Multiple sclerosis and COVID-19: Assessing risk perception, patient behaviors and access to disease-modifying therapies

BACKGROUND

Following the outbreak of COVID-19, global healthcare systems have had to rapidly adapt. People with multiple sclerosis (pwMS) were required to make decisions about their individual risk and consequent work and social behaviors. This study aimed to evaluate risk perception and patterns of shielding behavior amongst pwMS at the onset of the COVID-19 pandemic and the subsequent impact on patients' employment and access to disease modifying therapies (DMTs).

METHODS

Postal surveys were sent to 1690 people within a UK population-based MS cohort during the first wave of the COVID-19 pandemic. Patients were surveyed on: (i) perceived vulnerability to COVID-19; (ii) isolation behavior; (iii) interruption to DMT; (iv) employment status; (v) level of satisfaction with their current working arrangement.

RESULTS

Responses were received from 1000 pwMS. Two thirds of patients reported isolating at home during the first wave of the pandemic. This behavior was associated with increased age (p<0.0001), higher disability (p<0.0001) and use of high-efficacy DMTs (p = 0.02). The majority of patients reported feeling vulnerable (82%) with perceived vulnerability associated with higher EDSS (p<0.0001) and receiving a high-efficacy DMT (p = 0.04). Clinician-defined risk was associated with shielding behavior, with those at high-risk more likely to self-isolate/shield (p<0.0001). Patients on high-efficacy DMTs were more likely to have an interruption to their treatment (50%) during the first wave of the pandemic. Most pwMS experienced a change to their working environment, and most were satisfied with the adjustments.

CONCLUSION

This study highlights the risk perception, social behavioral practices and changes to treatment experienced by pwMS during the first wave of the COVID-19 pandemic in a large, well-described UK cohort. The results may help inform management of pwMS during future pandemic waves.

Applying Adult Learning Best Practices to Design Immunization Training for Health Care Workers in Ghana

INTRODUCTION

A 2016 assessment of frontline health care workers (HCWs) in Ghana identified knowledge, skill, and attitude gaps related to immunization during the second year of life (2YL). The U.S. Centers for Disease Control and Prevention subsequently supported the Ghana Health Service Immunization Program to apply best practices of adult learning and training of trainers (TOT) for a cascade training program for 2YL.

METHODS

Five districts from each of the 3 regions (Greater Accra, Northern, and Volta) were selected for the TOT based on key measles and rubella vaccination coverage indicators. The design incorporated best practices of adult learning and TOT. The curriculum integrated 3 major topical themes: technical (immunization topics), operational, and training adults. The technical and operational content was based on HCW tasks most directly affecting 2YL objectives. A cross-functional team developed all classroom, field activity, and training evaluation materials.

RESULTS

Seventy-four participants attended TOT workshops in 2017. Based on a rubric defined by the course designers, 99% of the participants reported an acceptable level of confidence to apply and teach the course content. After the TOTs, participants conducted 65 workshops, 43 field visits, and 4 review meetings, reaching 1,378 HCWs within 7 months. Fifty-four percent of HCWs who received training from TOT participants reported an acceptable level of confidence in using the skills, and 92% reported they would prioritize applying the skills acquired during the training.

DISCUSSION

The success factors for effective adult learning and TOT can be applied to design and implement high-quality TOT even in resource-limited settings. The factors include using a variety of approaches, spending enough class time to prepare TOT participants for their training role, setting specific expectations for cascading the training, and following up through mentorship and reporting. Strong collaboration across the administrative levels of the Ghana Health Service enabled cascade training.

Building health workforce capacity for planning and monitoring through the Strengthening Technical Assistance for routine immunization training (START) approach in Uganda

INTRODUCTION

The Global Vaccine Action Plan identifies workforce capacity building as a key strategy to achieve strong immunization programs. The Strengthening Technical Assistance for Routine Immunization Training (START) approach aimed to utilize practical training methods to build capacity of district and health center staff to implement routine immunization (RI) planning and monitoring activities, as well as build supportive supervision skills of district staff.

METHODS

First implemented in Uganda, the START approach was executed by trained external consultants who used existing tools, resources, and experiences to mentor district-level counterparts and, with them, conducted on-the-job training and mentorship of health center staff over several site visits. Implementation was routinely monitored using daily activity reports, pre and post surveys of resources and systems at districts and health centers and interviews with START consultants.

RESULTS

From July 2013 through December 2014 three START teams of four consultants per team, worked 6 months each across 50 districts in Uganda including the five divisions of Kampala district (45% of all districts). They conducted on-the-job training in 444 selected under-performing health centers, with a median of two visits to each (range 1-7, IQR: 1-3). More than half of these visits were conducted in collaboration with the district immunization officer, providing the opportunity for mentorship of district immunization officers. Changes in staff motivation and awareness of challenges; availability and completion of RI planning and monitoring tools and systems were observed. However, the START consultants felt that potential durability of these changes may be limited by contextual factors, including external accountability, availability of resources, and individual staff attitude.

CONCLUSIONS

Mentoring and on-the-job training offer promising alternatives to traditional classroom training and audit-focused supervision for building health workforce capacity. Further evidence regarding comparative effectiveness of these strategies and durability of observed positive change is needed.

Development of a valid and reliable scale to assess parents' beliefs and attitudes about childhood vaccines and their association with vaccination uptake and delay in Ghana

BACKGROUND

Parents' attitudes and beliefs in vaccination are important to understand for shaping vaccine acceptance and demand interventions. Little research has focused on developing a validated scale to measure parents' attitudes towards vaccinations in low and middle-income countries; Ghana provided an opportunity develop a caregiver vaccination attitudes scale (CVAS) validated against childhood vaccine compliance.

METHODS

We conducted a cluster survey of 373 households with children aged 12-35 months of age from Northern Region, Ghana. Caregivers responded to 22 vaccination behavior and belief survey items and provided the child's vaccination status. In exploratory factor analysis (EFA) to assess CVAS content validity, we used parallel analysis to guide the number of factors to extract and principal axis factor analysis for factor extraction. Reliability of the scale was assessed using McDonald's Omega coefficient. Criterion validity of scale and subscales was assessed against receipt of vaccinations by 12 months of age and vaccination delay, using number of days undervaccinated.

RESULTS

EFA of CVAS responses resulted in removing 11 of 22 survey items due to loadings <0.30 and development of a 5-factor structure with subscales for Vaccine-Preventable Disease (VPD) Awareness, Vaccine Benefits, Past Behavior, Vaccine Efficacy and Safety, and Trust. The 5 factors accounted for 69% of the common variance and omega coefficients were >0.73 for all subscales. Validity analysis indicated that for every unit increase in the parent's scale score, the odds of the child being vaccinated decreased by 0.58 (95% confidence interval [CI]: 0.37, 0.68) and the number of days under-vaccinated increased by 86 (95%CI: 28, 143). The final 3-factor scale included Vaccine Benefits, Past Behavior, and Vaccine Efficacy and Safety.

DISCUSSION

The final CVAS included three factors associated with vaccine compliance in Ghana, although several survey items suggested for use in vaccine acceptance scales were dropped. Replicating this study in several country settings will provide additional evidence to assist in refining a tool for use in routine vaccine acceptance and demand surveillance efforts.

Multiple sclerosis: long-term outcomes in ethnic minorities. Analysis of a UK population-based registry

BACKGROUND AND PURPOSE

Multiple sclerosis (MS) is most frequent in Caucasian populations. However, studies of MS in other ethnic groups may offer unique insights into genetic and environmental influences on the disease, and data on long-term outcomes in these patients is limited. In this work clinical features and time to disability milestones were investigated in ethnic minority (EM) patients with MS in a UK population and comparisons were made to a Caucasian cohort from the same region.

METHODS

In all, 1949 MS patients (1866 Caucasian, 83 EM) were identified from a regional disease registry. Cox proportional hazards regression was used to analyse the time to Expanded Disability Status Scale (EDSS) 3.0, 4.0 and 6.0.

RESULTS

Ethnic minority patients were younger at disease onset (28.6 years vs. 32.8 years, P = 0.001), and primary progressive MS was less common (EM 4.8%, Caucasian 11.6%, P = 0.03). After correction for clinical variables, ethnicity was associated with time to EDSS 3.0 [EM: hazard ratio (HR) 1.75, P < 0.0001] and 4.0 (HR 1.46, P = 0.03), but not 6.0 (HR 1.5, P = 0.05).

CONCLUSIONS

Ethnic minority patients reach early levels of fixed disability more rapidly than Caucasian patients, but this effect diminishes at later stages of the disease. This has implications for clinical management of these patients.

Clinical characterization of dystonia in adult patients with Huntington's disease

BACKGROUND AND PURPOSE

Huntington's disease (HD) is an autosomal dominant, neurodegenerative movement disorder, typically characterized by chorea. Dystonia is also recognized as part of the HD motor phenotype, although little work detailing its prevalence, distribution, severity and impact on functional capacity has been published to date.

METHODS

Patients (>18 years of age) were recruited from the Cardiff (UK) HD clinic, each undergoing a standardized videotaped clinical examination and series of functional assessment questionnaires (Unified Huntington's Disease Rating Scale, Burke-Fahn-Marsden Dystonia Rating Scale and modified version of the Toronto Western Spasmodic Torticollis Rating Scale). The presence and severity of dystonia were scored by four independent neurologists using the Burke-Fahn-Marsden Dystonia Rating Scale and Unified Huntington's Disease Rating Scale. Statistical analysis included Fisher's exact test, Wilcoxon test, anova and calculation of correlation coefficients where appropriate.

RESULTS

Forty-eight patients [91% (48/53)] demonstrated evidence of dystonia, with the highest prevalence in the left upper limb (n = 44, 83%), right upper limb most severely affected and eyes least affected. Statistically significant positive correlations (P < 0.05) were observed between dystonia severity and increasing HD disease stage and motor disease duration. Deterioration in functional capacity also correlated with increasing dystonia severity. No significant relationship was observed with age at motor symptom onset or CAG repeat length.

CONCLUSIONS

We report a high prevalence of dystonia in adult patients with HD, with worsening dystonia severity with increasing HD disease stage and motor disease duration. The recognition and management of dystonic symptoms in routine clinical practice will aid overall symptomatic treatment and functional improvement.

Immunization delivery in the second year of life in Ghana: the need for a multi-faceted approach

Introduction

in 2012, pneumococcal conjugate vaccine (PCV), rotavirus vaccine and a second dose of measles-containing vaccine (MCV2) were introduced into the Expanded Program on Immunization (EPI) in Ghana. According to Ghana’s EPI schedule, PCV and rotavirus vaccine are given in the first year of life and MCV2 in the second year of life (2YL) at 18 months. Although coverage with the last doses of PCV and rotavirus vaccine reached almost 90% coverage within four years of introduction, MCV2 coverage did not rise above 70%. The World Health Organization Global Measles and Rubella Strategic Plan established a 2020 milestone to achieve at least 95% coverage with the first and second doses of measles-containing vaccine in each district and nationally. We developed a project to address challenges to delivery of immunizations and other child health services at the 18-month visit and throughout the 2YL.

Methods

from March to April 2016, we conducted a cluster survey of households (HHs) with children 24-35 months of age in three regions in Ghana to assess knowledge, attitudes and beliefs among caregivers about immunization during the 2YL and to collect childhood vaccination history data using vaccination cards. Three independent samples were selected from the Northern (NR), Volta (VR), and Greater Accra (GAR) regions. A survey and direct observations were performed a ta representative sample of health facilities (HFs) providing immunization services in the same regions to further characterize barriers to immunization access, utilization and delivery in the 2YL.

Results

data on a total of 464 children ages 24-35 months were collected in the HH survey: 211 in NR, 153 in VR, and 100 in GAR (response rate > 99%). First dose of measles-containing vaccine (MCV1) coverage was (NR: 87%, VR: 96%, GAR: 99%); however, MCV2 coverage was lower (NR: 60%, VR: 83%, GAR: 70%). MCV1 to MCV2 dropout was 32% in NR, 14% in VR, and 31% in GAR. Caregiver awareness of immunization against measles was 69% in NR, 75% in VR, and 68% in GAR yet less than half knew the recommended ages for receiving the vaccine, (NR: 4%, VR: 9%, GAR: 44%). Among 160 HFs participating in the survey (>50 in each region), most lacked a defaulter tracing system (NR: 94%,VR: 76%,GAR: 85%). A varying proportion of HCWs correctly indicated how to record a catch-up first dose of MCV administered to an 18-month-old child in the 12-23 month immunization register (NR: 38%, VR: 55%, GAR: 67%) and on the vaccination card (NR: 54%, VR: 53%, GAR: 76%). Although more than half of caregivers would accept text messages, (NR: 57%, VR: 78%, GAR: 96%) including reminders, related to their child’s immunizations, < 10% HFs were utilizing this practice.

Conclusion

challenges encountered with the establishment of an immunization visit beyond the first year of life included knowledge gaps among caregivers, high dropout rates between MCV1 and MCV2 in all study regions, and a lack of defaulter tracing systems in most healthcare facilities providing childhood immunizations. Targeted strategies that promote behavioral, cultural, and policy changes are needed to strengthen 2YL child health service delivery and improve vaccination coverage.

Alemtuzumab for multiple sclerosis: Long term follow-up in a multi-centre cohort

Background:

Alemtuzumab has recently been approved for treatment of relapsing MS, but concerns remain about its use since long-term studies of adverse events remain limited. Furthermore, a clear understanding of its application and durability of effect in clinical practice has yet to evolve.

Objectives:

To investigate long-term efficacy and safety outcomes in a multicentre cohort of patients treated with alemtuzumab.

Methods:

Patients treated from 2000 and followed-up at three regional centres were identified. Baseline and prospective data were obtained and validated by clinical record review.

Results:

One hundred patients were identified with a mean follow-up of 6.1 years (range 1–13). Forty patients were retreated with at least one further treatment cycle. Annualized relapse rates fell from 2.1 to 0.2 ( p<0.0001) post-treatment and were sustained for up to eight years of follow-up. Mean change in EDSS score was +0.14. Forty-seven patients developed secondary autoimmunity.

Conclusion:

Observed reduction in relapse rates reflected those reported in clinical trials, but we were unable to corroborate previous observations of disability reversal. 40% of patients required additional treatment cycles. Autoimmune adverse events were common, occurring at a higher rate than previously reported, but were largely predictable, and could be managed effectively within a rigorous monitoring regime.

Multivariate meta-analysis using individual participant data

When combining results across related studies, a multivariate meta-analysis allows the joint synthesis of correlated effect estimates from multiple outcomes. Joint synthesis can improve efficiency over separate univariate syntheses, may reduce selective outcome reporting biases, and enables joint inferences across the outcomes. A common issue is that within-study correlations needed to fit the multivariate model are unknown from published reports. However, provision of individual participant data (IPD) allows them to be calculated directly. Here, we illustrate how to use IPD to estimate within-study correlations, using a joint linear regression for multiple continuous outcomes and bootstrapping methods for binary, survival and mixed outcomes. In a meta-analysis of 10 hypertension trials, we then show how these methods enable multivariate meta-analysis to address novel clinical questions about continuous, survival and binary outcomes; treatment–covariate interactions; adjusted risk/prognostic factor effects; longitudinal data; prognostic and multiparameter models; and multiple treatment comparisons. Both frequentist and Bayesian approaches are applied, with example software code provided to derive within-study correlations and to fit the models.

Site-specific clinical disease onset in multiple sclerosis

BACKGROUND AND PURPOSE

Multiple sclerosis is a chronic inflammatory disorder of the central nervous system characterized by acute episodes of neurological dysfunction thought to reflect focal areas of demyelination occurring in clinically eloquent areas. These symptomatic relapses are generally considered to be random clinical events occurring without discernible pattern. The hypothesis that relapses may follow a predetermined sequence and may provide insights into underlying pathological processes was investigated.

METHODS

Employing prospective clinical database data from 1482 patients who had experienced one or more consecutive relapses were analysed. Using regression analysis, site and symptom of index event were compared with those of first relapse.

RESULTS

It is demonstrated that following disease ignition subsequent relapses may not be random events but dependent on characteristics of the index event. All anatomical sites were more likely to be affected in the first relapse if that site had been involved in the index event with a similar association observed when comparing by symptoms.

CONCLUSION

These findings have importance in understanding the evolution of the disease and predicting individual disease progression and may aid with patient counselling and management.

74 MHz nonthermal emission from molecular clouds: evidence for a cosmic ray dominated region at the galactic center

We present 74 MHz radio continuum observations of the Galactic center region. These measurements show nonthermal radio emission arising from molecular clouds that is unaffected by free–free absorption along the line of sight. We focus on one cloud, G0.13-0.13, representative of the population of molecular clouds that are spatially correlated with steep spectrum (α(327MHz)(74MHz) = 1.3 ± 0.3) nonthermal emission from the Galactic center region. This cloud lies adjacent to the nonthermal radio filaments of the Arc near l 0.2° and is a strong source of 74 MHz continuum, SiO (2-1), and Fe I Kα 6.4 keV line emission. This three-way correlation provides the most compelling evidence yet that relativistic electrons, here traced by 74 MHz emission, are physically associated with the G0.13-0.13 molecular cloud and that low-energy cosmic ray electrons are responsible for the Fe I Kα line emission. The high cosmic ray ionization rate 10(–1)3 s(–1) H(–1) is responsible for heating the molecular gas to high temperatures and allows the disturbed gas to maintain a high-velocity dispersion. Large velocity gradient (LVG) modeling of multitransition SiO observations of this cloud implies H2 densities 10(4–5) cm(–3) and high temperatures. The lower limit to the temperature of G0.13-0.13 is 100 K, whereas the upper limit is as high as 1000 K. Lastly, we used a time-dependent chemical model in which cosmic rays drive the chemistry of the gas to investigate for molecular line diagnostics of cosmic ray heating. When the cloud reaches chemical equilibrium, the abundance ratios of HCN/HNC and N2H+/HCO+ are consistent with measured values. In addition, significant abundance of SiO is predicted in the cosmic ray dominated region of the Galactic center. We discuss different possibilities to account for the origin of widespread SiO emission detected from Galactic center molecular clouds.

Cognitive therapy for people with a schizophrenia spectrum diagnosis not taking antipsychotic medication: an exploratory trial

BACKGROUND

Although antipsychotic medication is the first line of treatment for schizophrenia, many service users choose to refuse or discontinue their pharmacological treatment. Cognitive therapy (CT) has been shown to be effective when delivered in combination with antipsychotic medication, but has yet to be formally evaluated in its absence. This study evaluates CT for people with psychotic disorders who have not been taking antipsychotic medication for at least 6 months.

METHOD

Twenty participants with schizophrenia spectrum disorders received CT in an open trial. Our primary outcome was psychiatric symptoms measured using the Positive and Negative Syndromes Scale (PANSS), which was administered at baseline, 9 months (end of treatment) and 15 months (follow-up). Secondary outcomes were dimensions of hallucinations and delusions, self-rated recovery and social functioning.

RESULTS

T tests and Wilcoxon's signed ranks tests revealed significant beneficial effects on all primary and secondary outcomes at end of treatment and follow-up, with the exception of self-rated recovery at end of treatment. Cohen's d effect sizes were moderate to large [for PANSS total, d=0.85, 95% confidence interval (CI) 0.32-1.35 at end of treatment; d=1.26, 95% CI 0.66-1.84 at follow-up]. A response rate analysis found that 35% and 50% of participants achieved at least a 50% reduction in PANSS total scores by end of therapy and follow-up respectively. No patients deteriorated significantly.

CONCLUSIONS

This study provides preliminary evidence that CT is an acceptable and effective treatment for people with psychosis who choose not to take antipsychotic medication. An adequately powered randomized controlled trial is warranted.

A preliminary exploration of predictors of outcome and cognitive mechanisms of change in cognitive behaviour therapy for psychosis in people not taking antipsychotic medication

BACKGROUND

Cognitive behaviour therapy (CBT) has been shown to be effective in an open trial for people with psychotic disorders who have not been taking antipsychotic medication. There is little known about predictors of outcome in CBT for psychosis and even less about hypothesised mechanisms of change.

METHOD

20 participants with schizophrenia spectrum disorders received CBT in an exploratory trial. Our primary outcome was psychiatric symptoms measured using the PANSS. Secondary outcomes were dimensions of hallucinations and delusions, self-rated recovery and social functioning, and hypothesised mechanisms of change included appraisals of psychotic experiences, dysfunctional attitudes and cognitive insight. We also measured patient characteristics that may be associated with outcome.

RESULTS

T-tests revealed that several of the hypothesised mechanisms did significantly change over the treatment and follow-up periods. Correlational analyses showed that reductions in negative appraisals of psychotic experiences were related to improvements on outcome measures and that shorter duration of psychosis and younger age were associated with greater changes in symptoms.

CONCLUSIONS

CBT based on a specific cognitive model appears to change the hypothesised cognitive mechanisms, and these changes are associated with good outcomes. CBT may be more effective for those who are younger with shorter histories of psychosis.

Dentatorubral pallidoluysian atrophy in South Wales

BACKGROUND

Dentatorubral pallidoluysian atrophy (DRPLA) is a rare, autosomal dominant, clinically heterogeneous neurodegenerative disorder characterised clinically by progressive dementia, ataxia, chorea, myoclonic epilepsy and psychiatric disturbance and pathologically by combined degeneration of the dentatorubral and pallidoluysian systems. DRPLA has a marked ethnic predilection, most commonly reported in Japan and thought to be rare in Caucasian populations.

METHODS

We describe the clinical and genetic characteristics of 17 patients with DRPLA segregating in four families in South Wales.

RESULTS

There was marked clinical heterogeneity with considerable overlap of symptoms and signs between and within families. The age of onset ranged from 34 to 60 years with an earlier onset associated with myoclonic epilepsy and a later onset associated with a Huntington disease-like presentation. We identified a distinct haplotype within one family not present within the other three families, suggesting that the expansion in at least one family did not arise from an immediate common ancestor. Analysis of repeat length polymorphisms in 306 Welsh control patients identified 14 (4.6%) with repeat lengths in the high-normal range, compared with 0% and 7.4% in previously reported north American Caucasian and Japanese control populations, respectively.

CONCLUSIONS

DRPLA may not be as geographically or ethnically restricted as previously thought and the diagnosis should be considered in non-Asian patients presenting with a wide spectrum of neurological disease, especially if there is a dominant family history of dementia or movement disorder. The prevalence of high-normal length alleles may account for the relatively high prevalence of DRPLA in Wales.

Case control analysis of repeat expansion size in ataxia

Spinocerebellar ataxias (SCAs) are a group of clinically and genetically heterogeneous neurological diseases. The expansion of unstable microsatellite repeats has been identified as the underlying pathogenic cause of 10 subtypes of autosomal dominant SCAs. The aetiology of sporadic SCA is unknown. The aim of this study was to investigate the effect of large normal repeats in patients presenting with sporadic or familial ataxia compared to a control population. The size of the expansion was determined using a fluorescent PCR approach in 10 common SCA genes: SCA-1 (ATXN1), SCA-2 (ATXN2), SCA-3 (ATXN3), SCA-6 (CACNA1A), SCA-7 (ATXN7), SCA-8 (ATXN8OS), SCA-10 (ATXN10), SCA-12 (PPP2R2B), SCA-17 (TBP) and DRPLA (ATN1), in 165 ataxia patients and 307 controls of Welsh origin. There was no difference between cases and controls in the distribution of the large normal alleles, or in the distribution of the combined CAG repeats. The normal allele distribution in the Welsh population was largely similar to that of other Caucasian populations. Our study failed to demonstrate an effect of large normal repeats on the susceptibility to develop ataxia.

The galactic center: An interacting system of unusual sources

The region bounded by the inner tens of light-years at the center of the Milky Way Galaxy contains five principal components that coexist within the central deep well of gravitational potential. These constituents are a black hole candidate (Sgr A*) with a mass equivalent to 2.6 +/- 0.2 x 10(6) solar masses, a surrounding cluster of evolved stars, a complex of young stars, molecular and ionized gas clouds, and a powerful supernova-like remnant. The interaction of these components is responsible for many of the phenomena occurring in this complex and unique portion of the Galaxy. Developing a consistent picture of the primary interactions between the components at the Galactic center will improve our understanding of the nature of galactic nuclei in general, and will provide us with a better-defined set of characteristics of black holes. For example, the accretion of stellar winds by Sgr A* appears to produce far less radiation than indicated by estimates based on models of galactic nuclei.

Thermal Stability of Cold Clouds in Galaxy Halos

We consider the thermal properties of cold, dense clouds of molecular hydrogen and atomic helium. For cloud masses below 10-1.7 M middle dot in circle, the internal pressure is sufficient to permit the existence of particles of solid or liquid hydrogen at temperatures above the cosmic microwave background temperature. Optically thin thermal continuum emission by these particles can balance cosmic-ray heating of the cloud, leading to equilibria that are thermally stable even though the heating rate is independent of cloud temperature. For the Galaxy, the known heating rate in the disk sets a minimum mass of order 10-6 M middle dot in circle necessary for survival. Clouds of this type may in principle comprise most of the dark matter in the Galactic halo. However, we caution that the equilibria do not exist at redshifts z greater, similar1 when the temperature of the microwave background was substantially larger than its current value; therefore, the formation and the survival of such clouds to the present epoch remain open questions.

Enhanced OH in C-Type Shock Waves in Molecular Clouds

Cosmic-ray and X-ray ionizations in molecular gas produce a weak far-ultraviolet flux through the radiative decay of H2 molecules that have been excited by collisions with energetic electrons (the Prasad-Tarafdar mechanism). I consider the effect of this dissociating flux on the oxygen chemistry in C-type shocks. Typically, a few percent of the water molecules produced within the shock front are dissociated before the gas has cooled to 50 K. The resulting column density of warm OH rises from 1015 to 1016 cm-2 as the ionization rate is increased from 10-17 s-1 (typical of dark clouds) to 10-15 s-1 (adjacent to supernova remnants). These column densities produce substantial emission in the far-infrared rotational transitions of OH and are consistent with the OH/H2O ratios inferred from Infrared Space Observatory observations of emission from molecular shocks. For high ionization rates, the column of warm OH is sufficient to explain the OH(1720 MHz) masers that occur where molecular clouds are being shocked by supernova remnants. The predicted abundance of OH throughout the shock front will enable C-type shocks to be examined with high spectral resolution through radio observations of the four hyperfine ground-state transitions of OH at 18 cm and heterodyne measurements of emission in the far-infrared (e.g., from the Stratospheric Observatory for Infrared Astronomy).