Left-sided valvular heart disease associated with poor functional outcomes in patients with acute ischaemic stroke undergoing endovascular thrombectomy

Background

Valvular heart disease (VHD) may be an important comorbidity acute ischemic stroke (AIS), adversely affecting cerebral haemodynamics and consequently limiting functional recovery after endovascular thrombectomy (EVT). We examined the clinical profile and outcomes in patients with or without VHD who underwent EVT for AIS.

Methods

Consecutive patients (n=224) who underwent EVT for AIS were examined. All patients underwent transthoracic echocardiography within 6 months of AIS. The patients were divided into those with left-sided VHD (at least moderate severity of aortic valve or mitral valve stenosis or regurgitation) and those without. Univariate and multivariate analyses were employed to compare clinical profile and outcomes.

Results

There were 47 (20.9%) patients with VHD. Of these, 31 (66.0%) had mitral regurgitation, 15 (31.9%) had aortic stenosis, 8 (17.0%) had aortic regurgitation and 6 (12.8%) had mitral stenosis. VHD patients had a higher prevalence of atrial fibrillation (68.1% vs 43.5%, p=0.003) compared to those without, but stroke severity was similar. They had higher prevalence of pathological left ventricular (LV) remodelling, with larger end-diastolic volume index (77.9±34.5 vs 61.1±19.9ml/m2, p<0.001) and LV mass index (116.4±32.7 vs 99.8±32.2g/m2, p=0.002). Despite similar rates of recanalisation, VHD patients had poorer functional outcomes (mRS>2) at 3 months (68.1% vs 50.3%, p=0.029). On multivariable analyses, VHD remained independently associated with poor functional outcomes (adjusted OR 2.42, 95% CI 1.08–5.43), after adjusting for age, stroke severity and recanalisation rates.

Conclusion

VHD may be associated with poorer functional outcomes in patients with AIS undergoing EVT, despite similar rates of successful recanalisation.

Ordinal shift analyses

Funding Acknowledgement

Type of funding source: None

Can a Stent Retriever Damage the JET 7 Reperfusion Catheter?

We encountered 2 separate instances of damage to JET 7 reperfusion catheters when they were used in conjunction with a stent retriever during mechanical thrombectomy. On both occasions, after 1 or 2 passes with a stent retriever, we found that the distal end of the catheter was frayed and it ballooned up on flushing with saline. This mechanical failure could potentially lead to serious complications; hence, it should be shared with fellow neurointerventionalists.

Circulating β cell-specific CD8+ T cells restricted by high-risk HLA class I molecules show antigen experience in children with and at risk of type 1 diabetes

In type 1 diabetes (T1D), autoreactive cytotoxic CD8+ T cells are implicated in the destruction of insulin-producing β cells. The HLA-B*3906 and HLA-A*2402 class I genes confer increased risk and promote early disease onset, suggesting that CD8+ T cells that recognize peptides presented by these class I molecules on pancreatic β cells play a pivotal role in the autoimmune response. We examined the frequency and phenotype of circulating preproinsulin (PPI)-specific and insulin B (InsB)-specific CD8+ T cells in HLA-B*3906+ children newly diagnosed with T1D and in high-risk HLA-A*2402+ children before the appearance of disease-specific autoantibodies and before diagnosis of T1D. Antigen-specific CD8+ T cells were detected using human leucocyte antigen (HLA) class I tetramers and flow cytometry was used to assess memory status. In HLA-B*3906+ children with T1D, we observed an increase in PPI5-12 -specific transitional memory CD8+ T cells compared to non-diabetic, age- and HLA-matched subjects. Furthermore, PPI5-12 -specific CD8+ T cells in HLA-B*3906+ children with T1D showed a significantly more antigen-experienced phenotype compared to polyclonal CD8+ T cells. In longitudinal samples from high-risk HLA-A*2402+ children, the percentage of terminal effector cells within the InsB15-24 -specific CD8+ T cells was increased before diagnosis relative to samples taken before the appearance of autoantibodies. This is the first study, to our knowledge, to report HLA-B*3906-restricted autoreactive CD8+ T cells in T1D. Collectively, our results provide evidence that β cell-reactive CD8+ T cells restricted by disease-associated HLA class I molecules display an antigen-experienced phenotype and acquire enhanced effector function during the period leading to clinical diagnosis, implicating these cells in driving disease.

Color and power Doppler combined with Fetal Intelligent Navigation Echocardiography (FINE) to evaluate the fetal heart

OBJECTIVE

To evaluate the performance of color and bidirectional power Doppler ultrasound combined with Fetal Intelligent Navigation Echocardiography (FINE) in examining the fetal heart.

METHODS

A prospective cohort study was conducted of fetuses in the second and third trimesters with a normal heart or with congenital heart disease (CHD). One or more spatiotemporal image correlation (STIC) volume datasets, combined with color or bidirectional power Doppler (S-flow) imaging, were acquired in the apical four-chamber view. Each successfully obtained STIC volume was evaluated by STICLoop™ to determine its appropriateness before applying the FINE method. Visualization rates for standard fetal echocardiography views using diagnostic planes and/or Virtual Intelligent Sonographer Assistance (VIS-Assistance®) were calculated for grayscale (removal of Doppler signal), color Doppler and S-flow Doppler. In four cases with CHD (one case each of tetralogy of Fallot, hypoplastic left heart and coarctation of the aorta, interrupted inferior vena cava with azygos vein continuation and asplenia, and coarctation of the aorta with tricuspid regurgitation and hydrops), the diagnostic potential of this new technology was presented.

RESULTS

A total of 169 STIC volume datasets of the normal fetal heart (color Doppler, n = 78; S-flow Doppler, n = 91) were obtained from 37 patients. Only a single STIC volume of color Doppler and/or a single volume of S-flow Doppler per patient were analyzed using FINE. Therefore, 60 STIC volumes (color Doppler, n = 27; S-flow Doppler, n = 33) comprised the final study group. Median gestational age at sonographic examination was 23 (interquartile range, 21-27.5) weeks. Color Doppler FINE generated nine fetal echocardiography views (grayscale) using (1) diagnostic planes in 73-100% of cases, (2) VIS-Assistance in 100% of cases, and (3) a combination of diagnostic planes and/or VIS-Assistance in 100% of cases. The rate of generating successfully eight fetal echocardiography views with appropriate color and S-flow Doppler information was 89-100% and 91-100% of cases, respectively, using a combination of diagnostic planes and/or VIS-Assistance. However, the success rate for the ninth echocardiography view (i.e. superior and inferior venae cavae) was 33% and 30% for color and S-flow Doppler, respectively. In all four cases of CHD, color Doppler FINE demonstrated evidence of abnormal fetal cardiac anatomy and/or hemodynamic flow.

CONCLUSIONS

The FINE method applied to STIC volumes of normal fetal hearts acquired with color or bidirectional power Doppler information can generate successfully eight to nine standard fetal echocardiography views (via grayscale, color Doppler or power Doppler) in the second and third trimesters. In cases of CHD, color Doppler FINE demonstrates successfully abnormal anatomy and/or Doppler flow characteristics. Published 2017. This article is a U.S. Government work and is in the public domain in the USA. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of the International Society of Ultrasound in Obstetrics and Gynecology.

The Medina Embolic Device: Karolinska experience

Background The aim of this study was to report our single centre experience with the Medina Embolic Device (MED). Methods We performed a retrospective analysis of prospectively collected data to identify all patients treated with the MED. A total of 14 aneurysms (non-consecutive), in 13 patients, were treated including one ruptured and one partially thrombosed aneurysm. Fundus diameter was ≥5 mm in all cases. We evaluated the angiographic appearances, the clinical status, complications, and the need for adjunctive devices or repeat treatments. Results Aneurysm location was cavernous internal carotid artery (ICA; n = 1), supraclinoid ICA ( n = 1), terminal ICA ( n = 2), anterior communicating artery (AComA; n = 4), A2-3 ( n = 1), M1-2 junction ( n = 1), posterior communicating artery (PComA; n = 1), superior cerebellar artery (SCA; n = 1), and basilar tip ( n = 2). The average aneurysm fundus size was 8.6 mm (range 7-10 mm) and average neck size 3.75 mm (range 1.9-6.9 mm). Immediate angiographic results were modified Raymond-Roy occlusion classification (mRRC) I n = 2, mRRC II n = 1, mRRC IIIa n = 2, mRRC IIIb n = 2, the remaining 7 aneurysms showed complete opacification. At follow-up angiography (mean 5 months) mRRC I n = 5, mRRC II n = 5, mRRC IIIa n = 3, and persistent filling was seen in 1 aneurysm. Overall, four patients had repeat treatment and one is pending further treatment. Of the aneurysms treated with more than one MED, 75% showed complete occlusion at 6-month follow up whereas only one aneurysm treated with a single device showed complete occlusion. Overall, three patients had temporary complications and there were no deaths. Conclusions The MED is an intra-saccular flow-diverting device with satisfactory angiographic results and an acceptable safety profile. Use of a single MED cannot be recommended and further longer term studies are needed prior to widespread clinical use.

Expression of chemokines CXCL4 and CXCL7 by synovial macrophages defines an early stage of rheumatoid arthritis

Background and objectives

For our understanding of the pathogenesis of rheumatoid arthritis (RA), it is important to elucidate the mechanisms underlying early stages of synovitis. Here, synovial cytokine production was investigated in patients with very early arthritis.

Methods

Synovial biopsies were obtained from patients with at least one clinically swollen joint within 12 weeks of symptom onset. At an 18-month follow-up visit, patients who went on to develop RA, or whose arthritis spontaneously resolved, were identified. Biopsies were also obtained from patients with RA with longer symptom duration (>12 weeks) and individuals with no clinically apparent inflammation. Synovial mRNA expression of 117 cytokines was quantified using PCR techniques and analysed using standard and novel methods of data analysis. Synovial tissue sections were stained for CXCL4, CXCL7, CD41, CD68 and von Willebrand factor.

Results

A machine learning approach identified expression of mRNA for CXCL4 and CXCL7 as potentially important in the classification of early RA versus resolving arthritis. mRNA levels for these chemokines were significantly elevated in patients with early RA compared with uninflamed controls. Significantly increased CXCL4 and CXCL7 protein expression was observed in patients with early RA compared with those with resolving arthritis or longer established disease. CXCL4 and CXCL7 co-localised with blood vessels, platelets and CD68⁺ macrophages. Extravascular CXCL7 expression was significantly higher in patients with very early RA compared with longer duration RA or resolving arthritis

Conclusions

Taken together, these observations suggest a transient increase in synovial CXCL4 and CXCL7 levels in early RA.

Does early life exposure to antibiotics increase the risk of eczema? A systematic review

A number of studies have suggested that early life exposure to antibiotics can lead to an increased risk of developing eczema. This systematic review and meta-analysis of observational studies, involving children or young adults aged 0-25 years, assessed the impact of antibiotic exposure either in utero or during the first 12 months of life on subsequent eczema risk. Twenty studies examined the association between prenatal and/or postnatal exposure to antibiotics and development of eczema. The pooled odds ratio (OR) for the 17 studies examining postnatal antibiotic exposure was 1.41 [95% confidence interval (CI) 1.30-1.53]. The pooled OR for the 10 longitudinal studies was 1.40 (95% CI 1.19-1.64), compared with a pooled OR of 1.43 (95% CI 1.36-1.51) for the seven cross-sectional studies. There was a significant dose-response association, suggesting a 7% increase in the risk of eczema for each additional antibiotic course received during the first year of life [pooled OR 1.07 (95% CI 1.02-1.11)]. Finally, the pooled OR for the four studies relating to antenatal exposure was 1.30 (95% CI 0.86-1.95). We conclude that exposure to antibiotics in the first year of life, but not prenatally, is more common in children with eczema.

Expression of FcRL4 defines a pro-inflammatory, RANKL-producing B cell subset in rheumatoid arthritis

Objectives

The success of B cell targeting therapies has highlighted the importance of B cells in rheumatoid arthritis pathogenesis. We have previously shown that B cells in the RA synovium are capable of producing pro-inflammatory and bone-destructive cytokines including RANKL. Here we sought to characterise the nature and functional relevance of the RANKL-producing B cell subset in the RA synovium.

Methods

Synovial fluid and peripheral blood B cells from patients with RA were analysed by flow cytometry for markers of B cell differentiation and activation and for chemokine receptors. FcRL4⁺ and FcRL4⁻ B cells sorted from synovial fluid were analysed for cytokine expression using Taqman low-density arrays. Synovial tissue biopsies obtained from patients with RA were analysed by immunofluorescence for CD20, RANKL and FcRL4. FCRL4 mRNA expression was determined in synovial tissue of RA patients and non-inflammatory control subjects by real-time PCR.

Results

RANKL-producing B cells in RA synovial tissue and fluid were identified as belonging to a distinct subset of B cells defined by expression of the transmembrane protein FcRL4. FcRL4+ B cells express a distinct combination of cytokines and surface proteins indicating a function distinct from that of FcRL4− B cells. Notably, FcRL4+ B cells expressed high levels of TNF-α and RANKL mRNA.

Conclusions

We have identified a novel pro-inflammatory B cell population in the RA synovium which is defined by expression of FcRL4 and responsible for RANKL production. This B cell population expresses high levels of CD20, and its removal by rituximab may contribute to the anti-inflammatory effect of this drug.

Cross-hemispheric functional connectivity in the human fetal brain

Compelling evidence indicates that psychiatric and developmental disorders are generally caused by disruptions in the functional connectivity (FC) of brain networks. Events occurring during development, and in particular during fetal life, have been implicated in the genesis of such disorders. However, the developmental timetable for the emergence of neural FC during human fetal life is unknown. We present the results of resting-state functional magnetic resonance imaging performed in 25 healthy human fetuses in the second and third trimesters of pregnancy (24 to 38 weeks of gestation). We report the presence of bilateral fetal brain FC and regional and age-related variation in FC. Significant bilateral connectivity was evident in half of the 42 areas tested, and the strength of FC between homologous cortical brain regions increased with advancing gestational age. We also observed medial to lateral gradients in fetal functional brain connectivity. These findings improve understanding of human fetal central nervous system development and provide a basis for examining the role of insults during fetal life in the subsequent development of disorders in neural FC.

Prospective validation of fetal weight estimation using fractional limb volume

OBJECTIVES

To prospectively validate the use of fractional limb volume measurements for estimated fetal weight (EFW) during the second and third trimesters of pregnancy and to summarize the medical literature regarding application of fractional limb volume for fetal weight estimation.

METHODS

One hundred and sixty-four women prospectively underwent three-dimensional ultrasonography within 4 days of delivery. Birth weights (BWs) ranged from 390 to 5426 g. Fetal measurements were extracted using volume datasets for biparietal diameter, abdominal circumference, femur diaphysis length, fractional arm volume and fractional thigh volume. Fractional limb volumes were manually traced from a central portion of the humerus or femur diaphysis. Mean percentage differences and SDs of the percentage differences were calculated for EFW. The proportion of newborns with EFW within 5 or 10% of BW were compared with an estimate obtained using a Hadlock formula that was modified using model coefficients from the same local population sample.

RESULTS

Ultrasound scans were performed between 21.7 and 42 weeks' menstrual age. Optimal model performance (1.9 ± 6.6%) resulted from using a combination of biparietal diameter, abdominal circumference and fractional thigh volume. The precision of this model was superior to results obtained using a modified Hadlock model (1.1 ± 8.4%), although accuracy of these predictions was slightly decreased for female infants. For all fetuses, the prediction model that incorporated fractional thigh volume correctly classified a greater proportion of EFW within 5% (55.1 vs 43.7%; P = 0.03) or 10% (86.5 vs 75.9%; P < 0.05) of BW when compared with the modified Hadlock model.

CONCLUSIONS

Fractional thigh volume can be added to two-dimensional sonographic measurements of the head and trunk to improve the precision of fetal weight estimation. This approach permits the inclusion of soft tissue development as part of a weight estimation procedure for the assessment of generalized fetal nutritional status.

Magnetic resonance diffusion-weighted imaging: reproducibility of regional apparent diffusion coefficients for the normal fetal brain

OBJECTIVE

To evaluate the reproducibility of regional apparent diffusion coefficient (ADC) measurements of the normal fetal brain in the second and third trimesters of pregnancy.

METHODS

Fifty normal singleton fetuses from healthy pregnant women between 19 and 37 weeks' gestation were studied without sedation. Single-shot diffusion-weighted images of the fetal brain were obtained using a 1.5-Tesla magnetic resonance scanner and a six-channel body array coil. ADC maps were created using 0 and 1000 b-values along three orthogonal directions. Two examiners independently measured ADC values in the cerebellar hemispheres (CH), pons, thalamus, basal ganglia (BG), centrum semiovale (CSO), and frontal (FWM), parietal (PWM), temporal (TWM) and occipital (OWM) white matter. Correlation between ADC values and menstrual age was assessed by linear regression analysis. The bias and agreement of ADC measurements were determined using Bland-Altman plots.

RESULTS

ADC values either remained constant (BG, FWM, PWM, TWM, OWM, CSO) or decreased (CH, pons, thalamus) with advancing menstrual age. Mean intraobserver bias for ADC measurements was not significantly different from zero. Small interobserver differences in mean ADC measurements (i.e. a small mean bias) were detected for CH (1.26 ± 0.20 vs 1.20 ± 0.18 μm(2) /ms, P = 0.006), PWM (1.37 ± 0.29 vs 1.33 ± 0.26 μm(2) /ms, P = 0.02) and CSO (1.36 ± 0.29 vs 1.33 ± 0.28 μm(2) /ms, P < 0.0001). Measurement agreement was acceptable.

CONCLUSIONS

ADC measurements in normal unsedated fetuses in the second and third trimesters are reproducible except for small differences for PWM, CH and CSO between examiners.

Iliac crest angle: a novel sonographic parameter for the prediction of Down syndrome risk during the second trimester of pregnancy

OBJECTIVES

To validate a new sonographic technique for the display and measurement of fetal iliac crest angle (ICA), and to determine the relative contribution of standardized fetal pelvic measurements for estimating risk of Down syndrome during the second trimester of pregnancy.

METHODS

Three-dimensional ultrasonography of the fetal pelvis was performed during a second-trimester scan for genetic amniocentesis. A coronal view of the 'virtual pelvis' was obtained by applying a maximum intensity projection algorithm to visualize both iliac wings. Two different ICAs were measured from a coronal projection of the fetal pelvis (ICA-coronal 1 and ICA-coronal 2); the latter approach relied on using ischial tuberosities as reference landmarks. Next, a coronal projection of the fetal pelvis was rotated to demonstrate a rendered view of the axial fetal pelvis. Axial inner (ICA-inner), middle (ICA-middle) and outer (ICA-outer) ICAs were measured. The maximum anterior iliac distance measurement was defined as the widest separation between the most anterior points of the rendered iliac crests. The reproducibility of each ICA method was analyzed using Bland-Altman statistics. Simple and multiple logistic regression analysis determined the relative contribution of each parameter as a prenatal predictor of Down syndrome.

RESULTS

Ninety-four normal fetuses and 19 fetuses with Down syndrome were examined. The ICA-middle and ICA-coronal 2 parameters were the most reproducible angle measurements. The mean +/- SD ICA-middle measurement for fetuses with Down syndrome was significantly greater than that for normal subjects (94.5 +/- 9.8 degrees vs. 83.1 +/- 6.5 degrees ; P < 0.001). The mean +/- SD ICA-coronal 2 angle measurement for fetuses with Down syndrome was slightly greater than that for normal subjects (57.9 +/- 5.5 degrees vs. 51.9 +/- 7.1 degrees ; P = 0.0014). A multiple logistic regression model including ICA-middle and ICA-coronal 2 provided a predictive ability of 88.1% based on the area under the receiver-operating characteristics curve. This combination had a sensitivity of 94.4% for a false-positive rate of 5% in the detection of Down syndrome.

CONCLUSIONS

Standardized iliac crest measurements of the fetal pelvis can be used to identify some fetuses at risk for trisomy 21 during the second trimester of pregnancy.

Non-invasive fetal lung assessment using diffusion-weighted imaging

OBJECTIVES

The main goal was to develop a reproducible method for estimating the diffusion of water in human fetal lung tissue using diffusion-weighted imaging (DWI). A secondary objective was to determine the relationship of the apparent diffusion coefficients (ADCs) in the fetal lung to menstrual age and total lung volume.

METHODS

Normal pregnant volunteers were scanned on a 1.5-Tesla (T) magnetic resonance imaging (MRI) system. The MRI system was equipped with 40-mT/m gradients (slew rate 200 T/m/s, rise time 0.2 ms). A six-channel body array coil was used for signal reception. Single-shot DWI utilized TE/TR 125/3400 ms, slice thickness 4 mm, field of view 280 mm x 280 mm, interslice gap 0.8 mm and a matrix of 128 x 128. The voxel size was 2.5 mm x 2.5 mm x 4.0 mm. Two b-values (0 and 1000) were chosen along three orthogonal directions. ADC maps were created using assigned b-values. Simple linear regression was performed with Pearson correlation coefficient. Interexaminer and intraexaminer bias, and 95% limits of agreement (LOA) were determined using Bland-Altman plots.

RESULTS

Forty-seven scans were performed at a mean +/- SD of 29.2 +/- 4.5 weeks. The median coefficient of variation for ADC was 5.6% (interquartile range, 4.0-8.1%). No differences in ADC values were found between right and left lungs. Normally distributed ADC measurements were not significantly correlated with either total lung volume (r(2) = 0.0001, P = 0.94) or menstrual age (r(2) = 0.003, P = 0.70). The mean ADC value was 1.75 (95% CI, 1.63-1.86). Mean +/- SD intraexaminer bias was -0.15 +/- 2.3 (95% LOA, -4.7 to + 4.4) and interexaminer bias was 2.2 +/- 3.5 (95% LOA, -4.7 to + 9.1).

CONCLUSIONS

Our findings suggest that ADC measurements of the fetal lung are reproducible between blinded examiners and are independent of menstrual age, as well as lung volume.

New fetal weight estimation models using fractional limb volume

OBJECTIVES

The main goal of this study was to determine the accuracy and precision of new fetal weight estimation models, based on fractional limb volume and conventional two-dimensional (2D) sonographic measurements during the second and third trimesters of pregnancy.

METHODS

A prospective cross-sectional study of 271 fetuses was performed using three-dimensional ultrasonography to extract standard measurements-biparietal diameter (BPD), abdominal circumference (AC) and femoral diaphysis length (FDL)-plus fractional arm volume (AVol) and fractional thigh volume (TVol) within 4 days of delivery. Weighted multiple linear regression analysis was used to develop 'modified Hadlock' models and new models using transformed predictors that included soft tissue parameters for estimating birth weight. Estimated and observed birth weights were compared using mean percent difference (systematic weight estimation error) and the SD of the percent differences (random weight estimation error). The proportion of newborns with estimated birth weight within 5 or 10% of actual birth weight were compared using McNemar's test.

RESULTS

Birth weights in the study group ranged from 235 to 5790 g, with equal proportions of male and female infants. Six new fetal weight estimation models were compared with the results for modified Hadlock models with sample-specific coefficients. All the new models were very accurate, with mean percent differences that were not significantly different from zero. Model 3 (which used the natural logarithms of BPD, AC and AVol) and Model 6 (which used the natural logarithms of BPD, AC and TVol) provided the most precise weight estimations (random error = 6.6% of actual birth weight) as compared with 8.5% for the best original Hadlock model and 7.6% for a modified Hadlock model using sample-specific coefficients. Model 5 (which used the natural logarithms of AC and TVol) classified an additional 9.1% and 8.3% of the fetuses within 5% and 10% of actual birth weight and Model 6 classified an additional 7.3% and 4.1% of infants within 5% and 10% of actual birth weight.

CONCLUSION

The precision of fetal weight estimation can be improved by adding fractional limb volume measurements to conventional 2D biometry. New models that consider fractional limb volume may offer novel insight into the contribution of soft tissue development to weight estimation.

Assisted hatching reduces the age-related decline in IVF outcome in women younger than age 43 without increasing miscarriage or monozygotic twinning

PURPOSE

Our purpose was to examine the benefits of assisted hatching in our program and to determine whether the procedure increases the implantation of nonviable embryos or monozygotic twinning.

METHODS

Consecutive in vitro fertilization cycles using assisted hatching were compared with historical controls. The impact of assisted hatching was analyzed according to the woman's age. Outcome measures were clinical pregnancy, implantation rate, delivery rate, multiple pregnancy, spontaneous abortion, and incidence of monozygotic twins.

RESULTS

The implantation rate was increased in women aged 35-39 and markedly increased in women aged 40-42. There was no change in spontaneous abortions and no increase in monozygotic twins.

CONCLUSIONS

Assisted hatching is a safe and highly effective adjunct to in vitro fertilization for women aged 35-42 and did not increase spontaneous abortion or monozygotic twinning.

Definitive Structural Characterization of the Conventional Low-Temperature Host Structure in Urea Inclusion Compounds

Structural properties of the 1,10-dibromodecane/urea and 1,12-dibromododecane/urea inclusion compounds have been determined by single-crystal X-ray diffraction for both the high- and low-temperature phases. In the high-temperature phase both inclusion compounds have the conventional hexagonal urea tunnel structure, with substantial orientational disorder of the guest molecules. In the low-temperature phase the urea tunnel structure distorts to an orthorhombic structure, based on a distorted form of the orthohexaganol cell of the high-temperature structure and with the loss of the C centre. Within this tunnel structure there is evidence that the guest molecules have a narrow distribution of orientations (with respect to rotation about the tunnel axis) and the preferred orientation of the guest molecules correlates well with the observed distortion of the host tunnel. This represents the first accurate and reliable report of the conventional low-temperature structure of urea inclusion compounds. Previous powder X-ray diffraction studies have confirmed that the host structure in the low-temperature phase of 1,10-dibromodecane/urea is the same as that in the low-temperature phase of the alkane/urea inclusion compounds.