More preoperative flexibility implies adequate neural pliability for curve correction without prophylactic untethering in scoliosis patients with asymptomatic tethered spinal cord, a retrospective study

Scoliosis is often associated with a number of spinal cord malformations, such as tethered spinal cords, diastomyelia, lipomyelomeninggoceles, and Chiari deformities. In addition, it has been shown that 15–30% of patients with congenital scoliosis have spinal malformations [1‐3]. TCS is a intraspinal anomaly, caused by abnormal spinal cord fixation and resultant low-lying and immobile conus medullaris. The incidence of TCS is estimated to be 0.05 to 0.25 per 1000 births. It may result from many causes, including intraspinal lipoma, lipomyelomeningocele, diastematomyelia, spina bifida occulta, tight or thickened filum terminale, or scarring from previous myelomeningocele repair [4]. Although rare, a tethered spinal cord is one of the most common pediatric disorders [5]. Tethered cord refers to intradural thickening of the filum, shortening or adherence to the spinal cord, and a limitation of movement between the spinal cord and spinal canal. A tethered spinal cord can be diagnosed if the conus medularis is located below the L1–2 level and/or a thickened filum terminale is present (>2 mm) on radiologic images [6]. In the process of growth and development of the spinal cord, spinal cord tension is increased, resulting in spinal cord ischemia, hypoxia, and degeneration [7]. Although tethered spinal cord is a congenital disorder which is present from birth, the symptoms of tethered cord can appear later, during the teenage years and adulthood, even no symptoms lifelong. Once symptoms developed, the patient has a tethered cord syndrome (TCS) [8]. The syndrome is usually diagnosed in childhood and the presentation is variable and may be insidious. The main clinical manifestations of TCS were numbness, weakness, and urination and defecation dysfunction. However, some patients have no neurologic symptoms [9] or no clinical symptoms and a tethered cord are found during a routine physical examination.

For scoliosis patients with a tethered cord, limited movement between the tethered cord and spinal canal greatly increases the level of difficulty in performing a scoliosis correction procedure, and also aggravates neurologic dysfunction or causes new neurologic deficits in the operative process. Traditional theory has shown that surgical detethering should be performed first, followed by scoliosis correction several months later [10, 11]; however, the scoliosis may deteriorate before the correction procedure is performed. Some studies have shown that surgical detethering and scoliosis correction can be performed concurrently and the risk of complications is not increased [12, 13]. It is still controversial whether or not preventive untethering is necessary before scoliosis correction; further studies are warranted.

In the current study, 17 scoliosis patients with asymptomatic tethered cords who underwent a one-stage posterior scoliosis correction procedure without preventive untethering were retrospectively reviewed. The coronal Cobb angle, apical translation (AVR), the curve flexibility and the correction rate (CR) were recorded and analyzed to evaluate correction result. We also analyzed the Scoliosis Research Society (SRS)-22 questionnaire, and the mJOA score pre- and post-operatively to evaluate the clinical outcomes.

Spine deformities are always associated with spinal cord malformations [1, 14]. Common spinal cord malformations include diastematomyelia (types I and II), tethered spinal cords, syringomyelia, a low conus medullaris, spinal meningoceles, and lipomas [1, 15‐17]. Multiple malformations can exist in an individual patient. In this study, nine of fourteen (64%) congenital scoliosis patients are associated with additional anomalies, including three patients (21%) with spina bifida occulta, three patients (21%) with vertebral deformity, two patients (14%) with syringomyelia, and one patients (7%) with ribs deformity. Of these, two patients (14%) had multiple abnormalities, one is associated with syringomyelia and polycystic ovary syndrome, and the other is associated with spina bifida occulta, syringomyelia and ribs deformity. It has been shown that 47% of pediatric tethered spinal cord patients had congenital or juvenile scoliosis [17, 18].

Most patients with tethered cord can be identified in patients with abnormality of the skin of the lumbar and sacral region and/or neurological dysfunction. However, some patients exhibit only scoliosis, which can also be considered as the first symptom of a tethered cord syndrome, without neurologic dysfunction at an early age [14]. The neurologic deficits which arise in these patients in adolescence or later life are caused by other factors, such as spinal stenosis, trauma over time, and increased physical activity [19]. These patients can be considered to have asymptomatic tethered cords.. While these patients had severe spinal deformity and/or scoliosis progression, a surgical correction is imperative for them.

TCS is usually diagnosed in childhood and a aggressive treatment to children with TCS is reasonable because they would have severe pain and advanced neurological deficits in the future [4]. For scoliosis patients with symptomatic tethered spinal cords (TCS), surgical detethering has consistently been reported to be effective in treating symptomatic tethered cord syndrome [10, 11]. There are three main opinions for treating scoliosis with tethered spinal cords. First, untethering is performed during the first-stage and second-stage posterior scoliosis correction is followed 3–6 months later. For slow progressive scoliosis and non-severe scoliosis, some authors stated that an early untethering procedure can prevent further progression of scoliosis [18] and can clearly relieve the symptoms of back pain and neurologic dysfunction [20]. They stated a surgical correction procedure can lead to excessive traction on the conus medullaris and spinal cord because of a degenerated thickened filum fixes the conus medullaris to he spinal canal, which increases the risk of neurologic injury and greatly increases the operative difficulty [17, 21, 22]. Complications, such as cerebrospinal fluid leakage and neurologic deterioration, are also common [23]. Second, some studies have suggested that surgical detethering and scoliosis correction can be performed at the same time and the risk of complications is not increased [12, 13]. This approach reduces the economic and mental burden of patients without the trauma of a second surgical procedure. Additionally, the surgical complications were decreased compared with a two-stage operation [24]; however, this procedure requires neurosurgeon and spine surgeon collaboration, which means longer operative time, increased intra-operative blood loss, a longer recovery time. Moreover, this approach cannot avoid untethering complications and a recurrent tethered spinal cord [25]. Third, a vertebral column resection (VCR) or spine shortening osteotomy have been shown to be safe and effective methods for treating scoliosis with spinal cord malformations, and are widely used in spine surgery [26‐28]. A VCR or spine shortening osteotomy is especially applicable for a tethered spinal cord with severe kyphosis or kyphoscoliosis caused by a hemi-vertebra [29].

Whether a aggressive treatment and prophylactic untethering are reasonable and necessary are still controversial for school-aged children, adolescent patients with spinal cord who are neurologically intact [4, 14]. Some authors considered that releasing the tethered spinal cord may stabilize or even reverse the progression of scoliosis. A corrective procedure may cause irreversible neurological deficits unless untethering of the spinal cord has been performed previously [30]. A prophylactic untethering for an asymptomatic tethered spinal cord can reduce the traction of the spinal cord and avoid new neurologic dysfunction in a correction process. However, some authors had raised objections. They considered that a prophylactic untethering may cause post-operative complications and an uncertain long-term curative effect. Gharedaghi et al. [31] reported that those patients with early surgical untethering had the best results at the early follow-up evaluation, but the prognosis was nearly the same in both with or without untethering groups at a longer follow-up (> 3 years). Samdani et al. [32] also reported that a one-stage scoliosis correction without untethering is a safe and valuable approach for scoliosis patients with asymptomatic tethered spinal cords. Some authors also stated that school-aged children, adolescent and young adult patients have relatively advanced neurologic deficits, and the functional zone they have retained is very narrow, which means that it is necessary to maintain the balance between skeletal maturation and conus traction upon the TC. A aggressive treatment and prophylactic untethering could also result in a worsening of their neurologic deficits [14]. In this study, all the patients are school-aged children and adolescents with mean Risser’s score 4, (range: 9–17 years), which is a very critical age with spine growth and scoliosis correction. At this stage, they have limited growth potential and the cord tethering is unlikely to cause much problems. In these patients, 9 of 17 cases exhibited a scoliosis progression. Although it cannot be confirmed whether a tethered spinal cord induced a progressive scoliosis, a surgical correction was imperative. As such, one-stage posterior scoliosis correction without a prophylactic untethering were performed to improve deformity and stop curve progression. Based on our study, we further proved that in the school-aged children and adolescents with scoliosis combined with tethered cord, one-stage posterior scoliosis correction can be used as a surgical strategy for scoliosis patients with asymptomatic tethered spinal cords. A prophylactic untethering may be an appropriate procedure only when they had neurological symptoms.

For scoliosis combined with tethered cords, the most important issue is to maintain the balance between correction rate and stability of neurological status. Usually, one-stage posterior correction for treating scoliosis without untethering may cause neurologic injury or deterioration of tethered spinal cord symptoms. In this study; suspension radiograph, supine bending radiograph and fulcrum bending radiograph were used to evaluate the flexibility and expected correction rate, which can be partly considered as neural pliability for intraoperative correction. As a result, spine suspension, supine bending and fulcrum bending radiograph examination showed no apparent symptoms of a tethered spinal cord, which can be considered as there was no apparent changes of neurological status. Depending on these examinations, a better operative correction rate was achieved eventually. Thus, we suspected that the efficiency of a one-stage correction without untethering may be related to the negative results of spine suspension, supine bending and fulcrum bending radiograph examinations. Based on this study, we infer that while the flexibility of major curve was more than 30% in addition with the correction rate of fulcrum radiograph was more than 30%, it means that patients with scoliosis combined with tethered cord have a better neural function reserve, one-stage posterior correction is safe and effective without a prophylactic untethering.

Furthermore, whether a spinal cord might be changed is also concerned during the correction of a spinal deformity. If there exist some changes of spinal cord after scoliosis correction, then surgeons have to pay more attention to this phenomenon because this a correction procedure can cause some changes of the neurological status, which can lead to possible new symptoms and spinal cord injury. A study from Jae-Young Hong et al. [33] found that the conus medullaris level changed postoperatively in patients with severe scoliosis, and the spinal cord level changed after a large amount of correction. They also stated that the patients with scoliosis or lordosis had longer spinal column length to cord ratios, which meant a relative stretching of the cord. If the scoliosis or lordosis is corrected, then the length of spinal column can be decreased, following the decrease of the upward tethering force. In this study, all the patients were mild to moderate scoliosis (the cobb angle of major curve is less than 90 degrees), which means a limitation of correction, the conus medullaris probably will not be changed after a correction procedure, which means no excessive traction during correction procedure and after surgery.

Admittedly, there were several limitations in our research. First, the number of cases was small and the duration of follow-up was short. Whether or not long-term symptoms will arise warrants further observation. Second, for patients with severe scoliosis with a coronal Cobb angle >90^o, severe kyphosis and kyphoscoliosis, whether or not this approach is applicable also warrants further study. Third, the indication for one-stage correction without untethering has some limitations.

Our study demonstrated that, a one-stage posterior scoliosis correction did not increase the risk of immediate neurologic dysfunction in scoliosis patients with tethered spinal cords. A one-stage posterior correction without a prophylactic untethering can possibly be a safe and valuable surgical approach if the patients had adequate neurologic reserve. However, study limitations need to considered and further study is warranted.