Nerve entrapment involves a cascade of physiological changes caused by compression and tension. Some of these changes are irreversible. The magnitude and duration of the forces determines the extent of injury. In the acute form, mechanical injury and metabolic blocks impede nerve function. In the chronic form, there is a sequence of changes starting with a breakdown of the blood-nerve-barrier, followed by edema with connective tissue changes, followed by diffuse demyelination, and finally followed by axonmetesis. The injury will often be a mixed lesion where mild/moderate compression is a combination of a metabolic block and neuropraxia, while severe compression combines elements of neuropraxia and axonmetesis.
Peripheral nerve anatomy
Nerve cell Nerve cells comprise a small cell body and a very long segment called the axon. The cell body resides in the spinal cord and the axon extends all the way to the innervation target of the nerve. Peripheral nerve axons can be longer than 100 cm as they may need to travel along the full length of a limb to reach their innervation target, while the cell body is only 100 micrometers long. Nerves may be myelinated or unmyelinated. Myelinated nerves have the axon covered by segments of schwann cells, which are short and concentrically wrapped around the diameter of an axon to give the appearance of a sausage-like mass and called a myelin sheath. The schwann cells are arranged in pattern such all parts of the axon are wrapped in schwann cells and successive schwann cells are separated by a very small distance. This separation gap is called a node of Ranvier. Unmyelinated nerves are also surrounded by schwann cells but the schwann cells are not wrapped around the axon multiple times to form a myelin sheath.
Nerve fiber The axons of nerve cells are surrounded by various connective tissue layers and bundled together in a structure called a nerve fiber. At the surface of a nerve fiber is a tissue layer called the epineurium or sometimes external epineurium. Within the epineurium there is a connective tissue matrix called the internal epineurium and fascicles. The internal epineurium acts as soft cushion for the fascicles. A nerve fiber may have a variable number of fascicles, but there will be at least one (otherwise there would be no nerve cells). Fascicles are surrounded by a tissue layer called the perineurium which is a protective sheath that acts as a barrier. Inside the fascicles is the endoneurium, a tissue matrix analogous to the internal epineurium, and the nerve cells. The endoneurium has many small blood capillaries (endoneurial microvessels) which directly supply the nerves themselves. These capillaries have tight junctions to prevent the free flow of materials between cells and instead require substances to pass through the endothelial cells.
Blood nerve barrier The peripheral blood nerve barrier is analogous to the blood brain barrier. Like the blood brain barrier, the blood nerve barrier creates a stable, privileged environment where certain substances cannot pass through due to tight junctions. The blood nerve barrier is made up of inner cells of the perineurium and the endothelial cells of the endoneurial microvessels.
Physical forces which cause entrapment Nerve entrapment is caused primarily by two physical forces on soft tissue: compression and tension. Compression will squeeze the nerve and impair its local microcirculatory environment which commonly happens in anatomic tunnels. Tension is a pulling force, often caused by scarring which impedes nerve mobility during limb movements. Both the magnitude and duration of these forces can determine the extent of injury.
Compression
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