Types of Nerve Injuries
Functional Nerve Block
This is the most common individually experienced temporary nerve injury. When any person rests his or her elbow on a counter and the arm “falls sleep”, what has actually occurred is a temporary block of conduction of the ulnar nerve (the” funny bone”) from the compression. Normally, as soon as the compression is removed, the “feeling” returns to the arm and hand.
There are some occasions that even after the compression is removed, the nerve function never fully recovers. The most common scenario for this is the so called “Saturday night palsy”. The individuals who suffer this disorder are typically alcoholics who are so inebriated that they rest their elbow on a hard surface when they are “passed out”. If the elbow remains compressed on this hard surface for a period of time, the nerve will be permanently damaged. The amount of compression and the length of time the compression is present are the two determining factors for recovery.
Immediate recovery of function can occur after a surgical decompression of the nerve in the spinal canal. The immediate return of function is from this effect.
Injury to the Myelin Sheath Only
The myelin sheath is as noted earlier, the insulation around the nerve. Not every nerve has this insulation but most important nerves do. When the myelin sheath is damaged but the nerve is still intact, the nerve may conduct its message slowly or not conduct at all (called a complete conduction block). Fortunately, the myelin sheath which is made up of Schwann cells will generally heal given time. This time period for healing can take between 8 and 14 weeks. Only time will tell if the nerve will regain conduction ability as there is no way to determine if the nerve has this type of injury.
Axonal Regeneration (Regrowth of the Nerve Cell)
If the nerve cell itself is disrupted from a traction of compression injury but the insulation (myelin sheath) is still left intact, the nerve can regenerate. After the initial injury to the nerve and a delay of a day or two, the cut end of the nerve nearest the brain will develop new sprouts that will grow down the surface of the lining insulation cells (myelin sheath made up of Schwann cells). While this regrowth is going on, the cut end of the nerve that is farthest away from the brain will die back to the farthest endpoint of the nerve. This process is called Wallerian degeneration and even though the nerve dies back, the insulating tube will be left intact. The nerve ending may terminate in a muscle, a sensory organ (skin sensation) or a proprioceptive organ (coordination sensation that allows the brain to know where all its parts are in space). Again, if the insulating myelin sheath is still intact, the side of the nerve cell that is still alive (closest to the brain) can grow down this intact insulating sheath. This nerve will grow about 1mm/day or about one inch per month toward the end of the sheath.
Once the nerve grows down this tube to its end, it will reattach to the organelle it once was attached to. This means that sensation can become normal again, muscle strength will return or incoordination will be reduced.
There are three caveats to return of function. If the nerve and the insulating sheath are both damaged, the nerve will not have a pathway to regrow to the end organ. The cut end of the nerve nearest the brain will still grow new sprouts but these sprouts will have nowhere to go. These sprouts will “ball up” into a tangle of nerves called neuroma and can become quite sensitive to pressure and even touch. Individuals who have had a Morton’s neuroma in their foot will attest to the pain that can be generated.
The second caveat is that there is a time factor for regrowth. A muscle cell that has no nerve supply will atrophy and fibrose (turn into scar tissue). This occurs between 12 and 18 months. If the regrowing nerve has not reached the endpoint muscle cell in about 12 months, it will have no functioning muscle cell to work with. This means that very long nerves (L5, S1, C8, T1) have a much poorer chance of joining their target organelle and functioning normally again.
The third caveat is that injured nerve cells in the central nervous system (CNS) do not generally regrow as there are inhibitory factors within the spinal cord cells that prevent this regrowth. The oligodendrocytes (the support cells in the central nervous system that serve the same function as of the Schwann cells in the peripheral nervous system) inhibit regrowth.
Terminal Axonal Sprouting (Nerve Budding)
When the nerve is injured, some of the muscle cells lose nerve supply in a condition called “denervation”. Due to this condition, the internal workings of the muscle cell changes significantly. The denervated muscle starts to become irritable. In fact, three weeks after the nerve is disconnected from the muscle, the muscle demonstrates irritability changes that can be picked up on an EMG (electromyography), a nerve test for the muscle cell.
This irritability causes the muscle to secrete a neurochemotactic factor, essentially a chemical cry for help. Any nerve that is still functional nearby this muscle cell will form a new nerve branch in a process called budding or sprouting. These sprouts can connect to at least ten denervated muscle cells. This sprouting activity can take from 12-16 weeks after the initial injury to occur.
There have been some studies that indicate the use of electrical muscle stimulation (EMS) during this period will decrease the secretion of the neurochemotactic factors. Therefore, stimulation of muscle contraction through EMS should be avoided by therapists working with the recovery of the muscle strength.
Another possibility for motor strength recovery is muscle hypertrophy. Arnold Schwarzenegger is what many individuals imagine for an example of muscle hypertrophy and that thought is not far off. The residual functioning muscle cells can be conditioned to become stronger and last longer. Training is the key for this type of recovery and good results may take three or more months of hard work to achieve success.
To learn more about nerve damage and healing, or to discuss a peripheral nerve injury in more detail, please contact the practice of Dr. Donald Corenman, spine specialist and back doctor serving the communities of Vail, Aspen, Denver and Grand Junction, Colorado.