Idiopathic Hydrocephalus Research Articles

Valvuloplasty for tricuspid stenosis caused by a ventriculoatrial shunt

A 45-year-old lady developed significant symptomatic tricuspid valve stenosis due to a ventriculoatrial (VA) shunt, implanted in childhood for idiopathic hydrocephalus. This appeared to be a rare complication of a VA shunt. After detailed discussions in multidisciplinary meeting, she underwent successful treatment with a single balloon valvuloplasty. To date, she has not suffered any complications.

Diffusion Tensor Imaging in Patients With Adult Chronic Idiopathic Hydrocephalus

Diffusion tensor imaging (DTI) parameters were investigated in patients with chronic idiopathic hydrocephalus to evaluate microstructural changes of brain tissue caused by chronic ventricular dilatation. Eleven patients fulfilling the criteria for possible or probable idiopathic normal pressure hydrocephalus and 10 healthy control subjects underwent MRI at 3 Tesla, including DTI with 12 gradient directions. Patients were scanned before lumbar cerebrospinal fluid (CSF) withdrawal tests. Differences in fractional anisotropy (FA) and mean diffusivity (MD) between patients and controls were assessed using 2 different methods: manual definition of regions of interest and a fully automated method, TBSS (Tract-Based Spatial Statistics). DTI parameters were correlated with clinical findings. Compared with the control group, patients with chronic idiopathic hydrocephalus had significantly higher MD values in both the periventricular corticospinal tract (CST) and the corpus callosum (CC), whereas FA values were significantly higher in the CST but lower in the CC. DTI parameters of the CST correlated with the severity of gait disturbances. Microstructural changes in periventricular functionally relevant white matter structures (CSF, CC) in chronic idiopathic hydrocephalus can be visualized using DTI. Further studies should investigate the change of DTI parameters after CSF shunting and its relation to neurologic outcome.

Hyperemic hydrocephalus: a new form of childhood hydrocephalus analogous to hyperemic intracranial hypertension in adults

In the majority of adults with idiopathic intracranial hypertension (IIH), there is an elevation in venous pressure associated with a venous outflow stenosis. In about 15% of IIH patients the elevated venous pressure is associated with an elevation in blood flow but little or no evidence of a stenosis. Venostenotic IIH and idiopathic hydrocephalus in children with a normal blood inflow have been shown to be equivalent. The aim of this study was to test whether children with hydrocephalus and an elevated arterial inflow have a vascular pathophysiology that is analogous to the hyperemic form of IIH in adults. Nine children with idiopathic hydrocephalus underwent MR imaging with flow quantification and were found to have arterial inflows 2 SDs above the mean for normal controls. Measurements of the head circumference, ventricular enlargement, total blood inflow, superior sagittal sinus (SSS)/straight sinus (SS) outflow, and the degree of collateral venous flow were performed. The results were compared with findings in 14 age-matched controls. In hyperemic hydrocephalus the cerebral blood inflow was elevated but the SSS and SS outflows were in the normal range. The sinus outflow as a percentage of the inflow was reduced by 8 percentage points in the SSS territory and 5 percentage points in the SS territory compared with findings in the controls (p = 0.04, p = 0.003, respectively), suggesting blood was returning via collateral channels. Similar to patients with hyperemic IIH, children with hyperemic hydrocephalus show a significant elevation in collateral venous flow, indicating that the same venous pathophysiology may be operating in both conditions.

SARCOIDOSIS PRESENTING AS BRAINSTEM ISCHEMIC STROKE

Sarcoidosis is characterized by noncaseating granulomas that involve the nervous system in 5% of patients.1,2 Stroke is uncommon despite small vessel inflammation, but when present, is usually supratentorial and rarely the first symptom of neurosarcoidosis.3 We present two patients with brainstem strokes as the initial manifestation of sarcoidosis. ### Case reports. #### Case 1. A 35-year-old African American man suddenly developed left-sided weakness in September 2001. He had mild hypertension diagnosed 6 months earlier, but no other risk factors. MRI 6 days later demonstrated increased T2 signal and early enhancement in the pons; no diffusion imaging was performed. Stroke evaluation was not performed and he was treated with aspirin and clopidogrel. In July 2002, he developed lethargy and urinary incontinence. MRI without contrast showed communicating hydrocephalus and a shunt was placed; CSF was not examined. Over the next year, the patient required six shunt revisions for presumed idiopathic hydrocephalus. Over the subsequent 2 years, he developed bilateral deafness, ataxia, and memory impairment. Our initial examination in March 2004 revealed a blood pressure of 140/90 mm Hg, short term memory loss, hearing impairment, and left greater than right spastic hemiparesis. Brain and spine MRI showed encephalomalacia of the mid-pons and leptomeningeal enhancement (figure, A). CSF showed 24 leukocytes/mm …

Bmi-1 over-expression in neural stem/progenitor cells increases proliferation and neurogenesis in culture but has little effect on these functions in vivo

The polycomb gene Bmi-1 is required for the self-renewal of stem cells from diverse tissues, including the central nervous system (CNS). Bmi-1 expression is elevated in most human gliomas, irrespective of grade, raising the question of whether Bmi-1 over-expression is sufficient to promote self-renewal or tumorigenesis by CNS stem/progenitor cells. To test this we generated Nestin-Bmi-1-GFP transgenic mice. Analysis of two independent lines with expression in the fetal and adult CNS demonstrated that transgenic neural stem cells formed larger colonies, more self-renewing divisions, and more neurons in culture. However, in vivo, Bmi-1 over-expression had little effect on CNS stem cell frequency, subventricular zone proliferation, olfactory bulb neurogenesis, or neurogenesis/gliogenesis during development. Bmi-1 transgenic mice were born with enlarged lateral ventricles and a minority developed idiopathic hydrocephalus as adults, but none of the transgenic mice formed detectable CNS tumors, even when aged. The more pronounced effects of Bmi-1 over-expression in culture were largely attributable to the attenuated induction of p16Ink4a and p19Arf in culture, proteins that are generally not expressed by neural stem/progenitor cells in young mice in vivo. Bmi-1 over-expression therefore has more pronounced effects in culture and does not appear to be sufficient to induce tumorigenesis in vivo.

O.105 Survival rate and reliability of the ProGAV adjustable CSF shunt: Results of the German prospective multicenter observational study

El riñón pediátrico es sitio frecuente de tumores que exhiben alteraciones cromosómicas características. El más común es el nefroblastoma o tumor de Wilms (TW) que se asocia con dos loci: 11p13 (WT1) y 11p15 (WT2 ó BWS), este último ligado también del síndrome de Beckwith-Wiedemann. Otros dos genes que parecen estar implicados son WT3 y WT4; además, dos anomalías específicas (adquisición 1q y deleción 22) se han correlacionado de manera independiente con un peor pronóstico en TW. Otras neoplasias con rearreglos cromosómicos, tales como los carcinomas renales (CRs), son mucho menos frecuentes en niños (entre el 1.8 y el 6.3% de todos los tumores renales malignos). Entre estos, se han identificado “CRs con translocación” que afectan el locus Xp11, siendo los dos tipos más importantes t(X;1), y t(X;17). El nefroma mesoblástico congénito (NMC) es un tumor renal de recién nacidos y lactantes. Los NMCs de la variedad celular se caracterizan por una translocación específica t(12;15)(p13;q25), misma que se encuentra también en los fibrosarcomas congénitos extrarenales, y que permite establecer una correspondencia genética entre estos dos tumores (NMC y fibrosarcoma congénito). Los tumores rabdoides (TR) del riñón son neoplasias muy infrecuentes y muy agresivas, que aparecen con una edad promedio de 11 meses. Al menos 50% de los TRs muestran anormalidades en el gen hSNF5/INI1, situado en el locus 22q11.2. Este gen probablemente está involucrado en la modulación transcripcional de otros genes, tales como el oncogen c-Myc, y de la vía de transducción de la proteína RB-retinoblastoma.The pediatric kidney is a common site for tumors carrying specific chomosomal alterations. The most common of these is the nephroblastoma or Wilms tumor (WT), which is associated with anomalies in two loci: 11p13 and 11p15, the latter also linked to Beckwith-Wiedemann syndrome. Two other genes that seem to be implicated are WT3 and WT4. In addition, 1q gains or 22 deletions have been shown to independently be associated with a worst prognosis in WTs. Other neoplasms with chromosomal rearrangements, such as Renal Cell carcinomas (CRs) are much less frequent in children (between 1.8 and 6.3 % of all malignant renal tumors). Among these, the “translocation renal carcinomas” have been identified involving the locus Xp11 with two main types of translocations: t(X;1), and t(X;17). Congenital mesoblastic nephroma (NMC) is a renal tumor affecting newborns and young infants. NMCs of the cellular type feature a specific translocation t(12;15)(p13;q25), which is also present in congenital fibrosarcomas outside of the kidney. These findings have led to conclude that these two tumors (NMC and congenital fibrosarcoma) are genetically equivalent. Rhabdoid tumors (TRs) of the kidney are very rare and aggressive neoplasms that appear with a mean age of 11 months. At least 50% of these TRs carry abnormalities in the hSNF5/INI1 gene, at 22q11.2. This gene is probably involved in transcriptional modulation of other genes such as the oncogene c-Myc, and also of the retinoblastoma protein RB signaling pathway.

O.104 Treatment of normal pressure hydrocephalus using the GAV shunt system: Preliminary results of a Spanish multicentre study

Introduction: Surgical treatment of chronic hydrocephalus in an adult requires use of a shunt system that combines low-pressure opening with the absence of overdrainage phenomena. We present the preliminary results of a prospective multicentre study involving 13 hospitals in Spain of treatment of normal pressure hydrocephalus (NPH) with a gravity assisted shunt (GAV; Aesculap), the main purpose of which was to evaluate its efficacy and safety, as well as analyse possible factors influencing patient evolution. Material and Method Design: This prospective, multicentre study of clinical practice with a common protocol for treatment of adult idiopathic hydrocephalus included 136 patients (59% men; age: 73.4±6.4). Of these, six-month follow-up data are available for 62 patients and twelve months for 33. Data were recorded on epidemiological details, symptoms, evolution time, neuroradiological findings, vascular risk factors, and rout/ICP records. The main outcome variables were symptoms associated with hydrocephalus, measured with the NPH scale (gait, sphincter control and cognitive function). Secondary outcome variables included evolution according to radiological parameters and post-surgical complications. Statistics: ANOVA tests. Results: A significant improvement was noted in all the parameters of the NPH scale (gait: 3.2±1.0 vs 2.5±1.1 vs 2.2±1.1; cognitive function: 2.1±0.9 vs 1.8±0.9 vs 1.7±0.9; sphincter control: 2.9±1.2 vs 2.5±1.3 vs 2.4±1.3; p<0.05) highly evident in the third month and slightly less so in the sixth month, with a tendency to stabilise after 12 months. No association was found between improvement and demographic data, evolution time, vascular risk factors, rout value or radiological findings. The complications were less than 5%. Conclusion: The GAV shunt system proves highly efficient for the treatment of hydrocephalus, with a low rate of complications. Most patients showed improvement within six months.

Idiopathic hydrocephalus in children and idiopathic intracranial hypertension in adults: two manifestations of the same pathophysiological process?

Both idiopathic intracranial hypertension (IIH) in adults and idiopathic hydrocephalus in children have been shown to involve elevations in venous pressure that resolve once the cerebrospinal fluid pressure is reduced. It has been assumed that the venous pressure elevations in both conditions are not hemodynamically significant, but measurement of venous collateral flow in IIH has shown these pressure elevations to be of consequence. The authors used the same methodology to see if the venous pressure elevations noted in childhood hydrocephalus are important. Fourteen patients with idiopathic childhood hydrocephalus underwent magnetic resonance imaging with flow quantification. The degree of ventricular enlargement, total blood inflow, and superior sagittal/straight sinus outflow was measured. The degree of collateral venous flow was calculated for each venous territory. The findings were compared with findings in 14 age-matched controls. In children with hydrocephalus the cerebral blood inflow was normal, but the superior sagittal sinus (SSS) and straight sinus outflows were reduced by 27% and 38%, respectively, compared with measurements in controls (p = 0.03 and 0.002). These findings suggest that approximately 150 ml of blood per minute was returning via collateral channels from that portion of the brain drained by the SSS, and 60 ml/minute was returning from collaterals in the deep venous territory. Similarly to patients with IIH, children with hydrocephalus show a significant elevation in collateral venous flow, indicating that the same venous pathophysiological process may be operating in both conditions. Whether or not the ventricles dilate may depend on the differences in brain compliance between adults and children.

CSF Pulse Pressure and B Waves

Abstract Object. The appearance of numerous B waves during intracranial pressure (ICP) registration in patients with idiopathic adult hydrocephalus syndrome (IAHS) is considered to predict good outcome after shunt surgery. The aim of this study was to describe which physical parameters of the cerebrospinal fluid (CSF) system B-waves reflect and to find a method that could replace long-term B-wave analysis. Methods. Ten patients with IAHS were subjected to long-term registration of ICP and a lumbar constant-pressure infusion test. The B-wave presence, CSF outflow resistance (Rout), and relative pulse pressure coefficient (RPPC) were assessed using computerized analysis. The RPPC was introduced as a parameter reflecting the joint effect of elastance and pulsatory volume changes on ICP and was determined by relating ICP pulse amplitudes to mean ICP. Conclusions. The B-wave presence on ICP registration correlates strongly with RPPC (r = 0.91, p &lt; 0.001, 10 patients) but not with CSF Rout. This correlation indicates that B waves—like RPPC—primarily reflect the ability of the CSF system to reallocate and store liquid rather than absorb it. The RPPC-assessing lumbar short-term CSF pulse pressure method could replace the intracranial long-term B-wave analysis.

The syndrome of hydrocephalus in young and middle-aged adults (SHYMA)

Objectives: Currently, headache, nausea/vomiting, visual changes, and altered mental status are accepted as indications for the evaluation of hydrocephalus in children; while dementia, gait apraxia, and urinary incontinence remain indications in the elderly. The clinical presentation of hydrocephalus in young and middle-aged adults remains poorly described. Hence, middle-aged patients with mild gait, cognitive, or urinary symptoms unaccompanied by clear exam findings often remain undiagnosed and untreated.Methods: We report the clinical presentation, treatment, and outcomes of 46 adults (ages 16–55 years) presenting with congenital, acquired, or idiopathic hydrocephalus with imaging-documented ventriculomegaly and elevated CSF pressure.Results: Primary symptoms were related to gait (70%), cognition (70%), urinary urgency (48%), and headaches (56%). Eighty-four percent complained of impaired job performance. The exam findings were subtle or absent (no gait apraxia, minor gait changes in 42.9%, mildly abnormal Mini Mental State exams in only14.3%, and incontinence in only 3.6%). Twenty-nine patients underwent ventriculoperitoneal (VP) shunting, and 11 endoscopic third ventriculostomy, of whom six subsequently required a VP shunt. Symptomatic improvement was observed in 93% of patients 16± 11 months after shunting (56% complete resolution, 37% partial resolution). Patients had been followed for their symptoms an average of 6 years (range, 1–30) prior to diagnosis.Discussion: We propose that there exists a clinically distinct syndrome of hydrocephalus in young and middle-aged adults (SHYMA) that comprises hydrocephalus of all etiologies. SHYMA is characterized by complaints of impaired gait, cognition, bladder control, and headaches, with a discrepancy between the prominence of symptoms and the subtlety of clinical signs. Despite the subtlety of clinical signs, CSF diversion treatment is effective at resolving symptomatology.

Reasons for shunting and reasons for revision: a survey based on data from the UK Shunt Registry

Data from the UK Shunt Registry for procedures carried out between May 1999 and April 2002 was examined for the given reason for shunting and the given reasons for subsequent revisions. Sixty-eight per cent of patients receiving shunts had 'secondary' hydrocephalus, with congenital and idiopathic hydrocephalus accounting for 18% and 14% respectively. Patients with Spina Bifida represented 5.1% of shunted patients.

PET-studies in idiopathic chronic hydrocephalus before and after shunt-treatment: the role of risk factors for cerebrovascular disease (CVD) on cerebral hemodynamics.

To investigate the impact of cerebrovascular risk factors in idiopathic chronic hydrocephalus concerning cerebral hemodynamics and clinical outcome after shunting. Global cortical cerebral blood flow (CBF) and cerebrovascular reserve capacity (CVR) in 53 patients (67 +/- 11 yrs) were determined by 15-0-water-PET studies before and after administration of acetazolamide (1 g) prior (pre), one week (7 d) and seven months (7 m) after shunting. According to the prevalence of vascular risk factors (American subcommittee on reporting standards for cerebrovascular disease) patients were classified into a "low-risk" (n = 27) and "high-risk" (n = 20) group; patients with a history of stroke (n = 6) were separated. After 7 months, clinical outcome was assessed according to Stein and Langfitt. While CBF in "high-risk" patients prior to surgery was significantly lower in clinical responder compared to non-responder (32 +/- 5 vs. 42 +/- 15 ml/100 ml/min; p < 0.05), CVR was marginal in both outcome groups (< 30%). One week after shunting, CVR in responder of "high-risk" significantly increased (64 +/- 30 vs. 31 +/- 10% pre; p < 0.01). In "low-risk" patients, differences in CVR prior to shunting were found: CVR was lower in clinical responder than in non-responder (36 +/- 11 vs. 47 +/- 22% pre; p > 0.05) and deteriorated in non-responder (29 +/- 15% vs. 47 +/- 22 pre; p < 0.02) one week after shunting. Different peri-operative characteristics in global CVR regarding clinical response after shunting between both "risk-groups" were observed. Pathophysiological mechanisms upon clinical sequels after shunting in idiopathic hydrocephalus may not be unique.

The Prognosis of Children with Hydrocephalus and Congenital Heart Disease

The association of congenital hydrocephalus and heart disease in children is infrequent, but may present considerable dilemmas in management. This report describes the treatment and prognosis of 11 children with both clinical problems. There were 5 males and 6 females. Hydrocephalus occurred following aqueductal stenosis in 5 children and the Dandy-Walker malformation in 3. Three children were diagnosed with idiopathic hydrocephalus. Ten children underwent cerebrospinal fluid diversion procedures for control of hydrocephalus. Five children received pharmacological therapy for cardiac disease; 4 children required surgical correction. Two children died from medical conditions; 2 families declined treatment. Follow-up from 2 to 7 years in the remaining 7 children demonstrated moderate or severe neurodevelopmental disability in 5. One child at 2 years of age showed borderline developmental disability while 1 child is developing normally at 10 years of age. Overall the occurrence of symptomatic hydrocephalus and heart disease in the perinatal period resulted in mortality or neurodevelopmental disability in 9/11 children.

Prospective analysis by computed tomography and long-term outcome of 23 adult patients with chronic idiopathic hydrocephalus.

From 1986 to 1989, 23 adult patients (average age, 70 yr) with idiopathic chronic hydrocephalus received shunts with medium-pressure Pudenz-Schulte valves for suspected normal pressure hydrocephalus. Prospective clinical and computed tomographic monitoring was continued for at least 5 years. We observed the formation of a hypodense subdural collection in each of 10 patients (43%). Those collections that occurred early, i.e., within the first 9 postoperative days, evolved differently from those that occurred late; only early hypodense collections became subdural hematomas (three cases). In one case, a subdural hematoma was already present 9 days after surgery, so that four patients (17%) presented a subdural hematoma within the first 2 postoperative months. Our long-term follow-up revealed three patients (13%) with hypodense subdural collections, which appeared more than 2 months after surgery. None of the collection evolved into a subdural hematoma. Thirteen patients (57%) died between 9 and 68 months (average, 20 mo) after surgery, most often of an ischemic stroke. During the 1st postoperative year, there was improvement in the condition of 22 patients (96%) who had received a ventricular shunt; 21 of these patients (91%) remained improved until death or for at least 5 years.

Infantile hydrocephalus in the south-western region of Saudi Arabia.

During the period January 1988 to December 1990, the overall incidence of infantile hydrocephalus in the south-western region of Saudi Arabia was 0.81/1000. The series comprised 61 infants affected by infantile hydrocephalus. The total number of live births in this period was 74,923. Hydrocephalus associated with spinal dysraphism (spina bifida cystica and encephalocoele) constituted 24 cases (39.3%), and there was aqueduct stenosis in 10 cases (16.4%). Nine cases (11.9%) were post-meningitic, and seven (14.8%) post-haemorrhagic. There was Dandy-Walker malformation in five cases (8.2%), three (4.9%) had congenital idiopathic hydrocephalus, two (3.3%) congenital toxoplasmosis, and one (1.6%) isolated Arnold Chiari malformation. There were congenital causes in 45 cases, 73.7% of all the cases in this series (incidence: 0.6/1000 births). Of the remaining 16 cases (26.3%) which were due to postnatal factors, seven (11.5%) were caused by acquired cerebral haemorrhage, only two of them being premature, and the other nine (14.8%) were due to meningitis. The data indicate the predominance of prenatal causes of infantile hydrocephalus and the relatively low contribution of extreme prematurity. This is possibly due to the high mortality of this group of infants in this region.

Communicating hydrocephalus in adults

A series of adult patients with idiopathic hydrocephalus in whom shunts were placed are presented. Preoperative diagnostic and prognostic criteria were evaluated. Findings on computed tomographic scans (significant ventricular enlargement, the absence of gyral atrophy, and the absence of sylvian fissure enlargement) and the clinical triad associated with hydrocephalus in adults (dementia, ataxia, and especially incontinence of urine) all contributed individually and in combination to the diagnosis of hydrocephalus that could be corrected by surgery. The use of radionucleotide cisternography did not add any additional information. Its use as a diagnostic or prognostic aid is, therefore, suspect. The use of high-pressure shunting systems for the initial shunting procedure is emphasized.

Communicating Hydrocephalus in Adults: Prediction of Outcome after Ventricular Shunting Procedures

Abstract A series of adult patients with idiopathic hydrocephalus in whom shunts were placed are presented. Preoperative diagnostic and prognostic criteria were evaluated. Findings on computed tomographic scans (significant ventricular enlargement, the absence of gyral atrophy, and the absence of sylvian fissure enlargement) and the clinical triad associated with hydrocephalus in adults (dementia, ataxia, and especially incontinence of urine) all contributed individually and in combination to the diagnosis of hydrocephalus that could be corrected by surgery. The use of radionucleotide cisternography did not add any additional information. Its use as a diagnostic or prognostic aid is, therefore, suspect. The use of high-pressure shunting systems for the initial shunting procedure is emphasized.

Normal-pressure hydrocephalus in rheumatic patients. A diagnostic pitfall.

In our rheumatologic clinics the diagnosis of idiopathic hydrocephalus has been made in six patients during the past three years. This disorder is characterized by the triad of gait disturbance, urinary incontinence, and dementia, all variable in severity.1 2 3 Since this diagnosis had not been made previously in our department of rheumatology, we assume that it has hitherto been overlooked. The mean period between the development of the first symptoms of normal-pressure hydrocephalus in our patients and the time when the disorder was diagnosed was 3.3 years; symptoms and signs were at first ascribed to arthritis and age. The main . . .

Surgical treatment of idiopathic hydrocephalus in elderly patients.

We studied 45 adult patients with idiopathic hydrocephalus who had undergone a shunting procedure at the Mayo Clinic during a 16-year period. The follow-up period extended up to 157 months. Thirty-four of the 45 patients (75%) improved at some time after the operation, and 19 (42%) experienced continuous improvement. The median duration of improvement was 24 months. Patients who had symptoms for less than 2 years were most likely to improve. CT played an important role in the diagnosis and follow-up, but ventricular size or degree of atrophy did not correlate with response.

Hydrocephalus in the adult secondary to the rupture of intracranial arterial aneurysms.

HE interest paid during the last years to the hydrocephalic syndrome in the adult is reflected in the increasing number of publications dealing with this subject. 1,~,s,11,14,1~,1~,~~ The nomenclature is somewhat confusing, for terms such as occult hydrocephalus, non-pressure hydrocephalus, normal-pressure hydrocephalus, low-pressure hydrocephalus, idiopathic hydrocephalus, and nonobstructive adult have been used to refer, within some limitations, to the same clinical entity. Some new etiological aspects have been reported, TM and more active therapy is being carried out in most neurological centers. This report concerns the hydrocephalic picture that may develop in the adult secondary to subarachnoid hemorrhage from the rupture of arterial intracranial aneurysms. A classical study of the reactions caused by blood when injected in the subarachnoid space and the relation of these reactions to the development of hydrocephalus was reported in 1928 by Bagley. 4,~ Although since then both clinical ~,6,12,2~176 and pathological 3,6,2~ reports on short series have been published, there are still no statistics on the incidence of this complication in a large series of adults 3~ and the frequency of hydrocephalus due to basal arachnoiditis following subarachnoid hemorrhage is not known? ~ Statements like these, and the fact that during the treatment of our cases some interesting clinical, radiological, and etiological factors were found, have prompted this study.