Diagnosis and Management of Tethered Cord Syndrome.

Tethered cord syndrome is a condition in which the spinal cord is tethered by pathological structures such as a tight filum terminale, intradural lipomas with or without a connecting extradural component, intradural fibrous adhesions, diastematomyelia, and neural placode adhesions following closure of a myelomeningocele.It usually occurs in childhood and adolescence as the spine grows in length, but it can also develop in adulthood. Symptoms of tethered cord syndrome are slowly progressive and varied. Incorrect diagnosis and inappropriate treatment may be provided if the physician lacks knowledge and understanding of this disease.This chapter aims to describe the pathophysiology, syndromes, diagnostic imaging, surgical treatment, and prognosis of tethered cord syndrome to enhance the understanding of this condition.

Disparities in indications and outcomes reporting for pediatric tethered cord surgery: The need for a standardized outcome assessment tool.

PURPOSE: Tethered cord syndrome (TCS) is characterized by abnormal attachment of the spinal cord neural elements to surrounding tissues. The most common symptoms include pain, motor or sensory dysfunction, and urologic deficits. Although TCS is common in children, there is a significant heterogeneity in outcomes reporting. We systematically reviewed surgical indications and postoperative outcomes to assess the need for a grading/classification system. METHODS: PubMed and EMBASE searches identified pediatric TCS literature published between 1950 and 2023. Studies reporting surgical interventions, ≥ 6-month follow-up, and ≥ 5 patients were included. RESULTS: Fifty-five studies representing 3798 patients were included. The most commonly reported non-urologic symptoms were nonspecific lower-extremity motor disturbances (36.4% of studies), lower-extremity/back pain (32.7%), nonspecific lower-extremity sensory disturbances (29.1%), gait abnormalities (29.1%), and nonspecific bowel dysfunction/fecal incontinence (25.5%). Urologic symptoms were most commonly reported as nonspecific complaints (40.0%). After detethering surgery, retethering was the most widely reported non-urologic outcome (40.0%), followed by other nonspecific findings: motor deficits (32.7%), lower-extremity/back/perianal pain (18.2%), gait/ambulation function (18.2%), sensory deficits (12.7%), and bowel deficits/fecal incontinence (12.7%). Commonly reported urologic outcomes included nonspecific bladder/urinary deficits (27.3%), bladder capacity (20.0%), bladder compliance (18.2%), urinary incontinence/enuresis/neurogenic bladder (18.2%), and nonspecific urodynamics/urodynamics score change (16.4%). CONCLUSION: TCS surgical literature is highly variable regarding surgical indications and reporting of postsurgical outcomes. The lack of common data elements and consistent quantitative measures inhibits higher-level analysis. The development and validation of a standardized outcomes measurement tool-ideally encompassing both patient-reported outcome and objective measures-would significantly benefit future TCS research and surgical management.

Recurrent tethered cord: outcome and follow-up of 20 de-thetering for symptomatic spina bifida: choort study.

OBJECTIVE: The objective of this study was to evaluate the prevalence of tethered cord among patients in the postoperative period of open and occult spina bifida. To identify warning signs for its early diagnosis, as well as outcomes after the new surgical approach. METHODS: Retrospective study of patients followed at the Pediatric Neurosurgery Department of the Federal University of São Paulo with spinal dysraphism. Signs and symptoms indicating reoperation were collected, and postoperative results were classified as improved, unchanged, or worsened. RESULTS: 222 medical records of patients diagnosed with spinal dysraphism were evaluated. Symptomatic Tethered Cord Syndrome (STCS) was identified in 30 patients (13.51%), with clinical manifestations related to orthopedic deformities (66.7%), neurological deficits (56.7%), urological dysfunction (50%), and intestinal dysfunction (40%). 20 cases underwent surgery for tethered cord release. The mean age at the time of surgery was 7.7 ± 4.9 years, with 13 female patients (65%). In the postoperative evaluation, improvement in low back pain (90.9%), urological pattern, and urinary tract infection episodes (45.4%) were particularly noteworthy. 3 patients (33.3%) with constipation showed improvement, and one worsened (11.1%). Improvement in ambulation was seen in two cases (16.7%). Low back pain was the first symptom to improve after surgery, with an average time of 1.3 months, followed by changes in the urological pattern at 15.6 months. Improvement in constipation was observed in the first month in 2 cases (66.7%), positive changes in ambulation were observed around 7 months after surgery, and only one case showed improvement in clubfoot correction. CONCLUSIONS: The prevalence of tethered cord recurrence after primary correction surgery for open or occult neural tube closure defects was similar to that found in the literature. The results were encouraging, with good postoperative evolution of patients, especially in the improvement of low back pain and urological symptoms.

Comparison of the inter-laminar approach and laminotomy open approach for filum terminale lipoma: A retrospective analysis.

PURPOSE: Filum terminale lipoma (FTL) causes spinal-cord tethering and is associated with tethered-cord syndrome, which is treated by dissection of the entrapment. The conventional treatment for FTL involves dissection of the spinal cord through a laminotomy open approach (LOA). However, in recent years, the interlaminar approach (ILA) has gained popularity as a minimally invasive surgery. This study compares the effectiveness of the minimally invasive ILA with the conventional LOA in treating FTL. METHODS: We retrospectively evaluated data on the ILA and LOA for FTL at our center. In total, 103 participants were enrolled, including 55 in the ILA group and 48 in the LOA group. RESULTS: The ILA required significantly less surgical time and resulted in less blood loss. The improvement rate of symptoms in symptomatic patients was 84%, and for urinary symptoms and abnormal urodynamic study findings, it was 77%. The postoperative maintenance rate for asymptomatic patients was 100%. Postoperative complications of ILA included delayed wound healing in two patients (3.6%). CONCLUSION: Compared with LOA, ILA offers advantages in terms of shorter operative time and less blood loss, with no significant difference in long-term symptom-improvement rates between the groups.

Intramedullary spinal capillary hemangioma with secondary neurulation defect in children.

Intramedullary spinal capillary hemangioma is a rare occurrence in pediatric patients, and only limited cases have been reported. This study presents the first two cases of spinal capillary hemangioma co-present with retained medullary cord and one case of spinal capillary hemangioma with lumbosacral lipomatous malformation. Previous literature on ten patients with this pathology was reviewed. We speculated pathogenesis, imaging features, and histopathologic findings of the disease.

lVentral tethering-is the prognosis worse than in dorsal tethering in the dysraphic spine?

OBJECTIVE: To compare cases of dysraphism with ventral tethering of cord with those with dorsal tethering and to find out any differences in the outcome of surgery in them. METHODS: We collected the data of 188 consecutively operated tethered cord patients at our institute in the past 7 years and divided them into ventral tethering and dorsal tethering groups. Those that we felt had both dorsal and ventral tethering were excluded. Their preoperative clinical, radiological, and baseline neurophysiological parameters as well as postoperative clinical and radiological parameters were analyzed in a retrospective study. RESULTS: Among the 188 tethered cord patients, 52 (28%) had ventral tethering and 136 (72%) had posterior tethering. Preoperative neurodeficit and cord signal changes as well as absent baseline MEP (of any one muscle) were significantly more associated with ventral tethered cord than the dorsal tethered cord. The neurological deterioration after surgery occurred significantly in the ventral tethered cord group than in the dorsal tethered cord group. Also, the postoperative MRI had more incomplete detethering cases in the ventral group than in the dorsal tethered cord group. CONCLUSION: Ventral tethered cord is more likely to present with preoperatively neurological deficits. It should be carefully identified in the preoperative MRI, so that the intraoperative difficulties in complete detethering and postoperative deterioration can be anticipated.

Walking recovery after tethered cord release.

After myelomeningocele (MMC) repair, a secondary tethered spinal cord occurs in almost all patients. The tethered spinal cord may result in progressive neurological deterioration and walking disability. This retrospective cohort study aimed to highlight the walking recovery one year after tethered cord release and its relation to the preoperative conus level. We reviewed the medical records at our university hospital from January 2014 to December 2022. The patients who underwent spinal cord untethering following lumbosacral MMC repair were included. We assessed the walking recovery one year after cord release using the modified Benzel scale. Thirty-seven patients met our selection criteria. There were 19 girls (51.4%) and 18 boys (48.6%). Their mean age at presentation was 8.6 years. The preoperative conus vertebral levels ranged between L4 and S3. One year after spinal cord release, 37.8% of the patients regained their walking ability. All the patients whose preoperative conus level was at S2 or S3 regained their walking ability. In contrast, all the patients with preoperative conus levels at L4 or L5 didn't regain their ability to walk. One-third (33.3%) of patients whose conus was at the S1 level regained their walking ability one year after cord release. One year after tethered cord release, 37.8% of the patients regained their walking ability. We found that the walking recovery was statistically associated with the preoperative conus level. A multicenter prospective study is required to support the results of this study.

Tethered cord syndrome from pediatric and adult perspectives: a comprehensive systematic review of 6135 cases.

OBJECTIVE: This study aimed to investigate the differences in clinical features, diagnostic examination, treatment, and pathological results between adult-onset and pediatric-onset tethered cord syndrome (TCS). METHODS: The authors searched the PubMed, Embase, and Cochrane Library databases through January 2023 for reports on TCS, extracting information on clinical features, imaging data, treatment modalities, prognosis, and pathological research results. A total of 6135 cases from 246 articles were included in the analysis. This review was conducted in accordance with the 2020 PRISMA guidelines and registered on PROSPERO. RESULTS: The most common adult clinical manifestations were pain, urinary symptoms, and numbness; in children, they were urinary symptoms, skin lesions, bowel symptoms, and unspecific motor deficits. Surgical treatment was the primary approach for both adults and children, with a higher clinical improvement rate observed in adults. However, adults also had a higher rate of surgical complications than children. TCS pathological studies have not yet identified the differences between adults and children, and the pathogenesis of adult-onset TCS requires further investigation. CONCLUSIONS: Adult-onset and pediatric-onset TCS exhibit certain differences in clinical characteristics, diagnostic examinations, and treatments. However, significant differences have not been found in current pathological studies between adults and children. Systematic review registration no.: CRD42023479450 (www.crd.york.ac.uk/prospero).

Expanding allelic and phenotypic spectrum of ZC4H2-related disorder: A novel hypomorphic variant and high prevalence of tethered cord.

ZC4H2 (MIM# 300897) is a nuclear factor involved in various cellular processes including proliferation and differentiation of neural stem cells, ventral spinal patterning and osteogenic and myogenic processes. Pathogenic variants in ZC4H2 have been associated with Wieacker-Wolff syndrome (MIM# 314580), an X-linked neurodevelopmental disorder characterized by arthrogryposis, development delay, hypotonia, feeding difficulties, poor growth, skeletal abnormalities, and dysmorphic features. Zebrafish zc4h2 null mutants recapitulated the human phenotype, showed complete loss of vsx2 expression in brain, and exhibited abnormal swimming and balance problems. Here we report 7 new patients (four males and three females) with ZC4H2-related disorder from six unrelated families. Four of the 6 ZC4H2 variants are novel: three missense variants, designated as c.142T>A (p.Tyr48Asn), c.558G>A (p.Met186Ile) and c.602C>T (p.Pro201Leu), and a nonsense variant, c.618C>A (p.Cys206*). Two variants were previously reported : a nonsense variant c.199C>T (p.Arg67*) and a splice site variant (c.225+5G>A). Five patients were on the severe spectrum of clinical findings, two of whom had early death. The male patient harboring the p.Met186Ile variant and the female patient that carries the p.Pro201Leu variant have a relatively mild phenotype. Of note, 4/7 patients had a tethered cord that required a surgical repair. We also demonstrate and discuss previously under-recognized phenotypic features including sleep apnea, arrhythmia, hypoglycemia, and unexpected early death. To study the effect of the missense variants, we performed microinjection of human ZC4H2 wild-type or variant mRNAs into zc4h2 null mutant zebrafish embryos. The p.Met186Ile mRNA variant was able to partially rescue vsx2 expression while p.Tyr48Asn and p.Pro201Leu mRNA variants were not. However, swimming and balance problems could not be rescued by any of these variants. These results suggest that the p.Met186Ile is a hypomorphic allele. Our work expands the genotypes and phenotypes associated with ZC4H2-related disorder and demonstrates that the zebrafish system is a reliable method to determine the pathogenicity of ZC4H2 variants.

Tethered cord syndrome in KBG syndrome.

Tethered cord syndrome (TCS) is characterized by leg pain and weakness, bladder and bowel dysfunction, orthopedic malformations such as scoliosis, and motor deficits caused by the fixation of the spinal cord to surrounding tissues. TCS is surgically treatable and often found in conjunction with other syndromic conditions. KBG syndrome is caused by variants in the ANKRD11 gene and is characterized by short stature, developmental delay, macrodontia, and a triangular face. The current study explores the prevalence of TCS in pediatric KBG patients and their associated signs and symptoms. Patients with KBG were surveyed for signs and symptoms associated with TCS and asked if they had been diagnosed with the syndrome. We found a high proportion of patients diagnosed with (11%) or being investigated for TCS (24%), emphasizing the need to further characterize the comorbid syndromes. No signs or symptoms clearly emerged as indicative of TCS in KBG patients, but some the prevalence of some signs and symptoms varied by sex. Male KBG patients with diagnosed TCS were more likely to have coordination issues and global delay/brain fog than their female counterparts. Understanding the presentation of TCS in KBG patients is critical for timely diagnosis and treatment.

Urological Aspects of Spinal Dysraphism.

Micturition requires complex interplay involving bladder, peripheral neural network, spinal cord, and brain. Spinal dysraphism presents variety of neural lesions that may affect this interplay leading to neurogenic bladder. However, the diagnosis of neurogenic bladder in those with spinal dysraphism is often difficult and contrasted to other types of neurogenic bladder caused by complete neural lesions such as spinal cord injury or brain tumor. Typically, neurogenic bladder caused by spinal dysraphism shows lower motor neuron lesion and partial neural injury. However, upper motor signs can be seen with the occurrence of tethered cord syndrome and developmental immaturity of bladder control often complicated by fecal impaction. Thus, the diagnosis of tethered cord syndrome should be made cautiously. Several invasive and noninvasive diagnostic modalities could be applied for accurate diagnosis, preventing renal damage and controlling urinary incontinence. However, it should be borne in mind that no single study is definitive for accurate diagnosis, so it requires careful scrutiny in interpretation. The follow-up scheme of these patients should be determined to detect urological deterioration due to the development of tethered cord syndrome. Since the development of tethered cord syndrome shows diverse nature implicating urological and orthopedic issues, multidisciplinary collaboration is essential for comprehensive care.

Secondary Neurulation Defects: Retained Medullary Cord.

Retained medullary cord (RMC) is a defect resulting from impaired secondary neurulation. Intraoperatively, RMC is recognizable as an elongated cord-like structure caudal to the conus, that contains histologically confirmed neuroglial components and a lumen with an ependymal lining. It characteristically does not possess neurological function. This chapter aims to summarize (1) the mechanisms that lead to the occurrence of RMC; (2) the various forms of RMC, such as cystic RMC and 'possible RMC', and (3) the treatment strategies, especially untethering through limited exposure.

Updates on Intraoperative Neurophysiology During Surgery for Spinal Dysraphism.

Spinal dysraphism is a group of disorders resulting from an embryologic failure of spinal cord development which can lead to a radicular-medullary mechanical stretch that generates vascular compromise and hypoxic-ischemic damage to the nervous structures of the conus-cauda region.Thus, the clinical relevance of the different types of spinal dysraphism is related to the possible neurologic deficits resulting from spinal cord tethering. The clinical presentation is heterogenous: from asymptomatic to very compromised patients. The indications and the time of a detethering surgery are still subject of debate, although there is an agreement on the high standards of treatment that have to be offered by the surgery. Intraoperative neurophysiology (ION) contributes to the safety of tethered cord surgery in reducing the risks of iatrogenic neurological damages.

Chiari Type 1 Malformation and Syringomyelia in Children: Classification and Treatment Options.

Chiari type 1 malformation (CIM) is defined as tonsillar ectopia of >5 mm, while syringomyelia (SM) is defined as a cerebrospinal fluid (CSF)-filled cavity larger than 3 mm dissecting the spinal cord. Over the last decades, our understanding of these pathologies has grown; however, many controversies still exist almost in every aspect of CIM and SM, including etiology, indication for treatment, timing of treatment, surgical technique, follow-up regime, and outcome. This chapter provides a comprehensive overview on different aspects of CIM and SM and on the still existing controversies, based on the evidence presently available. Future directions for clinical research concerning CIM and SM treatment and outcome are elaborated and discussed as well.

Endoscopic spinal cord untethering using a 1 cm skin incision technique in pediatrics: a technical case report.

BACKGROUND: Spinal cord untethering by sectioning the filum terminale is commonly performed in tethered cord syndrome patients with minor abnormalities such as filar lipoma, thickened filum terminale, and low conus medullaris. Our endoscopic surgical technique, using the interlaminar approach, allows for sectioning the filum terminale through a very small skin incision. To our knowledge, this procedure has not been previously reported. This is the first case report involving a 1 cm skin incision. CASE PRESENTATION: A 9-month-old male patient was referred to our neurosurgical department due to a coccygeal dimple. MRI revealed a thickened fatty filum. After considering the treatment options for this patient, the parents agreed to spinal cord untethering. A midline 1 cm skin incision was made at the L4/5 vertebral level. Untethering by sectioning the filum terminale was performed by full endoscopic surgery using the interlaminar approach. The procedure was uneventful and there were no postoperative complications. CONCLUSIONS: In terms of visibility and minimizing invasiveness, our surgical technique of using the interlaminar approach with endoscopy allows for untethering by sectioning the filum terminale through a very small skin incision.

Current status and challenges of neurosurgical procedures for patients with myelomeningocele in real-world Japan.

BACKGROUND: Little is known about the real-world status of neurosurgical treatment of myelomeningocele patients. OBJECTIVE: To investigate the real-world status of neurosurgical treatment of myelomeningocele patients, medical claims data provided by the Japan Medical Data Center (JMDC) were analyzed. METHODS: The health claims data of 556 patients with myelomeningoceles from January 2005 to March 2020 were examined. The number of neurosurgical procedures, including myelomeningocele repair, tethered cord release, cerebrospinal fluid (CSF) shunt, CSF drainage, and endoscopic third ventriculostomy (ETV), was determined. RESULTS: A total of 313 neurosurgical procedures were performed for 135 patients in 74 institutions during the study period. The shunt survival rate was most affected by shunts that were revised when the patient was less than 1 year old, which had a significantly lower survival rate than all of the initial shunts performed when the patient was less than on1 year old; the 1-year shunt survival rate was 35 vs 64% (P = 0.0102). The survival rate was significantly lower in patients younger than 1 year who had CSF drainage before shunting compared to those younger than 1 year who did not have CSF drainage before shunting; the 1-year shunt survival rate was 27 vs 59% (P = 0.0196), and 81% of patients remained free of tethered cord release 10 years later. CONCLUSIONS: In this study, a revised shunt of less than 1 year of age and CSF drainage before shunting were the factors that lowered the shunt survival rate in the real world for CSF shunts for hydrocephalus associated with myelomeningocele.

Tethered cord release in patients after open fetal myelomeningocele closure: intraoperative neuromonitoring data and patient outcomes.

PURPOSE: The purpose of the study was to better understand the clinical course and impact of tethered cord release surgery on patients who have previously undergone open spinal dysraphism closure in utero. METHODS: This is a single-center retrospective observational study on patients undergoing tethered cord release after having previously had open fetal myelomeningocele (MMC) closure. All patients underwent tethered cord release surgery with a single neurosurgeon. A detailed analysis of the patients' preoperative presentation, intraoperative neuromonitoring (IONM) data, and postoperative course was performed. RESULTS: From 2009 to 2021, 51 patients who had previously undergone fetal MMC closure had tethered cord release surgery performed. On both preoperative and postoperative manual motor testing, patients were found to have on average 2 levels better than would be expected from the determined anatomic level from fetal imaging. The electrophysiologic functional level was found on average to be 2.5 levels better than the anatomical fetal level. Postoperative motor levels when tested on average at 4 months were largely unchanged when compared to preoperative levels. Unlike the motor signals, 46 (90%) of patients had unreliable or undetectable lower extremity somatosensory evoked potentials (SSEPs) prior to the tethered cord release. CONCLUSION: Tethered cord surgery can be safely performed in patients after open fetal MMC closure without clinical decline in manual motor testing. Patients often have functional nerve roots below the anatomic level. Sensory function appears to be more severely affected in patients leading to a consistent motor-sensory imbalance.

A comparison of MRI appearance and surgical detethering rates between intrauterine and postnatal myelomeningocele closures: a single-center pilot matched cohort study.

INTRODUCTION: Intrauterine myelomeningocele repair (IUMR) and postnatal myelomeningocele repair (PNMR) differ in terms of both setting and surgical technique. A simplified technique in IUMR, in which a dural onlay is used followed by skin closure, has been adopted at our institution. The goal of this study was to compare the rates of clinical tethering in IUMR and PNMR patients, as well as to evaluate the appearance on MRI. METHODS: We conducted a retrospective review of 36 patients with MMC repaired at our institution, with 2:1 PNMR to IUMR matching based on lesion level. A pediatric neuroradiologist blinded to the clinical details reviewed the patients' lumbar spine MRIs for the distance from neural tissue to skin and the presence or absence of a syrinx. An EMR review was then done to evaluate for detethering procedures and need for CSF diversion. RESULTS: Mean age at MRI was 4.0 years and mean age at last follow-up was 6.1 years, with no significant difference between the PNMR and IUMR groups. There was no significant difference between groups in the distance from neural tissue to skin (PNMR 13.5 mm vs IUMR 17.6 mm; p = 0.5). There was no difference in need for detethering operations between groups (PNMR 12.5% vs IUMR 16.7%; RR 0.75; CI 0.1-5.1). CONCLUSIONS: There was no significant difference between postnatal- and intrauterine-repaired myelomeningocele on MRI or in need for detethering operations. These results imply that a more straightforward and time-efficient IUMR closure technique does not lead to an increased rate of tethering when compared to the multilayered PNMR.

Gestational age-specific reference standards of low-lying conus medullaris level in fetuses: a prospective cohort study.

PURPOSE: To explore the position change of fetal conus medullaris by ultrasound, and to propose gestational age-specific references for the lower limits of fetal conus medullaris level. METHODS: We prospectively collected the imaging and clinical data of fetuses whose mothers accepted routine prenatal ultrasonic follow-ups in the Department of Medical Ultrasonics, Chinese PLA General Hospital, between November 2020 and April 2021. By assigning to the conus medullaris levels, calculating statistical data, and performing linear regression analysis, we determined the correlation between the conus medullaris level and gestational week, as well as between the 95th percentile of the conus medullaris level, i.e., the lower limit of the conus medullaris level, and gestational week. RESULTS: We included 1202 different fetuses at 17-40 gestational weeks in the study. Both the conus medullaris level and the 95th percentile of the conus medullaris level were linearly correlated with gestational week. We calculated the adjusted values of the lower limits of fetal conus medullaris levels, that is, the theoretical references of the lower limits, according to the linear regression equation, and composed a comparison table. CONCLUSION: The fetal conus medullaris position continues changing cranially with gestational weeks during the whole pregnancy. The conus medullaris of a term fetus should not lie below the L2 vertebra level at birth. We proposed reference criteria of fetal low-lying conus medullaris for each gestational week from 17 to 40 weeks of gestational age, which potentially help prompt diagnosis and improve prognosis of fetal tethered cord syndrome.

Syndrome of anterior neural stalk, vertebral abnormality, enteric duplication cyst, and diaphragmatic hernia related to persistent ventral neurenteric canal: report of two cases.

PURPOSE: Abnormalities in notochordal development can cause a range of developmental malformations, including the split notochord syndrome and split cord malformations. We describe two cases that appear related to unusual notochordal malformations, in a female and a male infant diagnosed in the early postnatal and prenatal periods, which were treated at our institution. These cases were unusual from prior cases given a shared constellation of an anterior cervicothoracic meningocele with a prominent "neural stalk," which coursed ventrally from the spinal cord into the thorax in proximity to a foregut duplication cyst. METHODS: Two patients with this unusual spinal cord anomaly were assessed clinically, and with neuroimaging and genetics studies. RESULTS: We describe common anatomical features (anterior neural stalk arising from the spinal cord, vertebral abnormality, enteric duplication cyst, and diaphragmatic hernia) that support a common etiopathogenesis and distinguish these cases. In both cases, we opted for conservative neurosurgical management in regards to the spinal cord anomaly. We proposed a preliminary theory of the embryogenesis that explains these findings related to a persistence of the ventral portion of the neurenteric canal. CONCLUSION: These cases may represent a form of spinal cord malformation due to a persistent neurenteric canal and affecting notochord development that has rarely been described. Over more than 1 year of follow-up while managed conservatively, there was no evidence of neurologic dysfunction, so far supporting a treatment strategy of observation.