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18.1 Prevalence

Anorectal malformations (ARM) are commonly associated with maldevelopment of neighbouring structures derived from the caudal cell mass. Bony spinal anomalies and malformations of the spinal cord are among the more frequent findings. The association of ARM and tethered cord has been repeat- edly documented [1–5]. The reported prevalence of spinal cord abnormalities in patients with ARM varies between around 10% (4/44 [5], 15/106 [6], 22/223 [7]), and 50% and more (40/76 [8], 25/50 [9], 54/89 [10]). A prevalence of between 20 and 35% appears most plausible (27/111 [2], 22/63 [11]). The majority of tethered cord lesions will be missed unless routine screening with ultrasound and magnetic resonance imaging (MRI) is applied, as was shown by Tuuha et al. in a retrospective 10-year analysis. With systematic screening, the percentage of spinal cord abnormalities found jumped from 4% to 20% [7]. In surgical series of tethered cord, associated ARM are comparatively rare (7/435 [7], 35/480 own experience). Historically, a risk stratification subdividing between low, interme- diate and high level of ARM has been proposed, with supposedly higher incidence in high versus low lesions [1,6,12]. However, similar incidences of tethered cord were found in those subgroups in more recent surveys [7,9,10]; one study even recorded a higher incidence in low lesions [11]. The incidence of intraspinal anomalies is significantly higher in patients with anomalies of the sacrum on plain x-rays, typically

Contents

18.1 Prevalence . . . 281

18.2 Clinical Presentation, Diagnosis . . . 281 18.3 Pathophysiology, Malformation Entities . . . 282 18.4 Tethered Cord Surgery . . . 284

18.5 Results/Outcome . . . 284 18.6 Conclusion . . . 285

References . . . 285

hypoplasia of lower sacral segments [2,13]. Classifica- tion of ARM is a complex topic that is covered exten- sively in this book. While the frequent association of cloacal exstrophy and spinal cord malformation has long been established [1,8,14], there are no compre- hensive assessments so far. There are strong indica- tions in line with embryological considerations, however, that the prevalence of tethered cord is correlated with the severity of the ARM, as was proposed and demonstrated by Peña’s workgroup [2]. Tethered cord prevalence was found in 11% of vestibular versus 18%

of bulbar and prostatic fistulas, 27% of cloacal common channels, 40% of cloacal exstrophies, and 43%

of complex malformations [2]. The risk of tethered cord increased with the number of associated anomalies. Presacral mass, sacral hemivertebrae, and single or ectopic kidney were each associated with tethered cord in 50–60% of cases, reflux only in 10% [2].

18.2 Clinical Presentation, Diagnosis

Closed spinal dysraphism reveals itself through skin abnormalities in 50% to more than 80% of cases [15–

18]. In the context of ARM, vascular nevus, lumbosacral subcutaneous mass, skin dimple and deviation of the natal cleft should be searched for. The manifestations of tethered cord have rightfully been called protean by Hoffman and coworkers in 1976 [19]. Pre- senting symptoms may include motor and sensory deficits in the lower extremities, gluteal and genital region, leg or back pain (especially in older children and adults), foot deformities, leg length discrepancies and scoliosis (neuro-orthopaedic syndrome) or blad- der and bowel dysfunction (Fig. 18.1). The majority of our patients had abnormal findings in one or several of these aspects (27/35), but only a minority (6/35) presented with clear-cut progressive symptoms. Most authors agree, that faecal incontinence and urinary abnormalities are more common in patients with tethered cord [2,5,20], whereas constipation is more common in ARM without tethered cord [2]. When the type of ARM is accounted for, faecal incontinence

18 Tethered Spinal Cord in Patients with Anorectal Malformations

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and constipation are similar; only urinary dysfunction remains slightly more common [2]. This relationship and the commonly found dysplasia of the sacral bone point to maldevelopment of the respective innerva- tion rather than secondary factors.

Abnormalities of the sacral bone and lumbar spine point to the presence of intraspinal pathology, but plain x-rays are less sensitive than ultrasound and MRI and are therefore no longer advocated as a screening procedure for intraspinal pathology. All neonates with ARM should be subjected to an ultrasound examination of the spine and spinal canal, with emphasis on conus shape and position as well as ex- clusion of irregular intraspinal tissue and abnormal fluid collections (syrinx, arachnoid cyst, meningocele, myelocystocele). An MRI is indicated at 3 months of age if either ultrasound or clinical signs (skin lesion, neurological or neuro-orthopaedic abnormalities of the lower extremities) are positive. In the neonatal period, the image quality of MRI is significantly reduced. From a neurosurgical standpoint, early MRI is indicated only in cases requiring early interven- tion, such as large myelocystoceles precluding supine positioning of the infant. Whether all infants with negative ultrasound and without clinical signs and symptoms should still be screened with MRI at about 1 year of age is not entirely clear. The sensitivity of MRI is higher than that of ultrasound, estimated at 95.6% and 86.5%, respectively. Specificity is similar,

at 90.9% and 92.9%, respectively [21,22]. It appears prudent, therefore, to screen all patients using MRI, at least those patients with higher risk (e.g. cloacal and complex malformations).

18.3 Pathophysiology, Malformation Entities

Tethered cord is not synonymous with malformation of the spinal cord. In caudal regression syndrome, absence of the lower sacral spinal cord segments and ac- cording neurological deficits often occur without associated tethering lesions. Tethered cord describes a potentially harmful pathological fixation of the spinal cord, either directly or via inelastic tissue (bone, fat, fibrous or fatty filum terminale). It is typically located in the lumbosacral region and most commonly caused by spinal cord malformations. The negative impact on the spinal cord is believed to result from a stretching effect brought about by everyday activities, especially flexion of the spine and pelvis. Elongation has been demonstrated to be most severe at the point of fixation and extends upward for up to five segments, but not beyond the lowest insertion of the dentate liga- ment at the thoracolumbar junction. Such traction on the spinal cord causes impairment of capillary blood flow and results in reduced oxygenation of the spinal cord. Experimental studies in animals have shown a

Fig. 18.1 Signs and symptoms of tethered cord. A Subcutane- ous mass and reddish vascular naevi point to a lumbosacral lipoma of the spinal cord. B Neuro-orthopaedic syndrome.

C Club feet. D Pressure sore due to sensory loss and trophic problems on deformed foot

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close correlation between the severity and duration of traction, and reduction of blood flow, deterioration of interneuron potentials and neurological deficits [23].

These results have been corroborated by intraoperative measurements of spinal cord blood flow [24] and cytochrome-C redox state [23]. Intraoperative improvement of those parameters was correlated with the neurological outcome.

Spinal cord malformations causing tethered cord are a heterogeneous group of lesions resulting from maldevelopment of mesoderm invagination and pre- neurulation midline formation, errors of primary and secondary neurulation and post-neurulation events.

As would be expected, the spectrum of entities associated with ARM consists of the subgroup related to secondary neurulation and subsequent processes (see Fig. 18.2 for typical MRI findings). Most common is a thickened, shortened or fatty filum terminale, fol- lowed by lumbosacral lipoma or lipomyelomeningocele. Other lesions are terminal myelocystocele and ventral sacral meningocele. Caudal regression syndrome, also labelled caudal suppression syndrome or, better, caudal dysplasia syndrome, occurs without tethering lesion or in combination with the lesions mentioned above. The complexity of these lesions and their impact on neurological function, natural history, difficulty of surgical repair, outcome and related long-term problems varies enormously, similar to the spectrum of ARM.

Pathological filum is the prototype of pure spinal cord tethering. Dysplasia of the spinal cord and cauda is minor or absent, unless caused by associated malformations. The dura is intact, the arachnoid mem- branes only slightly abnormal if at all. Patients are less likely to have neonatal neurological deficits, deteriorate later than patients with more severe malformations, but continue to be at risk for deterioration throughout their life [25,26].

Spinal cord lipomas and lipomyelomeningoceles occur in a wide spectrum of mild to severe cases in terms of malformation size, degree of spinal cord in- volvement, extent of dysplasia of the spinal cord and cauda and intensity of spinal cord tethering and/or compression. At birth, about 20% have functional deficits attributable to the malformation, the majority will deteriorate during infancy and childhood [27–30].

Terminal myelocystocele is a malformation that is not rarely associated with cloacal malformations [31,32]. It has all the features of a lipomyelomeningocele (i.e. extension of the spinal cord into a meningocele, where it is infiltrated by and tethered to fat tissue). In addition, the central canal of the spinal cord is distended into a terminal ventricle, which forms another component of the cele. The spinal cord tends to be both dysplastic and hypertrophic at the level of the malformation, a syrinx may extend upwards into functionally normal spinal cord segments. Patients

Fig. 18.2 Magnetic resonance imaging findings typical for tethered cord in patients with anorectal malformations.

A Thickened filum terminale (see arrow in A2), absence of the lower segments of the sacral bone. B Lipoma of the spinal cord (arrow). C Terminal myelocystocele. Note the terminal ven-

tricle (v) dorsal to the malformed spinal cord, which is pushed out far above the fascial plane by an enlarged subarachnoid space (sa). D Ventral sacral meningocele with large dermoid (d) adjacent to the subarachnoid space

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tend to be more severely affected at birth and throughout life than average spinal cord lipoma patients.

Ventral sacral meningocele together with ARM and dysplasia of the sacral bone (scimitar sacrum) often presents as a component of the Currarino syndrome. A high proportion of cases are hereditary with autosomal dominant inheritance [33]. Tethered cord is a common finding in ventral sacral meningoceles.

The spinal cord is attached to the caudal wall of the meningocele, either directly or by way of a pathological filum terminale. The spinal cord can be elongated to an extent that it may be addressed as a mere pathological filum on imaging. Dermoid tumours are commonly associated, located in or adjacent to the caudal wall of the meningocele. Comorbidities other than faecal abnormalities are uncommon. Early progres- sion of tethered cord syndrome is uncommon, but we have seen some cases with deterioration even in adult age.

18.4 Tethered Cord Surgery

The decision for surgical treatment of tethered cord in patients with ARM is controversial. Patients suf- fering from progressive motor or sensory deficits are unanimously considered good candidates for surgery [7–9,11]. The relationship, however, between faecal and urinary abnormalities and spinal cord tethering is highly questionable in the context of caudal dysplasia with ARM [2,13]. The natural history of patients with ARM appears to harbour only a low risk of neurological deterioration [2,7]. Tuuha found that only 2% of 435 patients with ARM had tethered cord symptoms and only 18% of the subgroup were diagnosed with tethered cord [7]. It is concluded by some authors, therefore, that patients with ARM may form a subgroup of tethered cord patients less prone to deterioration who may not profit enough from untethering to outweigh the risks of this procedure [2,7], while other authors and most of the paediatric neurosurgical community keep recommending prophylactic surgery for all tethered cord patients [9,34]. When tethered cord is diagnosed late and the patient has been stable so far, further observation is preferred.

Warf et al. did so in patients beyond 10 years of age [8]; non-progressive patients should certainly not be operated on in adult age [35]. The controversy regard- ing prophylactic surgery is less applicable for terminal myelocystocele, where large lumps on the back and the threat of direct transcutaneous pressure transmis- sion through the terminal ventricle and syrinx into the normal lumbar spinal cord warrant early inter-

vention. Expanding intrapelvic meningocele or detec- tion of an inclusion tumour are additional reasons for prophylactic surgery in ventral sacral meningoceles.

Constipation is not an indication for surgery, at least in our experience, since we never saw improvement after untethering plus closure or removal of the meningocele.

18.5 Results/Outcome

Data on the outcome of tethered cord surgery specific for patients with ARM remain scarce. There is no reported mortality, and permanent morbidity such as injury to neurological or urological function, while certainly possible, appears to be low. Specifically, there is no reported iatrogenic deficit in 6 series with an accumulated number of 87 patients [1,2,7,8,11,34], which parallels our own results in 35 patients. Motor and sensory deficits usually stop deteriorating. Post- operative motor improvement is reported in a cumu- lative 8/15 patients [1,2,7,8]. Unanimously, however, very little influence of surgery on manifest urinary or faecal incontinence has been found, as is our experience. A surprising exception is the paper by Muthu- kumar, who reported improvement of urinary, but not faecal incontinence in all eight patients operated on, without giving details or plausible explanations [34]. A 9% rate (2/22) of late deterioration ascribed to retethering has been reported in one series [11].

In the general tethered cord population, the com- plication rate and long-term results are significantly different in patients with pathological filum compared to lipomas of the conus. In the former, surgery is straightforward and the rate of neurological injury approaches zero in large series. Secondary deterioration during follow-up is rare [29,30]. Deficits present at the time of surgery will usually stop worsening, but will improve in only 53% of cases [29] and resolve completely in only 19–41% [29,30]. Prophylactic surgery of pathological filum terminale and of lipomas of the filum therefore continues to be unanimously recommended [29,30]. Conus lipoma (and lipomyelomeningocele), on the other hand, often pose sig- nificant surgical problems; complete untethering is impossible in up to 20% of the patients and the risk of neurological injury is not negligible, at around 4%

[29]. Figure 18.3 illustrates the different situations encountered intraoperatively. There is a disturbingly high rate of secondary deterioration during follow-up, ranging from 28% [30] to 36% after 5 years or more [29]. Some have been moved by these results to offer surgery only when symptoms evolve [29], while most

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of the neurosurgical community continue to advocate prophylactic surgery. The main arguments for that position remain that existing symptoms are only sta- bilized after surgery in the majority of patients, rather than reversed, and that deterioration over time, while not completely preventable, appears to be mitigated [30]. In our experience (122 patients, unpublished data), deterioration during a 5-year follow-up period was less common after complete untethering (16%) than after incomplete untethering (48%).

18.6 Conclusion

Spinal cord malformations and tethered cord are no less heterogeneous a group as are ARM. While the specific aspects of tethered cord in patients with ARM warrant more extensive investigation, these patients cannot be adequately addressed and treated as a homogeneous subgroup of tethered cord. Exist- ing data are insufficient to either reject or mandate prophylactic tethered cord surgery conclusively. The true natural history remains unknown. The lifelong deterioration risk, sufficiently demonstrated for the general tethered cord population, however, cannot be completely denied, although its incidence and dy- namics need further quantification. Prospective studies, while welcome, would need to span decades to

properly balance the risks and benefits of operative versus conservative treatment. Since the risk:benefit ratio of tethered cord surgery is so much better and the retethering risk so much lower for pathological filum compared to lipomas of the spinal cord, prophylactic surgery appears to be difficult to reject at least for this subgroup.

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