The normal spinal cord begins at the junction of the skull with the cervical spine. The spinal cord fills the spinal canal throughout the neck and mid-back regions. At the upper portion of the lower back (the lumbar region) the spinal cord itself comes to an end. This is at the level of the first lumbar vertebral body. At this point, the end of the spinal cord is free, i.e. it is not attached to any of the surrounding structures. It is free to move as the person grows and as the lower back moves and bends. From the end of the spinal cord at the first lumbar vertebra level to the very tip of the spine at the level of the lower sacrum, there are only nerves that continue through the remainder of the spinal canal. A "tethered" spinal cord is a spinal cord that is tightly fixed at the distal or lower end so that there is not a normal amount of movement of the lower end of the spinal cord. During the formation of the embryonic spinal cord, the spinal cord fills the entire length of the spinal canal, from the first cervical vertebra (C1) to the end of the sacrum (S5). As the fetus grows during embryonic life, the bones of the vertebral column grow faster than the spinal cord itself. Thus, the distal end of the spinal cord comes to be located at the level of the first lumbar vertebral body (L1). If there is an abnormality affecting this normal "ascension" of the lower end of the spinal cord, something that binds the cord down toward the sacral level, the spinal cord is said to be tethered. This results in a tight pull or stretching on the lower portion of the spinal cord and can cause neurological damage as the tightness increases due to continued growth of the spinal column.
The neurological deterioration caused by tethering is unlikely to stabilize while there is still potential growth of the spine. Growth puts further stretch on the tethered spinal cord. If neurological findings are already present then further deterioration can be anticipated. Children are obviously more at risk than adults. An adult spine is no longer growing so if the neurological condition is stable, there is less chance of change than with a growing child. However, even adults with tethered cords can deteriorate. This deterioration is believed to be due to the daily "wear and tear" on the tethered and stretched spinal cord. Even though the adult is less likely to show neurological worsening, one should always be aware of the possibility of change. Some patients have required untethering surgery in their 50's or 60's.
What are the signs and symptoms of a tethered cord?
The signs and symptoms of tethering may vary from patient to patient. However, the signs and symptoms of a tethered cord are relatively constant despite the underlying cause of the tethering. Some of the more common symptoms are pain in the lower back region,fatigue, change in gait or walking pattern, or recurrent bladder infections. Some patients may not exhibit any symptoms. Signs (findings on examination) may include muscle weakness, sensory loss, incontinence or loss of control of bowel and bladder function and scoliosis or curvature of the spine. Tethering may cause an obvious neurological deficit by the time of birth but many children do not experience neurological changes until growth of the spine causes further stretching and tension on the lower spinal cord. It is important to recognize tethering of the spinal cord at an early age. The majority of patients with a tethered cord will experience deterioration of their neurological function as they grow if they do not have release of the tethering. Once neurological deficits have occurred, many patients will not have recovery of lost function. Since loss of neurological function is often subtle and slowly progressive in these patients, and because it may be difficult to notice changes such as loss of bladder control in an infant, early recognition of tethering is essential. It is important to recognize this condition and treat it early.
- Pain caused by a tethered spinal cord is usually located in the lower back region. It rarely radiates into the legs. The pain is increased with activity and relieved by rest. The typical patient will have to stop during activity to rest so that the pain will subside. This produces an "off again, on again" pattern of pain and rest so that it is usually easily apparent when this type of pain is occurring.
- Weakness. In a patient with a tethered cord, any change in strength of the muscles of the legs or feet is a sign of concern. Many patients will have a certain amount of weakness that has been present throughout their life. However, any deterioration from their baseline examination is a sign of progression of the tethering. There should not be progressive loss of function in a patient with a tethered cord.
- Sensory Loss. Loss of sensation may be seen when a patient is experiencing signs of tethering. The typical sensory loss is in the feet and around the genital area.
- Incontinence, or loss of bowel and/or bladder control, is a common sign of progressive tethering. Children who show early signs of tethering may have never gained good control. Children who gain normal toilet training and then begin to experience incontinence are showing signs of progression of the tethering process. Any documented loss of control is a relatively urgent indication for surgical intervention.
- Scoliosis, or curvature of the spine, is another sign associated with tethered cord. Progressive curvature of the spine in a patient with a known tethered cord is a sign of neurological progression. Occasionally scoliosis will be the initial symptom that there is a tethering process.
Cutaneous Markers. Most patients with a tethered cord have a mark or discoloration of sometype on the skin of their lower back, usually in the midline. This is referred to as a "cutaneous marker". This may be a tuft of dark hair, a red coloration of the skin, a dimple that appears to go deep into the back, a localized subcutaneous collection of fat, or other similar abnormalities. The image below shows two different cutaneous markers in the same patient in the lumbar region of the back: a small area of very thin skin surrounded by a red discoloration and a separate area of red discoloration below. Other than the presence of a cutaneous marker, many children with a tethered cord are neurologically normal in their early years.
Which diagnostic tests are important?
When evaluating the spinal cord, the most helpful diagnostic images are obtained by Magnetic Resonance Imaging (MRI). These images give a clear view of the spinal cord and distal nerve roots as well as the surrounding structures. Since the images can be formatted in 3 different planes, a clear understanding of the anatomy is more easily achieved. Other imaging studies, including plain x-rays and computed tomography (CT) scans may also be helpful.
What is the usual treatment for tethering of the spinal cord?
Once signs and symptoms of spinal cord tethering are present, especially if progressive, conservative (non-surgical) management is rarely effective. Rest and physical therapy may be effective in relieving some of the early symptoms but the young patient is essentially doomed to worsen as time passes. The only effective treatment is surgical untethering of the underlying cause. The surgery for untethering of a spinal cord has two goals: first, to stop any further neurological deterioration, second, to hopefully see some improvement in any lost function. Once neurological function is lost it may never recover. This is why surgery is indicated when there are early signs of neurological change.
Although the causes of tethering vary, the general principles of the surgery are similar. The overwhelming majority of tethering is in the lumbar or sacral regions of the spine so the surgery is usually performed on the low portion of the back. A midline incision is made in the skin. The muscles are retracted away from the midline and exposure of the bones of the posterior spine is accomplished. The spinous process and arch of bone covering the posterior or back portion of the spinal canal (lamina) is removed in order to gain exposure to the cause of the tethering. The dura, the covering over the spinal cord and nerves) is opened, exposing the tethering process. The operation is usually done through the operating microscope or with the surgeon wearing special magnifying glasses so that vision is enhanced. Lasers or other specialized surgical equipment may be used. Depending on the cause of the tethering, the surgery may last many hours. Six to eight hours is not uncommon in complex cases. This operation is not one that usually has a lot of bleeding. Blood transfusions are rarely required.
The patient may be kept flat in bed for 24-48 hours. After that there is usually no need for prolonged bed rest. Depending on the neurological condition of the patient and the extent of the surgery, the patient may be restricted from vigorous activity for a few weeks.
Common Causes of Tethering of the Spinal Cord
Tethering of the spinal cord is almost always caused by a congenital condition, i.e. a birth defect. It occurs during the first 4-6 weeks of pregnancy. Tethering does not have one single cause. There are several different causes of spinal cord tethering. Although the underlying cause may vary, the signs and symptoms of tethering are generally the same among the various causes. Some of the more common causes of a tethered cord are as follows:
Lipoma or lipomyelomeningocele causes tethering due to fatty tissue connecting to the lower end of the spinal cord. This produces tethering by the attachment of the fat to the surrounding tissues of the back. It is important to recognize that the fatty tissue in a lipoma is normal fat, not tumor tissue. It is normal fat in an abnormal place. It will only grow in proportion to the remainder of the fat in the body. In the surgical procedure, the fatty tissue is disconnected from the attachment to the spinal cord and removed, thus untethering the cord.
Split cord malformation (diastematomyelia) is a complex congenital condition where the spinal cord is split into two halves, each half usually functioning normally (see the MRI images below).The split is in a plane running front to back. The split in the cord can occur at any level and the split cords may reunite or not at some point below the split. Occasionally there is no obvious cause of the split but usually there is bone, cartilage or fibrous tissue that is between the two halves of the spinal cord. The presence of the tissue splitting the spinal cord causes tethering. The surgical procedure is to remove any tissue that is between the two split cords, thus releasing any tethering that is present.
Dermal sinus tract is a small dimple-like opening in the midline of the spine that may connect deep into the spinal cord. The majority of dermal sinus tracts are located at the level of the sacrum or the lumbar region. The drawing on the right illustrates such a condition. Since these tracts may be openings that communicate with the contents of the spinal canal there is the possibility of meningitis developing. The dermal sinus attaches to the end of the spinal cord, causing tethering. The surgery is to remove any portions of the sinus tract that go into the spinal canal and to disconnect the sinus tract from the spinal cord, thus untethering it.
Fatty filum or thickened filum is a small, threadlike piece of connective tissue that connects the lower end of the spinal cord to the sacral end of the spinal canal. This is called the filum terminale. If the filum is thickened and is shorter than normal, it is usually filled with fat and it pulls down on the spinal cord, causing tethering. This surgical procedure is usually the simplest of all untethering operations. A single level of bone is removed in order to allow access to the tight and thickened filum. The filum is easily identified and cut. The filum has no neurological function so the procedure is unlikely to cause any neurological damage.
Myelomeningocele or spina bifida is the most common cause of tethered cord. Because of the open exposed end of the spinal cord at the time of birth, there is considerable scar tissue that develops at the end of the spinal cord and the area of the myelomeningocele known as the neural placode. Thus all children with myelomeningocele have tethering of the spinal cord from the time of birth. Because of this, these patients are watched closely as they grow for signs of neurological deterioration.
How many children with myelomeningocele will require untethering as they grow? We don't know the final answer to that question. It is apparent, however, that the percentage is relatively high. If you follow children with spinal bifida from birth through adolescence and teenage years, current data would suggest that approximately 25 percent will have symptoms suggestive of tethered cord and benefit from surgical intervention. Although children with myelomeningocele can have multiple anomalies and problems, their problems are related to the malformations that occur prior to birth and, in general they should not worsen or deteriorate as time passes. Obviously there are circumstances where deterioration may occur, but the natural history of a person with myelomeningocele should not be loss of function with advancing age. If neurological function is being lost a search for a treatable cause is required. Deterioration in bladder control, such as decreasing ability to maintain some continence with medication and intermittent catheterization, is a common sign of tethering. Increasing weakness in the legs, changes in the strength of the arms and hands, and progressive curvature of the spine are symptomatic of tethered cord.
It is unusual for a child with myelomeningocele to need tethered cord release at an early age. Some children have required untethering by 2 years of age, but this is quite uncommon. The more typical circumstance is for the child to be in the later childhood years before symptoms occur. However, if a child is operated at an early age and there is a significant growth of the spinal column yet to occur, the chance of needing a repeat procedure later in life is increased. A few children have required untethering two or three times because of the growth of their spine and due to their sensitivity to neurological changes caused by the tethering.
Once spine growth has been achieved (this is usually achieved at approximately age 13 in girls and age 16-17 in boys) there is less likelihood of symptomatic spinal cord tethering. If a child has to be untethered at 10-12 years of age they are much less likely to need repeat untethering because there is not as much spine growth left to occur. So the question of the number of surgeries and the interval between surgeries is dependent upon many factors. It is dependent upon the sensitivity of the given individual to neurological changes that may be occurring, it is related to spine growth and it is related to the potential for scar tissue formation that the individual may manifest. The overwhelming numbers of children with myelomeningocele who require untethering require only one procedure.
There is another point that is important to make. In the myelomeningocele population, when we speak of tethering, we are really speaking of symptomatic tethering. All children with myelomeningocele have a tethered cord caused by the scar tissue that forms due to the open myelomeningocele at birth and the surgery to close the opening. However, because the cord looks tethered, as seen on MRI, this does not necessarily mean that there will be symptomatic changes related to the tethering. In children with myelomeningocele the untethering operation is reserved for those who show deterioration. Surgery is not performed just because the MRI suggests that tethering is present. With this in mind, an untethering procedure will again produce tethering of the spinal cord because scar tissue naturally forms. Once the untethering has been accomplished the healing process starts and is associated with scarring. This leads to repeat tethering. However, it may not lead to symptomatic re-tethering. This is a very important distinction. The spinal cord will always look tethered on the MRI in patients with a myelomeningocele. An MRI is obtained prior to untethering operations to look for associated conditions. There may be evidence of a syrinx (loculated fluid within the central cavity of the spinal cord), cyst formation outside the spinal cord or small retained dermoid tumors that can occur from elements of the skin.
Once tethering symptoms have begun in the patient with a myelomeningocele, it still remains a judgment call as to how soon to perform surgery. If the primary symptom is pain and the patient is relatively stable and not requiring significant medication, then it is certainly reasonable to wait. If there are bladder changes, i.e. the child is no longer able to have any periods of continence with intermittent catheterization, this is a much more urgent situation and surgery should be scheduled relatively soon. Any loss of strength can be devastating to a patient who already has weakness and this is a signal to proceed with untethering. As a general rule any neurological function that is lost may not recover.
In patients who have shunted hydrocephalus, it is not uncommon for a shunt malfunction to mimic symptoms of a tethered cord. Patients with myelomeningocele may be very sensitive to any type of change affecting the lower spinal cord. In the case of a shunt malfunction, the pressure may build up in the spinal canal affecting the distal spinal cord and symptoms can mimic tethering. It is important to check shunt function before making a decision to proceed with untethering.
There is no technique for closure of the myelomeningocele at the time of the original surgery that will prevent tethering. However, there are some techniques that may minimize the amount of tethering that occurs. The open neural placode at the distal end of the spinal cord can be folded over and anatomically made into a tube by suturing the edges of the open placode together. This makes the distal portion of the spinal cord tubular in nature, shaped like the rest of the spinal cord and tends to minimize the amount of scarring that occurs. It is easier for the flat, open neural placode to form more dense scar to the overlying tissue of the lower back than a closed tubular-shaped distal spinal cord. It doesn't prevent tethering, but it seems to make the surgery for untethering easier and perhaps less risky.
Scar tissue formation is a normal response of the body to tissue injury. There is no way at present to prevent scar formation. Many different types of tissue have been tried in an effort to minimize the scarring as a result of tethered cord surgery. Silastic, Gore-Tex™, artificial dura, and many others have been tried as closure over the area of the surgery. None have been found to prevent scar tissue formation. There is nothing a present that can prevent this process.
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