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By Kevin K. Haussler, DVM, DC, PhD, DACMSR

Dermatomes are specific areas of skin that are primarily supplied by single spinal nerves. In horses, dermatomes are essential for diagnosing neurological conditions and guiding therapeutic interventions, including acupuncture and myofascial treatments.

Embryology

As spinal nerves begin to form embryologically, they progressively elongate and provide innervation to skin (i.e., dermatomes), muscles and fascia (i.e., myotomes), and bones and joints (i.e., sclerotomes). 

Spinal nerves originate from specific spinal cord segments, which allow neuroanatomic localization of individual nerves as they exit the respective intervertebral foramen. However, like blood vessels, the spinal nerves then branch extensively into the surrounding tissues with overlapping innervation patterns. This is especially evident as several spinal nerves merge to form the brachial plexus and lumbosacral plexus.

Dermatomal maps

While a general dermatomal pattern is similar within a species, significant variation can exist in the dermatome maps between individual patients. In humans, discrepancies exist among published dermatome maps based on the methodologies used to identify skin segment innervation.

Autonomous zones: These are regions of the skin that are primarily supplied by a single spinal nerve without significant overlap from neighboring dermatomes. The sensory changes in these zones can be particularly telling because any impairment to these areas often directly reflects a problem with the corresponding spinal nerve. 

For example, injury to a specific spinal nerve would lead to sensory deficits localized to its autonomous zone, offering valuable clues for diagnosis. Autonomous zones are known to only be present within certain regions of the fore and hind limbs in horses.

Overlapping zones: Dermatomes do not operate in isolation; they often overlap with one another. This overlap means that a single area of skin may be innervated by multiple spinal nerves. 

The overlapping of sensory input can be a safeguard against complete sensory loss in cases of nerve damage, but it can also complicate diagnostic efforts. 

For example, a lesion affecting one nerve might show only mild sensory changes in a region because the sensory input is still partially maintained by adjacent dermatomes. 

Testing for sensory loss in specific dermatomal patterns helps to determine the level of spinal cord involvement or nerve root damage. However, due to significant overlap between adjacent dermatomes, complete sensory loss typically requires damage to multiple neighboring spinal nerves.

Cutaneous innervation

Horses have 8 cervical, 18 thoracic, 6 lumbar, 5 sacral, and multiple caudal spinal nerves. Each of these nerves supply a dermatomal region, creating a comprehensive sensory map across the horse’s body. 

While most people associate dermatomes with nociceptive stimulation (i.e., pain or numbness), it must be remembered that cutaneous innervation also includes mechanoreceptors (e.g., touch, pressure, vibration), chemical, and thermal receptors. In addition, sympathetic innervation is responsible for changes in blood vessel tone (i.e., vasoconstriction) and local sweat patterns.

Segmental changes

Changes in sensory perception within dermatomes can manifest as hypersensitivity, numbness, or altered sensation. 

For example, a horse with a neurological disorder affecting a specific dermatome might exhibit heightened sensitivity or an inability to perceive touch in that area. These clinical signs are crucial diagnostic indicators that help to localize and diagnose spinal cord and peripheral nerve disorders. Spinal reflexes involve sensory (i.e., dermatomal) stimulation, spinal cord synapsis, and motor responses (e.g., spontaneous muscle contraction). The lack of local sensory or motor responses may be indicative of spinal nerve injury or dysfunction. 

Clinical significance

Dermatomes are useful to help localize neurologic deficits, especially when an isolated spinal nerve is compressed or irritated as evidenced by pain or sensory loss within the corresponding dermatome. Neurologic assessment is conducted by tracing the affected dermatome back to a spinal nerve, vertebral segment, and the associated spinal cord segment. 

A manual therapist often evaluates spinal segmental function and its relationship to specific dermatomes. This involves evaluation of vertebral segment joint ranges of motion and local soft tissue changes in tone or texture. The goal is to identify specific vertebral segments or soft tissue restrictions that correlate with changes in dermatomal sensory patterns.

Diagnostic methods

Manual therapy: Dermatomes help identify areas of sensory dysfunction or hypersensitivity, guiding manual therapists in locating specific regions for treatment, such as myofascial release or deep tissue therapy. By testing dermatomal areas, therapists can identify abnormal cutaneous sensations or pain patterns. Different types of cutaneous stimulation (e.g., light touch, pinpoint, or deep pressure) can produce variable patient responses.

Acupuncture: The application of gentle mechanical stimulation is often used to localize sites of increased sensitivity associated with reactive acupuncture points, many of which are associated with cutaneous nerves.

Neurologic examination: Veterinarians conduct thorough neurological exams to assess sensory changes associated with specific dermatomes. This includes testing responses to stimuli (such as touch or pinprick) across different regions of the body.

Diagnostic imaging: Thermography is a useful modality for identifying potential sources of neurological impairment affecting dermatomes due to changes in superficial blood flow and visualized heat patterns.

Electrodiagnostics: Electromyography (EMG) and nerve conduction studies can assess nerve function and identify abnormalities affecting dermatomal areas. These tools are the only reliable methods for mapping dermatomal innervation patterns in animals.

In closing…

Understanding dermatomes in horses is crucial for effectively diagnosing and treating neurological disorders. A working knowledge of overlapping and autonomous zones is essential for accurately diagnosing spinal injuries or nerve compression. Through a combination of neuroanatomical knowledge, clinical assessment, advanced diagnostics, and integrative therapies like acupuncture and myofascial treatments, practitioners can provide comprehensive care tailored to the individual neurological needs of horses.