Axonotmesis
Encyclopedia
Axonotmesis is a disruption of nerve cell axon, with Wallerian degeneration
occurring below and slightly proximal to the site of injury. If axons, and their myelin sheath are damaged, but schwann cells, the endoneurium
, perineurium
and epineurium
remain intact is called axonotmesis. Axonotmesis is usually the result of a more severe crush or contusion than neuropraxia.. In axonotmesis, the proximal section is repaired by creating a sprout with its growth cone
, but in the distal section occurs axonal degeneration. The rate of outgrowth of regenerating nerve fibers is about 1 mm to 2 mm per day, so that the recovery of conduction to a target structure depends on not only regrowth into the appropriate endoneurial tube (endoneurium
), but also on the distance involved.
Assessment items include:
In assessment, sensory-motor defects may be mild, moderate, or severe. Damage to motor fibers results in paralysis of the muscles. Nervous plexus injuries create the more signs and symptoms from sensory-motor problems (such as brachial plexus injuries). In these cases, the prognosis depends on the amount of damage and the degree of functional recovery.
Electromyography
(EMG) is a medical test performed to evaluate and record the electrical activity (electromyogram) produced by skeletal muscles using an instrument called electromyograph. In axonotmesis, EMG changes (2 to 3 weeks after injury) in the denervated muscles include:
Changes in NCV (nerve conduction velocity
):
EMG test is often performed together with another test called nerve conduction study, that measures the conducting function of nerves. NCV study shows loss of nerve conduction in the distal segment (3 to 4 days after injury). According to NCV study, in axonotmesis there is an absence of distal sensory-motor responses.
Wallerian degeneration
Wallerian degeneration is a process that results when a nerve fiber is cut or crushed, in which the part of the axon separated from the neuron's cell body degenerates distal to the injury. This is also known as anterograde degeneration, or orthograde degeneration...
occurring below and slightly proximal to the site of injury. If axons, and their myelin sheath are damaged, but schwann cells, the endoneurium
Endoneurium
The endoneurium, also referred to as an endoneurial channel, sheath or tube, is a layer of delicate connective tissue made up of endoneurial cells that encloses the myelin sheath of a spinal cord nerve fiber. These are bundled up into groups called nerve fascicles, which have a protective sheath...
, perineurium
Perineurium
In the peripheral nervous system, nerve fibers are each wrapped in a protective sheath known as the endoneurium. These are bundled together into groups known as fascicles, each surrounded by a protective sheath known as the perineurium. Several fascicles may be in turn bundled together with a blood...
and epineurium
Epineurium
The epineurium is the outermost layer of connective tissue surrounding a peripheral nerve. It is made of dense irregular connective tissue and usually contains multiple nerve fascicles as well as blood vessels which supply the nerve...
remain intact is called axonotmesis. Axonotmesis is usually the result of a more severe crush or contusion than neuropraxia.. In axonotmesis, the proximal section is repaired by creating a sprout with its growth cone
Growth cone
A growth cone is a dynamic, actin-supported extension of a developing axon seeking its synaptic target. Their existence was originally proposed by Spanish histologist Santiago Ramón y Cajal based upon stationary images he observed under the microscope...
, but in the distal section occurs axonal degeneration. The rate of outgrowth of regenerating nerve fibers is about 1 mm to 2 mm per day, so that the recovery of conduction to a target structure depends on not only regrowth into the appropriate endoneurial tube (endoneurium
Endoneurium
The endoneurium, also referred to as an endoneurial channel, sheath or tube, is a layer of delicate connective tissue made up of endoneurial cells that encloses the myelin sheath of a spinal cord nerve fiber. These are bundled up into groups called nerve fascicles, which have a protective sheath...
), but also on the distance involved.
Assessment
A peripheral nerve contains sensory fibers, motor fibers, or both. Sensory fibers lesions cause the sensory problems below to the site of injury. Motor fibers injuries may involve lower motor neurons, sympathetic fibers, and or both.Assessment items include:
- Sensory fibers that send sensory information to the central nervous systemCentral nervous systemThe central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
. - Motor fibers that create the contraction of skeletal muscleSkeletal muscleSkeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
. - Sympathetic fibers that innervate the skin, and blood vessels of the upper and lower limbs.
In assessment, sensory-motor defects may be mild, moderate, or severe. Damage to motor fibers results in paralysis of the muscles. Nervous plexus injuries create the more signs and symptoms from sensory-motor problems (such as brachial plexus injuries). In these cases, the prognosis depends on the amount of damage and the degree of functional recovery.
EMG and NCV findings
Changes in EMG:Electromyography
Electromyography
Electromyography is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph, to produce a record called an electromyogram. An electromyograph detects the electrical potential generated by muscle...
(EMG) is a medical test performed to evaluate and record the electrical activity (electromyogram) produced by skeletal muscles using an instrument called electromyograph. In axonotmesis, EMG changes (2 to 3 weeks after injury) in the denervated muscles include:
- Fibrillation potentials (FP)
- Positive sharp waves
Changes in NCV (nerve conduction velocity
Nerve conduction velocity
Nerve conduction velocity is the speed at which an electrochemical signal propagates down a neural pathway. Many things can affect this, including axon diameter, myelination, the internal resistance of the axon, and temperature. Nerve conduction velocity differs from species to species, and to a...
):
EMG test is often performed together with another test called nerve conduction study, that measures the conducting function of nerves. NCV study shows loss of nerve conduction in the distal segment (3 to 4 days after injury). According to NCV study, in axonotmesis there is an absence of distal sensory-motor responses.
Treatment
Treatment of axonotmesis consists of :- Physical therapyPhysical therapyPhysical therapy , often abbreviated PT, is a health care profession. Physical therapy is concerned with identifying and maximizing quality of life and movement potential within the spheres of promotion, prevention, diagnosis, treatment/intervention,and rehabilitation...
. Physical therapy aims include:- Pain relief
- Maintain range of motion
- Reducing muscular atrophy
- Patient education
- Use of assistive devices (Orthotic needs)
- Surgical treatment (For example, in cases of surrounding scar tissue)
Prognosis
The prognosis is usually good in terms of recovery. Rate of recovery depends on the distance from the site of injury, with axonal regeneration occurring at 1 to 4 mm/day. Peripheral nerves regeneration may take several months.See also
- Seddon's classification
- NeurapraxiaNeurapraxiaNeurapraxia is a disorder of the peripheral nervous system in which there is a temporary loss of motor and sensory function due to blockage of nerve conduction, usually lasting an average of six to eight weeks before full recovery...
- NeurotmesisNeurotmesisNeurotmesis is part of Seddon's classification scheme used to classify nerve damage.It is the most serious nerve injury in the scheme.In this type of injury, both the nerve and the nerve sheath are disrupted....
- Nerve fiberNerve fiberA nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system , myelin...
- Peripheral nerve
- Neuroregeneration
- Connective tissue in the peripheral nervous systemConnective tissue in the peripheral nervous systemA peripheral nerve contains two types of tissue: nerve fibers, and connective tissue. Dendrites and axons with schwann cells and myelin sheath are surrounded by connective tissue. A nerve fiber in the peripheral nervous system consists of an axon or long dendrite, myelin sheath and their schwann...
- Wallerian degenerationWallerian degenerationWallerian degeneration is a process that results when a nerve fiber is cut or crushed, in which the part of the axon separated from the neuron's cell body degenerates distal to the injury. This is also known as anterograde degeneration, or orthograde degeneration...
- Peripheral nerve injuryPeripheral nerve injuryPeripheral nerve damage is categorized in the Seddon classification based on the extent of damage to both the nerve and the surrounding connective tissue since the nervous system is characterized by dependence of neurons on their supporting glia. Unlike in the central nervous system, regeneration...
( Nerve injuryNerve injuryNerve injury is injury to nervous tissue. There is no single classification system that can describe all the many variations of nerve injury. Most systems attempt to correlate the degree of injury with symptoms, pathology and prognosis...
)