Muscle overgrowth and sural nerve entrapment
Like other tissues in the body, nerves are susceptible to disease or trauma. This is particularly true of peripheral nerves, which act as a communications highway linking the brain and spinal cord with the rest of the body. As a subtype of peripheral neuropathy, sural nerve damage has important characteristics related to the anatomy and function of the structures involved.
Neuron Types and Anatomy
Neurons, or nerve cells, fit in three major categories: sensory, motor or interneurons. Sensory neurons pick up signals and transmit them to the brain and spinal cord. In contrast, motor neurons receive commands from the brain and spinal cord, which they relay to muscles or glands. Regardless of their function, neurons share certain structural components: a cell body, axon and dendrites. The cell body contains the nucleus and is the seat of a neuron’s metabolism. The axon is a long nerve fiber that carries electrical signals away from the cell body, while the short, multiple projections named dendrites carry incoming signals. An insulating myelin sheath often coats the axon.
A nerve is essentially a cord-like bundle of axons. The term mono-neuropathy refers to a neurological disorder resulting from a single malfunctioning nerve. When several nerves simultaneously become dysfunctional, the resulting disease is polyneuropathy. Scientists and clinicians generally agree that widespread peripheral neuropathy—or polyneuropathy—often involves the sural nerve. However, isolated sural neuropathy—or true mono-neuropathy—is very rare.
In a 2006 issue of the “Muscle & Nerve” journal, physicians described the sural nerve as a purely sensory nerve with an anatomical trajectory that makes it particularly susceptible to injury. This is because of its convoluted downward course to innervate the ankle and outer part of the foot. Indeed, the sural nerve arises from the tibial nerve in the back of the lower leg, passes between the heads of the biggest calf muscle and travels deep in the tissues before running superficially, close to the Achilles tendon.
Clinical data featured in the “Muscle & Nerve” journal point to trauma as the most common cause of sural nerve neuropathy. Possible sources of trauma include: ankle fracture, ankle sprain, vein stripping, knee surgery, long-distance running, ankle laceration, gunshot and external compression. In a 2009 issue of Orthopaedics and Trauma, physicians remarked that nerve compression may result from calf muscle tears as well as muscle overgrowth. Other causes of sural neuropathy may include: vasculitis, or blood vessel inflammation; ganglion cyst; sural nerve biopsy and diabetes.
Signs & Symptoms
Nerve damage—and therefore symptoms—can occur anywhere in the distribution of the sural nerve. Sural neuropathy typically translates into pain and abnormal sensations in the skin of the foot and ankle, such as numbness and tingling. According to the Institute of Neurological Disorders and Stroke, neuropathic pain usually worsens at night, to the point of disrupting sleep.
The National Institute of Neurological Disorders and Stroke lists several diagnostic procedures for peripheral neuropathy, including thorough neurological testing, general physical examination, detailed patient history, blood tests, nerve or skin biopsies and imaging studies. In the American Journal of Physical Medicine & Rehabilitation journal, Dr. Manouchehr and colleagues note that isolated sural neuropathy often poses a greater diagnostic challenge than other types of peripheral neuropathy and requires electro-physiologic evaluation.
For sural neuropathy that results from nerve entrapment or compression, the Orthopaedics and Trauma journal lists surgical release as the best treatment. Sural neuropathy related to ankle problems may require physiotherapy, bracing or reconstruction. Overall, the key to disease management is to eliminate or control the specific causes, treat underlying diseases and address behaviors that may lead to further damage.