Intraoperative neurophysiological monitoring in tethered cord surgery

27.1

Introduction

Tethered cord syndrome (TCS) is a complex and progressive clinical and radiological condition, secondary to an embryologic failure of spinal cord development, which induces various types of spinal dysraphisms. These defects ultimately cause a radicular-medullary stretch that generates a hypoxic-ischemic damage to the nervous structures of the conus–cauda region. This distress may lead to functional disorders, which tend to worsen with age .

It is difficult to distinguish patients who will remain asymptomatic for their whole life from those whose conditions will deteriorate. For this reason the treatment of patients with a congenitally asymptomatic tethered cord remains controversial. Thus while the debate on the natural history of spina bifida occulta (SBO) is still open, children with SBO frequently undergo prophylactic surgery in order to prevent neurological deficits. In contrast, adult patients are rarely asymptomatic at the time of surgical treatment .

Among the most challenging SBO disorders are the lumbosacral lipomas (LSL) . A conservative management, consisting of a close clinical–radiological–urological follow-up, is based on a French study in which the authors compared the incidence of neurological decay in patients treated with prophylactic surgery to those who have not been treated. After a 9-year follow-up, Kulkarni et al. observed that 33% of conservatively treated patients and 46% of surgically treated patients presented neurological symptoms. While this difference was not statistically significant, it was still suggested that surgical treatment was not better than the natural history . Further studies have demonstrated that not all asymptomatic pediatric patients will develop symptoms during life. In some cases, symptoms will appear only during adulthood, with more favorable progress. On the other hand, the incidental finding of truly asymptomatic LSLs in adult patients is anecdotal, indicating that sooner or later patients with a tethered cord will ultimately deteriorate. For this reason, prophylactic surgery has been offered, but it should warrant high treatment standards in order to avoid neurological deficits.

One of the problems that emerged from reviewing the results of studies on the long-term outcome of children surgically treated for lipomyelomeningoceles (LMMs) was that surgeons often performed partial or subtotal resection of conus lipomas, with the goal of untethering the conus but not of removing the lipoma. In 2013 Pang evaluated the efficacy of subtotal/total resection of LSL: patients treated with partial resection of the lipoma presented neurologic worsening in 57% of the cases. The ones who benefitted from a total/near total resection remained asymptomatic in 98.8% of the cases. This was a seminal paper that outlined the value of a “total” removal of LSLs, providing results that were clearly superior to the French study .

Interestingly, something which is rarely pointed out whenever this debate is addressed is the fact that Pang’s approach is strongly based on the extensive use of intraoperative neurophysiology (ION). Vice versa, no ION was used in the French cohort, suggesting that ION could be, in fact, one of the major variables in achieving optimal results. ION gives the surgeon an immediate feedback and warning signals during manipulation of the spinal cord and nerve roots. Some studies suggest that this will ultimately reduce the risk of inadvertent nervous injuries and improve long-term outcome both in children and adults .

The different ION techniques that could assist the neurosurgeon whenever the conus and cauda equina are at risk of injury have been already extensively described in Chapter 6 , Intraoperative neurophysiological monitoring of the sacral nervous system.

To follow, we will review some of these techniques in the clinical context of surgery for tethered cords.

27.2

Tethered cord etiopathogenesis

Neural tissues originate from ectoderm, together with skin, eye, and inner ear. The development of neural system starts during the second and the third week of gestational age with the formation of the mesoderm between the endoderm and the ectoderm (gastrulation). From mesodermal mesenchymal cells arise the notochord, which stimulates the thickening of overlying ectoderm and the growth of neural plate . The neural plate folds up and separates itself from the overlying ectoderm, becoming the primitive neural tube. Its extremities, called neuropores, close at day 25 (the cranial one) and at day 28 (the caudal one), determining the ends of the primary neurulation and the beginning of the secondary neurulation at the caudal end of spinal cord ( Fig. 27.1 ) .

This process, following the process of mesenchyme-epithelium transformation, condensation, and intrachordal cavitation, conduces, at day 48, to the formation of the caudal conus and the nonneural terminal filum . Meninx and vertebral column grow faster than neural tube; therefore the conus ascends progressively, reaching its definitive position during the second month of postnatal life, on average between the inferior third of T12 and the medial third of L2 ( Fig. 27.2 ) .

Spinal dysraphisms originate from abnormalities occurring during either primary or secondary neurulation and are divided into “open” defects—characterized by the presence of the neural placode exposed over the skin and “close” defect—when the neural placode is covered under the skin ( Fig. 27.3 ) .

Spinal dysraphism can be classified both according to clinical and radiological aspects, and to the time of neurulation arrest ( Table 27.1 ) .

In 1981 Yamada demonstrated that these developmental impairments determine a constant or intermittent traction at the level of the conus–cauda region, causing a hypoxic-ischemic damage to the nervous structures of the lumbosacral cord. This presents, clinically, with the so-called TCS. Yamada demonstrated further that untethering the cord could improve mitochondrial oxidative metabolism, reducing the hypoxic stress .

TCS has an incidence of 0.004%–0.008% inhabitant, and it arise up to 0.1% when we consider only primary school children . Spinal lipomas represent 70% of the lesions associated with tethering . The timing of the onset of clinical symptoms depends on the amount of stretch on the conus medullaris and cauda equina, and it is not predictable. Adult patients are rarely asymptomatic and the risk of a neurologic deterioration increases with time .

When symptomatic, TCS is characterized by several neurological deficits including lower limb sensory-motor impairments, low-back pain, sphincter dysfunction, and musculoskeletal deformities.

The goals of surgery are to avoid or arrest the progression of symptoms and, to a lesser degree, to revert the existing neurological deficits. After surgery, low-back pain improves between 86% and 95% of cases and motor impairments improve up to 71% of cases . Sensory deficits rarely improve, but they tend to stabilize after surgery .

Bladder and bower dysfunction are the most socially disabling condition, and their prevalence is more than 70% in adult patients and go from 22% to 60% in pediatric ones . These conditions tend to be refractory to surgery and improve in only 16%–60% of the cases; this is one reason not to delay untethering once the diagnosis of TCS is made .

Surgery for tethered cords ranges from simple cutting of the filum terminale to more complex resection of lipomas, which often strictly adhere to and include neural structures. Dissecting and releasing scar tissue and fibrous adhesion may be challenging and expose to the manipulation of neural roots and conus. In addition, medullary anatomy is often distorted, the roots abandon the conus in different directions and, often, anatomy does not reflect function .

The incidence of transient complications following tethered cord surgery has been reported between 10.9% and 13%, but it drops to 0%−4.5% when only permanent neurological injuries are considered .

The ION techniques utilized in tethered cord surgery include a combination of monitoring and mapping techniques: mapping techniques are used since the early 1980s, in order to recognize and distinguish cauda equina functional structures. Historically, somatosensory evoked potentials (SEPs) and free-running electromyography (EMG) have been the first techniques introduced to monitor the functional integrity of nervous structures. This was further implemented, at the end of 1990s, by the introduction of the monitoring of muscle motor-evoked potentials (mMEPs) and the bulbocavernosus reflex (BCR) .

Tethered cord surgery is mainly performed in the pediatric population. As such, the developing nervous system may still be immature from a neurophysiological standpoint, and this can affect ION techniques, especially in children below the age of 4–5 years. Therefore tailoring stimulation and recordings to the child’s age is critical .

27.3

Anesthesia

Anesthetic agents interfere with synaptic function and alter signals’ conduction, therefore challenging the success of ION techniques.

Short to intermediate term myorelaxants are used only for the intubation, because they would prevent motor mapping and monitoring during the course of the surgery. Also the use of halogenate agents is not recommended because it significantly interferes with SEP and MEP monitoring.

In adults the maintenance of anesthesia is reached with a constant infusion of propofol [100–150 (μg kg)/min] and fentanyl [1 (μg kg)/h].

In the pediatric population, especially in the first years of life, the prolonged infusion or high dosage of propofol is linked to PRIS (propofol infusion syndrome), characterized by reversible lactic acidosis associated with acute refractory bradycardia, potentially leading to asystole. For this reason, within the pediatric population, the use of total intravenous anesthesia is controversial. However halogenated gases, such as isoflurane, sevoflurane and, recently, desflurane, are still used for the maintenance of anesthesia, but these could all compromise neuromonitoring, to some degree.

Vice versa, direct motor mapping of the cauda equine is less affected by anesthetics because it does not involve polysynaptic pathways .

It is known that the myelinization of corticospinal motor pathways completes around the age of 11–13 years, and so ION in young children (especially below age 6) is challenging. The low monitorability of transcranial MEPs in younger children can be overcome by techniques aimed to improve temporal and spatial facilitation . Some protocols consider the reduction or elimination of propofol with the addition of ketamine. This approach seems to increase the success in acquiring responses from 78% to 96% in children over the age of 6 and up to 86% in children under the age of 6 .

27.4

Intraoperative neurophysiology techniques during tethered cord surgery

The goals of ION in tethered cord surgery are to identify ambiguous neural tissue through the use of mapping techniques and to assess the functional integrity of motor and somatosensory pathways as well as reflex circuits such as the BCR.

In general, mapping techniques play a major role in tethered cord surgery because these are essential to identify functional neural structures that must be preserved from nonfunctional vestigial roots or fibrous bands that could be cut to achieve cord untethering. Monitoring techniques are certainly valuable, especially with regards to mMEPs and the BCR, while SEPs might be more difficult to monitor, especially in very young children, and are less relevant in guiding the surgical strategy ( Fig. 27.4 ) .

The sensitivity of SEPs ranges between 28.6% and 50% and specificity between 94.7% and 100% .

The low SEP sensitivity to detect postoperative radiculopathy can be overcome by the use of MEPs, reported to have a sensitivity range between 75% and 100%, with a specificity from 25% to 100%. Muscle MEPs are characterized by a positive predictive value (PPV) ranging from 63% to 100%, and a negative predictive value (NPV) ranging from 75% to 97% .

Free-running EMG sensitivity is reported as high as 87.5% and specificity as high as 83.3%, with a PPV of 87.5% and NPV of 83.3%, respectively .

Although is not the aim of this chapter to address ION techniques in detail, to follow we will shortly review those aspects more relevant to tethered cord surgery.