Nerve Injuries in Surgery and Nerve and/or Spinal Cord Complications
Nerves are sensitive structures. Nerves are not really like wires we think of in an electrical circuit. They are actually similar to individual ”hoses” made out of permeable membranes that continuously pump ions into and out of the center of the hose. This pumping action keeps the membrane electrically charged and ready to conduct a signal from one end of the nerve to the other. Many of the nerves are also insulated by a myelin sheath somewhat like the plastic coating found insulating wires.
The structure of the nerve membrane is fragile and can be damaged by compression, retraction, stretch, inflammation and even infection. When the membrane is damaged, two separate problems can result. A block of conduction can occur. That is, the signal will not travel down the nerve. Numbness from a blocked sensory nerve or lack of motor strength from a motor nerve block can result. The other injury problem is that the nerve can transmit unwanted signals due to a leaky membrane. This is similar to a short circuit. The damaged nerve can send false messages to the brain by depolarizing (triggering) at the injured site from the leak in the membrane. For example, a pain signal can be transmitted to the brain without damage to any tissue the nerve is connected to. This is called neuropathic pain.
The nerves exit the spine and “attach into” the leg and arm through the insertions to muscles, tendons and joints. A nerve can be injured simply by a slip and fall when the leg or arm becomes overextended. This action stretches the nerve beyond its normal length. Nerves are designed to stretch somewhat but have stretch limitations. Nerves can also fibrose or develop scar tissue around them which blocks the conduction of the signal.
Nerves can become stretched when the spine is “put back into normal alignment” during surgery to straighten out a deformity of the spine (scoliosis or hyperkyphosis). Rarely, the simple act of retracting a nerve can cause some pain and dysfunction. It depends upon how sensitive the nerve is at the time. Manipulation of a nerve is part of the procedure in spine surgery and most often, the nerve tolerates retraction quite well. In fact, nerves are designed to be able to stretch at least ½ inch from their normal resting position. This is due to the increased tension on the nerve when an arm or leg is moved forward or backwards.
A nerve that is injured from compression (a common scenario with a disc herniation or bone spur) may not fully recover even after surgery that was designed to decompress the nerve. Under very careful and meticulous surgical manipulation, a nerve can still become damaged. The nerve injury may heal quickly or unfortunately may generate long standing symptoms.
BMP (Bone Morphogenic Protein)
The use of bone morphogenic protein for fusion can cause nerve irritation. BMP is a growth factor that induces bone formation and can also promote inflammation which aggravates nerves. In surgery, great care is taken to keep the BMP away from the nerve but rarely, some of the BMP protein can leech out of the area it is placed and contaminate a nerve. The result is normally a nerve irritation that can last as much as 3 months but there have been some reported cases of permanent irritation.
Instrumentation can also cause nerve irritation or damage. The nerves lie on either side of the vertebral pedicle. Screws are commonly placed into the pedicle, used to gain control of the position of the vertebra and immobilize the segments. Although rare, inadvertent misplacement of a screw in a pedicle can compress or injure the nerve or even the spinal cord. There can be unintended direct damage to the nerve roots through placement of pedicle screws. The O-arm intraoperative CT scanner and Stealth device helps to significantly reduce the risk of this potential complication.
Cervical Cord and Root Injuries
The spinal cord is a very sensitive structure and easily disturbed and injured. Cervical spine surgery has to be very precise and specific. Some patients have such tight (stenotic) canals that the act of opening the canal to decompress the spinal cord can cause injury. The procedure called a laminoplasty, performed for severe narrowing of the spinal canal, opens the back of the canal to make more room for the cord. This procedure allows the cord to drift backwards into the open space. This drift can allow stretch of the C5 nerve roots and can cause temporary and rarely permanent dysfunction of these roots.
Arachnoiditis is inflammation of the membrane that surrounds the nerves in the canal. Arachnoiditis is a combination of two words taken from the Latin root. “Arachnoid” means spider and “itis” means inflammation of. The arachnoid membrane has hundreds of small spider web type attachments from nerve to nerve and this explains why it was named as such.
There are three membranes, the dura mater, pia mater and the arachnoid that surround the spinal cord and nerves. The dura mater, which is the outer most membrane, contains the cerebral spinal fluid (CSF) and nerves or spinal cord depending upon the level of the spine. The pia mater lines the inside of the dural membrane. The final arachnoid layer “coats” the nerve roots within the dural sac.
The pathology of arachnoiditis is that the small spider web connections scar, contract and stick the nerves together in clumps. This reduces the nerve membrane exposure to the CSF and injures the small blood vessels that line and feed these nerves. This lack of blood circulation and the direct scar injury to the nerve can cause chronic nerve discharge or block signals. The nerves also do not have normal excursion or movement when scarred. A nerve without scar can move back and forth in the canal up to an inch when the leg is flexed or extended, just like a cable moving over a pulley. If the nerve is tethered by scar, it cannot stretch and the tension on the nerve can cause symptoms.
Arachnoiditis can rarely occur with any type of spinal surgery in the lumbar spine. Interestingly enough- like many conditions of the spine, some people have this condition and never develop symptoms.
The dura is the thin, tissue-paper like membrane that encases the nerve roots in the lumbar spine and spinal cord (located up in the thoracic and cervical spines) and contains the cerebral spinal fluid. During spine surgery, there exists the possibility that a small rent (or tear) of the dura will occur. A tear of the dura can occur for a variety of reasons. Most commonly, it occurs when attempting to remove material such as bone, ligament (ligamentum flavum), cyst, or disc material that is scarred or stuck to the dura.
The incidence of dural tear increases with a history of prior surgery in the same area from scar formation. A dural tear is a relatively minor complication of spine surgery that rarely causes a long term problem. The dura membrane can leak after surgery however if a small hole is present. Suturing the small rent in the dura generally repairs this problem. The nerve roots can be exposed with a larger dural tear. This exposure and the resultant manipulation to reduce the nerves into the sheath and repair the dura can induce nerve injury or arachnoiditis.
- Symptoms of a dural leak: most commonly, patients describe a positional headache in which standing aggravates and laying down improves and/or resolves the headache. The headache can vary in intensity, ranging from mild to severe. It often involves the forehead region, yet can involve the back of the head or simply be widespread/diffuse. Other commonly reported symptoms include the following: nausea, dizziness, ringing in the ears and a feeling of malaise/fatigue. Physical exam findings may note a swollen incision.
- Treatment: Almost always, a dural leak is discovered immediately as it occurs. In very rare cases, a dural leak may occur in the post-operative period and thus not be discovered in surgery. When a dural leak occurs in surgery, it can normally be sutured (primary closure) and/or be sealed with a variety of products on the market such as Tisseal and Duragen. In regard to the fluid that has leaked out, it is comforting to know that your body produces approximately 500cc of cerebrospinal fluid (CSF) daily while the circulating volume of CSF is 90-150 cc – and only a small portion of this fluid typically leaks out when a dural leak occurs. Following surgery in which a dural leak has occurred and has been repaired, it is recommended to lay flat for 12-24 hours (to be determined by your doctor). Similar to reducing the pressure within a water balloon, this flat position helps reduce the pressure within the dura, thereby enhancing the chance of healing. Patients who have had a dural leak repaired during surgery and followed by strict bed rest typically experience minimal or no symptoms. Symptoms, if they do occur, are listed above. Further treatments, if a dural leak were to persist despite primary repair during surgery, include the following: blood patch, use of caffeine, and consideration of a ‘diverting drain’. On rare occasions, re-operation must be considered. A blood patch involves drawing blood from the arm and having it injected into the space just outside the spinal canal. The technique is similar to an epidural steroid injection, and is designed to essentially form a blood clot to act as a patch.