Adherence to Objective Therapeutic Monitoring and Outcomes in Patients with Inflammatory Bowel Disease with Adalimumab Treatment. A Real-world Prospective Study

BACKGROUND AND AIMS

Objective monitoring and effective early treatment using a treat-to-target approach are key to improving therapeutic outcomes in IBD patients. This study aimed to assess adherence to objective monitoring (clinical, biomarkers, and endoscopy) and its impact on clinical outcomes.

METHODS

A prospective, multicenter study included consecutive IBD patients starting on adalimumab therapy between January 2019 and December 2020. Disease activity, assessed by the Harvey-Bradshaw index (HBI), partial Mayo, C-reactive protein (CRP), fecal calprotectin (FCAL), and endoscopy were evaluated at adalimumab initiation and 3, 6, 9 and 12 months. Therapeutic drug monitoring, changes in treatment, drug sustainability, and clinical outcomes were assessed.

RESULTS

104 IBD patients were enrolled (78.8% CD, median age 34.3 years, disease duration 9 years). During the 12 months follow-up, high adherence to clinical activity assessment was observed in both CD (81.3%- 87.7%) and UC patients (76.5-90.9%). CRP measurement decreased over time in both CD (37.3%-54.9%) and UC (29.4%-50.0%). The adherence to serial FCAL monitoring was low in CD (22.7-31.3%) and UC patients (17.6-56.0%). UC patients had higher adherence to early endoscopic assessment (<6 months) compared to CD patients (40.9% vs. 21.5%). Adherence to early combined clinical and biomarkers resulted in earlier dose optimization in CD and UC (log-rank<0.001), but drug sustainability was not different. The patients with early combined adherence had a significantly higher clinical remission rate at 1 year compared to non-adherence (70.2% vs. 29.8%, p=0.007) but no significant difference in UC patients.

CONCLUSIONS

The adherence to early objective monitoring with combined clinical and biomarkers assessment in IBD patients starting adalimumab therapy led to dose optimization and improved 1-year clinical remission in CD but did not change drug sustainability and clinical remission in UC.

Pan-Esophageal Submucosal Dissection following Transesophageal Echocardiography

A 73-year-old female underwent open mitral valve replacement with transesophageal echocardiography (TEE) guidance. She developed upper gastrointestinal bleeding postoperatively and was found on upper endoscopy to have a bleeding site at the gastric cardia with the appearance of focal trauma and a possible puncture site. A submucosal bluish protrusion was seen throughout the esophagus with a mucosal flap at the proximal esophagus. As a unifying diagnosis, it was suspected that the intraoperative TEE probe caused a submucosal dissection with point of entry at the proximal esophagus, running the entire length of the esophagus and exiting at the gastric cardia, giving rise to a clinical upper gastrointestinal bleed. Closure of the esophageal defect was achieved using an endoclip. A CT scan showed focal pneumomediastinum along the proximal esophagus, confirming the hypothesis. We report the first case to our knowledge of iatrogenic pan-esophageal submucosal dissection, which, in this case, presented as a clinical bleed from the exit point trauma to the gastric cardia mucosa caused by a TEE probe. Endoscopic management of the gastric injury as well as the esophageal defect led to resolution of the bleeding and avoidance of mediastinitis, respectively, allowing for an excellent recovery.

Fucosyltransferase 2 Mutations Are Associated With a Favorable Clinical Course in Crohn's Disease

BACKGROUND

Fucosyltransferase 2 (FUT2) participates in intestinal antigen secretion and bacterial adherence. FUT2 homozygous nonsense mutations (FUT2M) and subsequent nonsecretor status is associated with Crohn's disease (CD). The common null allele is rs601338. We assessed the relationship between FUT2M and disease course.

METHODS

In consecutive adult CD outpatients, clinical, biochemical, and genetic data were collected at baseline visits. Patients were longitudinally followed over 5 years. The primary outcome analyzed the relationship between FUT2M and rates of CD patients in persistent steroid-free clinical remission requiring neither surgery, biologics, nor immunomodulators.

RESULTS

Sixty-two CD patients were recruited. FUT2M homozygotes (rs601338 or any mutation in linkage disequilibrium) were detected in 27% of CD (17/62). Patients with rs601338 mutations had higher rates of the primary outcome (homozygous: 46.6%, heterozygous: 28.0%, wild-type: 5.3%, P=0.02). Similar findings existed for CD patients with homozygous mutations in any single-nucleotide polymorphism for FUT2 (homozygous: 41.2%, heterozygous: 25.9%, wild-type: 5.6%, P=0.04). On multivariable analysis, rs601338 mutation was associated with the primary outcome (odds ratio=3.4, 95% confidence interval: 1.3-8.7, P=0.01), while other parameters were not. Mutation of rs601338 was associated with lower rates of penetrating disease (homozygous: 13.3%, heterozygous: 28.0%, wild-type: 52.6%, P=0.05) and particularly in high-risk patients (homozygous: 0%, heterozygous: 37.5%, wild-type: 83.3%, P=0.01).

CONCLUSIONS

FUT2 mutation status is associated with a favorable clinical course in CD. Further confirmatory studies are needed.

A33 ENDOSCOPIC ULTRASOUND-GUIDED GASTROJEJUNOSTOMY VERSUS SURGICAL GASTROJEJUNOSTOMY AND ENTERAL STENTING FOR THE TREATMENT OF MALIGNANT GASTRIC OUTLET OBSTRUCTION: A SYSTEMATIC REVIEW AND META-ANALYSIS

Background

Gastric outlet obstruction (GOO), often encountered in advanced malignancy, is associated with debilitating symptoms and decreased quality of life. Traditional management of this condition has been surgical gastrojejunostomy (SGJ) or enteral stenting (ES). While SGJ is highly effective, it is invasive and associated with high rates of morbidity. ES provides a less invasive approach with a lower risk of adverse events; however, it is associated with a significant risk of stent dysfunction with increased need for reintervention. Endoscopic ultrasound-guided gastrojejunostomy (EUS-GJ) is a novel modality in the management of GOO that aims to endoscopically bypass the obstruction with a lumen-apposing metal stent, with early studies suggesting good effectiveness and safety outcomes; but the data are limited.

Aims

To perform a systematic review and meta-analysis comparing the clinical outcomes of EUS-GJ to more traditional treatments of malignant GOO.

Methods

The study protocol was prospectively registered with the PROSPERO international database. The literature was systematically searched using MEDLINE, EMBASE and Web of Knowledge databases from inception through May 2021. Studies comparing EUS-GJ to ES or SGJ in patients with malignant GOO were included. Meta-analysis was performed with results reported as odds ratios (ORs) with 95% confidence intervals (CIs) using random effects models. The two primary outcomes of interest were clinical success without GOO recurrence and adverse events. Secondary outcome was technical success.

Results

Ten studies with a total of 1016 patients were included. EUS-GJ was associated with higher clinical success without GOO recurrence compared to SGJ or ES [OR: 2.19, 95% CI: 1.18–4.09, heterogeneity: P = 0.10; I² = 59%]. Subgroup analysis showed higher clinical success without GOO recurrence compared to ES [OR: 5.31, 95% CI: 3.07–9.17], but no significant difference compared to SGJ [OR: 1.69, 95% CI: 0.76–3.72]. EUS-GJ was associated with fewer adverse events compared to SGJ and ES [OR: 0.28, 95% CI: 0.14–0.55] and compared to SGJ alone [OR: 0.20, 95% CI: 0.10–0.37], but no difference was noted when compared to ES alone [OR: 0.53, 95% CI: 0.15–1.87]. EUS-GJ was associated with decreased technical success compared to SGJ and ES [OR: 0.26, 95% CI: 0.09 – 0.75] and SGJ alone [OR: 0.14, 95% CI: 0.04–0.48]; however, there was no difference when compared to ES alone [OR: 0.43, 95% CI: 0.05–3.44].

Conclusions

EUS-GJ provides a robust bypass with lower risk of recurrent obstruction compared to ES and fewer adverse events compared to SGJ. High quality prospective studies are needed to further characterize the role of EUS-GJ in the management of malignant GOO.

Funding Agencies

None

Endoscopic ultrasound (EUS)-guided fine needle biopsy alone vs. EUS-guided fine needle aspiration with rapid onsite evaluation in pancreatic lesions: a multicenter randomized trial

BACKGROUND

Endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) is the standard in the diagnosis of solid pancreatic lesions, in particular when combined with rapid onsite evaluation of cytopathology (ROSE). More recently, a fork-tip needle for core biopsy (FNB) has been shown to be associated with excellent diagnostic yield. EUS-FNB alone has however not been compared with EUS-FNA + ROSE in a large clinical trial. Our aim was to compare EUS-FNB alone to EUS-FNA + ROSE in solid pancreatic lesions.

METHODS

A multicenter, non-inferiority, randomized controlled trial involving seven centers was performed. Solid pancreatic lesions referred for EUS were considered for inclusion. The primary end point was diagnostic accuracy. Secondary end points included sensitivity/specificity, mean number of needle passes, and cost.

RESULTS

235 patients were randomized: 115 EUS-FNB alone and 120 EUS-FNA + ROSE. Overall, 217 patients had malignant histology. The diagnostic accuracy for malignancy of EUS-FNB alone was non-inferior to EUS-FNA + ROSE at 92.2 % (95 %CI 86.6 %-96.9 %) and 93.3 % (95 %CI 88.8 %-97.9 %), respectively (P = 0.72). Diagnostic sensitivity for malignancy was 92.5 % (95 %CI 85.7 %-96.7 %) for EUS-FNB alone vs. 96.5 % (93.0 %-98.6 %) for EUS-FNA + ROSE (P = 0.46), while specificity was 100 % in both. Adequate histological yield was obtained in 87.5 % of the EUS-FNB samples. The mean (SD) number of needle passes and procedure time favored EUS-FNB alone (2.3 [0.6] passes vs. 3.0 [1.1] passes [P < 0.001]; and 19.3 [8.0] vs. 22.7 [10.8] minutes [P = 0.008]). EUS-FNB alone cost on average 45 US dollars more than EUS-FNA + ROSE.

CONCLUSION

EUS-FNB alone is non-inferior to EUS-FNA + ROSE and is associated with fewer needle passes, shorter procedure time, and excellent histological yield at comparable cost.

A157 REAL-WORLD TIGHT OBJECTIVE MONITORING WITH ADALIMUMAB LEADS TO EARLIER DOSE OPTIMIZATION AND HIGHER CLINICAL REMISSION RATES AT 12 MONTHS

Background

Data suggests that tight objective monitoring of inflammatory bowel diseases (IBD) may improve one-year clinical outcomes.

Aims

The goal of this study is to assess the adherence to serial tight objective monitoring, via clinical symptoms and biomarkers, and the effect of such tight monitoring on one year outcome in IBD patients at an academic and an university-affiliated center.

Methods

We retrospectively reviewed the chart of 428 consecutive IBD patients who started adalimumaby at the McGill University Health Center and Jewish General Hospital (Montreal, Canada) between January 1, 2015 and January 1, 2019 [338 Crohn’s disease(CD), 90 ulcerative colitis(UC)]. Clinical symptoms (assessed by Harvey-Bradshaw-Index and partial Mayo), C-Reactive Protein(CRP), and fecal calprotectin(FCAL) were captured at treatment initiation and at 3, 6, 9, and 12 months. Combined adherence was defined as the evaluation of ≥2 of 3 parameters(clinical, CRP, FCAL). Dose optimization and drug sustainability curves were plotted by Kaplan-Meier method.

Results

Clinical symptoms were assessed in nearly all patients at 3 (CD-UC:95-94%), 6 (90-83%), 9 (86-85%) and 12 (96-89%) months. CRP was also available for most patients but the frequency of assessment decreased in CD patients over the study period. In comparison, compliance to serial FCAL testing was low throughout the follow-up period. Clinical remission at one-year was significantly higher in patients who were adherent to early assessment visit at 3 months (p=0.001 both for CD and UC). Adherence to early follow-up also resulted in earlier dose optimisation in both CD and UC patients(pLogrank=0.026 for UC and p=0.09 for CD). However, the overall drug sustainability did not differ.

Conclusions

Clinical assessment and CRP, but not FCAL, were frequently assessed in patients starting adalimumab. Adherence to early objective combined follow-up visits resulted in earlier dose optimization and improved one-year clinical outcomes but did not change drug sustainability rates.

Funding Agencies

None

Endoscopist-Directed Propofol as an Adjunct to Standard Sedation: A Canadian Experience

Background

Sedation practices vary widely by region. In Canada, endoscopist-directed administration of a combination of fentanyl and midazolam is standard practice. A minority of cases are performed with propofol.

Aims

To describe the safety of nonanaesthetist administered low-dose propofol as an adjunct to standard sedation.

Methods

This was a single-centre retrospective study of patients having undergone endoscopic procedures with propofol sedation between 2004 and 2012 in a teaching hospital in Montreal. Procedures were performed by gastroenterologists trained in Advanced Cardiovascular Life Support. Sedation was administered by intravenous bolus by a registered nurse, under the direction of the endoscopist. Outcomes of procedures were collected in the context of a retrospective chart review using the hospital's endoscopy database.

Results

Of patients undergoing endoscopies at our centre, 4930 patients received propofol as an adjunct to standard sedation with fentanyl and midazolam. Cecal intubation rate for colonoscopies (n = 2921) was 92.0%. Gastroscopies (n = 1614), flexible sigmoidoscopies (n = 28), endoscopic retrograde cholangiopancreatography (n = 331) and percutaneous endoscopic gastrostomy insertion (n = 36) had success rates, defined as successful completion of the procedure within anatomical limits, of 99.0, 96.4, 94.0 and 91.7%, respectively. The average dose of propofol used for each procedure was 34.5 ± 20.8 mg. Fentanyl was used in 67.4% of procedures at an average dose of 94.3 ± 17.5 mcg. Midazolam was used in 92.7% of cases at an average dose of 3.0 ± 0.7 mg. Reversal agents (naloxone or flumazenil) were used in 0.43% of the cases (n = 21). Patients who received propofol were discharged uneventfully within the usual postprocedure recovery time. One patient required sedation-related hospitalization. For patients having received propofol in addition to standard sedation agents, 99.6% experienced no adverse events. There were no mortalities.

Conclusion

The use of low-dose propofol as an adjunct to fentanyl and midazolam, administered by a registered nurse under the direction of the endoscopist was safe and effective in patients at our centre.

TC-325 hemostatic powder versus current standard of care in managing malignant GI bleeding: a pilot randomized clinical trial

BACKGROUND AND AIMS

TC-325 (Hemospray; Cook Medical, Winston-Salem, NC, USA), an endoscopic hemostatic powder, exhibits possible benefits in patients with malignant GI bleeding. Our aim is to assess feasibility and determine estimates of efficacy of TC-325 compared with standard of care (SOC) in terms of initial hemostasis and recurrent bleeding rates in comparable groups of patients with malignant GI bleeding.

METHODS

Adult patients presenting with acute malignant upper or lower GI bleeding were randomized to TC-325 or SOC. Measured outcomes included feasibility of recruitment and randomization in the urgent care setting, immediate hemostasis, recurrent bleeding, need for additional treatment modalities, and mortality.

RESULTS

A preplanned 20 patients (upper GI source in 85%) were randomized 1:1 to TC-325 or SOC (25% women, age 67.2 ± 15.9 years, oozing in 95%) over 20 months. Immediate hemostasis was achieved in 90% of patients treated initially with TC-325 versus 40% in the SOC group (P = .057). Overall, 83.3% crossed over to TC-325, with hemostasis then achieved at index endoscopy in 80%. Overall, hemostasis at index endoscopy (before or after crossover) was obtained in 87.7% of patients treated with TC-325. Recurrent bleeding over the next 180 days was 20% in the TC-325 group compared with 60% in the SOC group (P = .170).

CONCLUSIONS

This pilot trial demonstrates the feasibility of TC-325 in malignant GI bleeding and provides results to help inform a larger randomized trial. Although not powered for such, results suggest that use of TC-325 is a very promising modality in malignant GI bleeding in achieving immediate hemostasis and may even result in decreased subsequent recurrent bleeding. (Clinical trial registration number: NCT02135627.).

High-Dose Infliximab Rescue Therapy for Hospitalized Acute Severe Ulcerative Colitis Does Not Improve Colectomy-Free Survival

BACKGROUND AND OBJECTIVE

Optimization strategies with infliximab (IFX) are increasingly used as rescue therapy for steroid refractory acute severe ulcerative colitis (ASUC). We aim to determine if intensified IFX induction improves colectomy rate and identifies outcome predictors.

METHODS

Hospitalized adult patients who received IFX for ASUC between 2010 and 2016 were identified. We compared standard inductions (5 mg/kg) vs high-dose induction (10 mg/kg) with 3-month colectomy rate as primary outcome.

RESULTS

Seventy-two patients (62.5% male, median age 38.5) were identified. Thirty-seven patients (51.3%) received 5 mg/kg IFX and 35 received 10 mg/kg. Baseline clinical, biochemical and endoscopic parameters were well matched between these two groups. 10 mg/kg was more likely to be used by clinicians from 2014 onwards (p < 0.001). Three-month colectomy rate was 9.7%; which was not significantly different between the standard (5.4%) and high-dose (14.3%) IFX induction (p = 0.205). CRP ≥ 60 (OR 10.9 [95% CI 1.23-96.50], p = 0.032), hemoglobin ≤ 90 g/L (OR 15.6 [95% CI 2.61-92.66], p = 0.036) and albumin < 30 g/L (OR 9.4 [95% CI 1.06-83.13], p = 0.044) were associated with increased risk of colectomy at 3 months in univariate regression analysis.

CONCLUSION

Use of high-dose infliximab rescue therapy did not improve 3-month colectomy-free survival in this cohort. Tailored use in high-risk patients may be beneficial although further validation is required.

Onsite evaluation of endoscopic ultrasound fine needle aspiration: the endosonographer, the cytotechnologist and the cytopathologist

Endoscopic ultrasound guided fine needle aspiration (EUS-FNA) has become an essential tool in the management of multiple diseases. Its accuracy is related to different aspects of the technique, one of the most important being the experience and interaction of the endosonographer and pathologist. Certain studies over the past years have highlighted the importance of having rapid on-site evaluation (ROSE) of samples obtained at the time of EUS-FNA. We have reviewed the role of ROSE, performed by the same endosonographer, a cytotechnologist and an expert cytopathologist. The available data suggest that ROSE (either by the endosonographer, the cytotechnologist, or the cytopathologist) improves sample adequacy and diagnostic yield, with the best option to have ROSE performed by an expert cytopathologist. However, if non-ROSE accuracy is already very high, any improvement is harder to achieve.

Accuracy of biopsies for Helicobacter pylori in the presence of intestinal metaplasia of the stomach

BACKGROUND/AIMS

Gastric cancer is the second leading cause of cancer-related death worldwide. The majority of gastric cancers is "intestinal-type" adenocarcinoma, caused in part by H. pylori infection. Chronic gastritis leading to atrophy and intestinal metaplasia (IM) can result in cancer. Studies have demonstrated reversibility of mucosal atrophy following H. pylori eradication. Concern has been raised regarding the sensitivity of gastric biopsy for H. pylori detection in the context of IM.

MATERIALS AND METHODS

All cases of IM on gastric biopsy from a single gastroenterologist's outpatient practice were retrospectively reviewed from February 1, 2006 until May 31, 2012.

RESULTS

In total, 105 IM cases were found, of which 37 (35.2%, 95% CI: 26.3-45.2) were H. pylori-positive on biopsy. Charts of the remaining 68 patients were reviewed for availability of other tests, namely urea breath test (UBT) and serology. Of 43 H. pylori-negative patients who underwent a UBT, 10 were positive for the infection (23.3%, 95% CI: 12.3-39.0). Amongst patients with coexisting autoimmune gastritis (AIG), 4 out of 9 (44.4%, 95% CI: 15.3-77.3) also had evidence of H. pylori infection by UBT, despite negative histology.

CONCLUSION

For cases of gastric IM with negative histology for H. pylori, UBT should be considered, even in cases of AIG, as this may alter the management and clinical course for patients.

Is routine second-look endoscopy effective after endoscopic hemostasis in acute peptic ulcer bleeding? A meta-analysis

BACKGROUND

Routine second-look endoscopy in modern-era peptic ulcer bleeding (PUB) remains controversial.

OBJECTIVE

To assess the effectiveness of routine second-look endoscopy in patients with PUB exhibiting high-risk stigmata after standard medical care and endoscopic therapy.

DESIGN

Comprehensive literature searches (1990-2011) were performed, seeking randomized trials comparing a routine with an as-needed second endoscopy.

MAIN OUTCOME MEASUREMENTS

The main outcome was rebleeding. Secondary outcomes were surgery and mortality. Subanalyses assessed the influence of study quality, rebleeding definitions, endoscopic hemostasis modality, and proton pump inhibitor (PPI) therapies. Analyses were performed with Revman 5.1. Results are shown as odds ratios (ORs) and 95% confidence intervals (CIs).

RESULTS

Only 4 published articles completely reporting studies and 4 abstracts (of 577 citations) were included (938 patients). Rebleeding was significantly decreased by a routine second-look endoscopy (OR 0.55; 95% CI, 0.37-0.81), as was surgery (OR 0.43; 95% CI, 0.19-0.96), but not mortality (OR 0.65; 95% CI, 0.26-1.62). Results remained robust with varying definitions of rebleeding, but not with varying endoscopic hemostasis modalities and PPI therapies; the only trial in which high-dose PPI was used did not show a benefit of a second-look endoscopy. When removing the 2 trials that included patients at highest risk of rebleeding, no significant benefit attributable to a second-look endoscopy was noted (OR 0.65; 95% CI, 0.42-1.00).

LIMITATIONS

The small number of trials and patients in each of these studies.

CONCLUSIONS

In the absence of high-dose PPI, especially in patients at very high risk (eg, active bleeding), routine second-look endoscopy appears effective in these selected patients with PUB. However, the generalizability of these results to the era of high-dose PPI and otherwise unselected patients with high-risk stigmata is unclear.

Inflammation and repair in viral hepatitis C

Hepatitis C viral infection (HCV) results in liver damage leading to inflammation and fibrosis of the liver and increasing rates of hepatic decompensation and hepatocellular carcinoma (HCC). However, the host's immune response and viral determinants of liver disease progression are poorly understood. This review will address the determinants of liver injury in chronic HCV infection and the risk factors leading to rapid disease progression. We aim to better understand the factors that distinguish a relatively benign course of HCV from one with progression to cirrhosis. We will accomplish this task by discussion of three topics: (1) the role of cytokines in the adaptive immune response against the HCV infection; (2) the progression of fibrosis; and (3) the risk factors of co-morbidity with alcohol and human immunodeficiency virus (HIV) in HCV-infected individuals. Despite recent improvements in treating HCV infection using pegylated interferon alpha (PEGIFN-alpha) and ribavirin, about half of individuals infected with some genotypes, for example genotypes 1 and 4, will not respond to treatment or cannot be treated because of contraindications. This review will also aim to describe the importance of IFN-alpha-based therapies in HCV infection, ways of monitoring them, and associated complications.

Evidence for the validity and utility of the Stages of Exercise Behaviour Change scale in young adults

This study examined the validity and utility of the Stages of Exercise Behaviour Change (SEBC) scale in 244 young British adults. One-way ANOVA revealed significant differences (F > 7.34, P < 0.01) between the Exercise Behaviour Change Categories of Precontemplation/Contemplation (n = 49), Preparation (n = 87) and Action/Maintenance (n = 108) in self-report levels of exercise behaviour. Significant differences (F > 3.14, P < 0.05) were also revealed in exercise self-efficacy, physical self-perception sub-domains and global self-esteem scores. Subsequent step-wise discriminant analyses revealed that discrimination between the Categories of Exercise Behaviour Change was possible on the basis of selected behavioural and psychological parameters (Canonical r = 0.76-0.82, Wilks' lambda = 0.30-0.33, chi 2 = 60.3-94.6, d.f. = 14, P < 0.0001). In both males and females, the most dominant discriminatory variables in the first Function were revealed to be perceived physical conditioning and 'strenuous' exercise behaviour. For males, the second Function comprised exercise self-efficacy and perceived bodily attractiveness, whilst for females it comprised perceived bodily attractiveness, perceived sports competence and perceived physical strength. Subsequent cross-validation analysis, using a randomly selected 40% sub-sample, revealed that 67.8-70.7% of subjects were assigned to the correct Category. These results appear to confirm the concurrent validity of the SEBC scale in terms of self-report of exercise behaviour. Furthermore, the utility of the SEBC scale was demonstrated via the ability to predict membership of specific Categories of Exercise Behaviour Change using a selection of behavioural and psychological parameters.

Immunocytochemical analysis of glial cells in the hypomyelinated optic nerve of the BW mutant rat

The Browman-Wyse (BW) rat is a mutant with structural defects of the visual system, including a failure of the proximal (retinal) end of the optic nerve to myelinate. This latter abnormality is correlated with an absence of CAII+ oligodendrocytes, but we have previously shown that astrocytes are normally distributed, as judged by morphological characteristics of GFAP+ cells in vivo. We have further examined in vitro the immunohistochemical characteristics of macroglia isolated from the BW optic nerve, either as cell suspensions or after 4 days in culture. Cell cultures derived from the hypomyelinated proximal segment of BW optic nerves contained very few 0-2A progenitor cells (from which oligodendrocytes and cells with the GFAP+/A2B5+ phenotype develop), whereas over 90% of the glia were Schwann cells. A proportion of these few 0-2A progenitor cells differentiated normally after 4 days in vitro into both progeny phenotypes in appropriate media. Accordingly, we conclude that the myelination deficiency in the BW optic nerve could be explained as a failure of 0-2A progenitor cells to populate fully the proximal extremity of the nerve during development. Since most glia isolated from adult optic nerves did not adhere to the culture substrate, we analysed the phenotypes of freshly isolated cells in suspension. Comparing optic nerves of normal adult rats with those of BW mutants, a significantly higher fraction of the GFAP+ cells reacted with A2B5 in cell suspensions of the latter. The double-labelled cells which are present in abnormally high numbers may be the differentiated progeny of 0-2A progenitors in the hypomyelinated segment of nerve. One explanation for these findings is that Schwann cells within the BW nerve induce the differentiation of 0-2A progenitor cells to the GFAP+/A2B5+ phenotype. We investigated this possibility using conditioned medium from cultured Schwann cells which increased tenfold the frequency of GFAP+/A2B5+ cells in normal neonatal rat optic nerve cultures. Oligodendrocyte numbers showed a concomitant decline with increasing concentration of Schwann cell conditioned medium. Hypomyelination in the BW rat optic nerve may therefore arise because Schwann cells, present in the proximal segment of the nerve, not only impede the migration of 0-2A progenitor cells but also release a factor which induces those 0-2A progenitor cells which arrive in the proximal segment of the nerve to differentiate into GFAP+ cells at a critical stage in oligodendrocyte development.