatomes is done to assess the sensory function of specific spinal nerves. Here are the common methods used for testing:
Light Touch: A cotton ball or piece of tissue is lightly brushed over the skin in specific dermatome areas to assess sensitivity. The patient is asked to confirm whether they feel the touch and whether it feels normal.
Pinprick (Pain Sensation): A pin or similar object is lightly pressed against the skin in different dermatome areas to test for pain sensation. The patient should be asked if they feel pain or whether the sensation feels dull or sharp.
Vibration: A tuning fork is placed on various bony prominences in specific dermatomes to test the vibration sense. The patient is asked if they can feel the vibration, and how long they feel it.
Temperature: A cold or warm object can be applied to the skin to assess temperature sensation in different dermatomes. The patient is asked to identify whether the temperature feels normal.
Two-Point Discrimination: This test involves applying two points to the skin and asking the patient to report whether they feel one or two points of contact. This helps assess the fine touch and tactile sensory function of the dermatomes.
What Does It Mean if One Experiences an Abnormal Response?
If a patient experiences an abnormal response during a dermatome test, it could indicate a sensory nerve problem, which might be due to various neurological issues. Some possible causes include:
Nerve Compression or Injury: If a specific dermatome does not respond appropriately to sensory tests, it may suggest compression, irritation, or damage to a particular spinal nerve root or the nerve itself. This could result from conditions like herniated discs, spinal stenosis, or trauma.
Radiculopathy: This refers to nerve root compression, often caused by a herniated disc, that leads to pain, numbness, or tingling in a specific dermatome. For example, cervical radiculopathy might affect the arm or hand, while lumbar radiculopathy could affect the leg.
Peripheral Neuropathy: Damage to the nerves outside of the spinal cord can lead to sensory deficits. For example, diabetic neuropathy or alcohol-related neuropathy can result in altered sensations in the feet or hands.
Spinal Cord Lesions: Damage to the spinal cord, whether from trauma, infection, tumours, or multiple sclerosis, can affect the sensation in certain dermatomes. This might cause complete loss of sensation or abnormal sensations such as tingling or numbness.
Central Nervous System Disorders: Conditions such as stroke, multiple sclerosis, or tumours affecting the brain or spinal cord could alter sensation in multiple dermatomes, potentially leading to mixed sensory loss across different regions of the body.
Infection: Some viral infections, such as shingles (herpes zoster), affect specific dermatomes, causing painful rashes and altered sensations in the area.
Interpretation of Results
Normal Response: If the patient responds correctly in all dermatomes, sensory function is considered to be intact.
Abnormal Response: If the response is reduced, increased, or absent in a specific dermatome, it could indicate nerve damage or irritation at the level of the spinal nerve or in the nerve pathways leading from the spinal cord to the brain.
Loss of sensation could suggest nerve injury or compression at the spinal level.
Hypersensitivity (increased sensation) may indicate conditions like nerve irritation or neuropathy.
Conclusion
Dermatomes are important for assessing the sensory function of the nervous system, especially when diagnosing nerve-related or neurological issues. If a dermatome test response is abnormal, it can indicate problems ranging from nerve compression, radiculopathy, or neuropathy to more complex issues like spinal cord lesions or central nervous system disorders. A thorough examination, including other neurological tests, will help determine the underlying cause of the abnormal responses.
