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Methods of surveillance for Barrett’s oesophagus
Although the early detection of high grade dysplasia, the precursor of oesophageal adenocarcinoma, remains a primary task in the management of patients with Barrett’s oesophagus, several other key end points of screening and surveillance need to be considered (table 1). As dysplasia is rarely visually recognised during routine fibreoptic or video endoscopy, extensive four quadrant biopsy sampling every 1–2 cm of the entire mucosal surface using jumbo biopsy forceps (Seattle protocol) has been extensively practised, validated, and is currently recommended.1 In a recent report by the pioneers of this approach, the use of this systematic jumbo biopsy protocol every 1 cm in patients with high grade dysplasia who eventually developed cancer, 100% of cancers were detected.2 However, because the technique is labour intensive and requires a therapeutic endoscope, it is used by less than 20% of US gastroenterologists.3 Recently, many strategies and innovative techniques have been developed to improve the sensitivity of dysplasia detection and to overcome the problems of sampling error. Such approaches aim at enhancing the image contrast of the mucosa, detecting biochemical changes associated with dysplasia, and increasing the image resolution. The purpose of all of these approaches has been to improve from the small degree (S) of resolution and tissue penetration to medium (M), large (L), or even extra large (XL) levels of refinement, sensitivity, and specificity not only for dysplasia but also for a wider range of end points of screening and surveillance (table 1).
For the uninitiated endoscopist, these novel techniques are briefly described here and summarised in table 2. Chromoendoscopy uses a standard video endoscope to visualise the mucosal surface after application of a dye, such as methylene blue or indigo carmine.4 High magnification endoscopy employs a special endoscope that has additional lenses in the distal tip for enlarging the image, using a special control knob to allow conversion from a standard (S) to high level (L-XL) magnification.5–7 Light induced fluorescence (autofluorescence) is generated by exciting endogenous biomolecules such as collagen, NADH, FAD, and porphyrins, which have greater accumulation in dysplasia than in normal mucosa.8 Photodynamic diagnosis involves collection of fluorescence images after administration of an exogenous agent, such as 5-aminolevulinic acid (5-ALA).9 5-ALA is a prodrug that concentrates in dysplasia and is converted to the highly fluorescent protoporphyrin IX.10 Optical coherence tomography (OCT) is a method of detecting backscattered infrared light from microstructures within the tissue layers of the oesophagus.11 High resolution ultrasound uses a 20 MHz transducer to evaluate mucosal changes by detecting the backscattering of acoustic waves.12 Confocal microscopy with a miniature probe can visualise subcellular structures below the surface of the mucosa by optical sectioning to reduce the effects of light scattering.13 Table 2 lists the relative strengths of different imaging methods, including sensitivity for detection of high grade dysplasia and Barrett’s metaplasia, resolution, field of view, light penetration depth, cost, and time.
Despite the wide range and potential of all of these approaches, an adequate method of surveillance of Barrett’s oesophagus has remained elusive to date because of the complexity and variable evolution of this epithelium. Firstly, the architecture of mucosa containing dysplasia in the oesophagus is flat, and its appearance on conventional endoscopy is indistinct from that of specialised intestinal metaplasia. Secondly, the distribution of dysplasia over the mucosal surface can be quite variable—that is, focal, patchy, or diffuse. Thus the method of surveillance must be sensitive over a wide area. Thirdly, dysplasia is a histopathological diagnosis that requires subcellular image resolution to visualise nuclear and perinuclear morphology. This level of detail cannot be achieved in practice by conventional imaging methods, such as ultrasound, computed tomography, or magnetic resonance imaging. The best resolution so far has been obtained by optical methods, but these techniques have yet to achieve subcellular resolution in vivo. Fourthly, detection of dysplasia is frequently needed in the setting of an oesophagus that may contain erosions, strictures, and inflammation. Inflammatory changes in the mucosa may obscure methods sensitive to tissue biochemistry such as fluorescence. Fifthly, neosquamous epithelialisation of the oesophagus can occur after prolonged acid suppression or ablative injury and thus methods of surveillance must be able to identify Barrett’s metaplasia present below the new (squamous) mucosal surface. Finally, for practical purposes, screening must be performed in a time and cost efficient manner. This set of diagnostic requirements is quite rigorous and is unlikely to be satisfied by any single technique of surveillance. Moreover, because the identification of high grade dysplasia may result in an oesophagectomy for the patient, a conventional biopsy is desired for confirmation. Thus a new technique that serves as a guide for biopsy as an adjunct to conventional endoscopy is greatly needed.
Since the early days of fibreoptic endoscopy, many techniques have been used to identify specific epithelia or to enhance mucosal surface characteristics. Magnification chromoendoscopy alone or combined with methylene blue or indigo carmine has been used to detect intestinal metaplasia in Barrett’s oesophagus since 1994 and characteristic patterns (villous, ridged) have since been described. Using an adjustable image magnification in a continuous range up to 35× (M), combined with 1.5% acetic acid instillation, Guelrud et al described four different mucosal surface patterns and a sensitivity for specialised intestinal metaplasia of up to 100% when the ridged pattern was noted.5 Highly magnified images (80× at maximum, L) with or without methylene blue using a magnifying endoscope fitted with a transparent cap allowed Endo et al to classify the superficial mucosal appearance of Barrett’s epithelium by histological (gastric or intestinal) phenotypes.6 In this issue of Gut,7 magnification chromoendoscopy (115×, XL) is proposed as a useful tool not only for the diagnosis of intestinal metaplasia but also for detection of high grade dysplasia [see page 24]. Also in this issue of Gut,8 high resolution standard video endoscopy combined with methylene blue staining and tissue autofluorescence imaging proved inferior to stepwise four quadrant biopsies for surveillance in Barrett’s oesophagus [see page 18]. How does the clinician reconcile these observations and incorporate the findings in their everyday practice? What does the future have to offer? Should endoscopists implement the newly proposed techniques? If yes, which one?
Chromoendoscopy has the advantages of simplicity, low cost, and safety but adds to the procedure time, requires reagents and supplies, and the volume, concentration, and dwell time for reagent use have not yet been standardised. Also, this method looks only at the mucosal surface and misses important subepithelial pathology. Furthermore, the interpretation of staining is still subjective due to differing definitions and staining criteria.14 Methylene blue selectively stains intestinal metaplasia with up to 90% accuracy.15 However, results of methylene blue directed biopsy were similar to conventional biopsy in detecting specialised intestinal metaplasia and low grade dysplasia.16 Light or absent methylene blue staining with heterogeneity of uptake is associated with high grade dysplasia or cancer.17 High magnification endoscopy also looks only at surface features and magnifies the image at the expense of reducing the field of view. A significant amount of additional time may be needed to adjust the image into focus, and a special endoscope adds cost to the procedure.
Light induced fluorescence is a promising method for guiding biopsy because it provides wide area surveillance and can visualise below the mucosal surface.18 Fluorescence detects the presence of biomolecules associated with dysplasia rather than subcellular morphology, and thus extra high (XL) resolution is not required. In a promising case series, autofluorescence endoscopy using the LIFE-II imaging system in which blue light excites tissue autofluorescence, identified focal high grade dysplasia in Barrett’s mucosa.19 However, the image contrast with endogenous fluorescence alone may not be sufficient to obtain high sensitivity for detecting high grade dysplasia, as found by Egger and colleagues8 in this issue of Gut, and further refinements may be needed. For example, addition of agents that label dysplasia such as 5-ALA15 or optical reporter peptides20 have the potential to significantly enhance high grade dysplasia detection. OCT has a sensitivity of 97% and a specificity of 92% for detection of intestinal metaplasia using specific architectural criteria.21 However, the resolution of this instrument is not sufficient to characterise subcellular structures, such as nuclei, that are important for histopathological evaluation. Higher resolution OCT systems are being developed but at a cost that is not practical for clinical use. High frequency ultrasound is good for detecting submucosal invasion and lymph node involvement, but also does not have sufficient resolution to detect dysplastic cells at an early stage. Confocal microscopy is an intriguing option that can image with subcellular resolution but this instrument is still in an early stage of development.
At this time, endoscopists and clinicians should resist the temptation to use these very promising technologies in making management decisions on their Barrett’s oesophagus patients. The available data, although highly encouraging, are insufficient to allow us to draw conclusions about the best way to screen and survey these patients. As these technologies develop, the sensitivity for detection of intestinal metaplasia and high grade dysplasia will improve. The optimum method for surveillance of Barrett’s oesophagus and dysplasia will likely evolve in the form of an imaging instrument that has wide area surveillance and penetration below the mucosal surface. A combination of approaches (autofluorescence enhanced by 5-ALA, magnification chromoendoscopy, etc) may be applied in the interim. It will take years before validation, comparison, and standardisation of all of these technologies brings the level of confidence that biopsy currently provides for all the end points listed in table 1. For the time being, in the deadly game of Barrett’s oesophagus surveillance, the American actor and comedian Mae West (1892–1980) reminds with wit and wisdom that: “It is better to be looked over than overlooked . . .”
Methods of surveillance for Barrett’s oesophagus
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