What is chromoendoscopy in gastroscopy?

The contrast method makes use of two physical principles: surface tension and gravitational deposition. The most commonly used agent in clinical practice is indigo carmine.

Its mechanism lies in the fact that indigo carmine is not absorbed by the gastrointestinal mucosa and has relatively low viscosity. When sprayed onto the mucosal surface, the dye settles into depressed areas such as gastric pits, crypt openings, and the edges of ulcers, while the surface of elevated lesions remains unstained because of surface tension. This light-dark contrast enhances the three-dimensional appearance of the mucosal surface, allowing physicians to clearly observe the demarcation line and the surface microstructure, or pit pattern. It is especially useful for outlining flat lesions.

Its function is much like drawing contour lines on a topographic map. The normal gastric mucosa contains many tiny grooves and depressions. After the dye is sprayed, it pools in these grooves and highlights the folds and surface pattern of the stomach. If an area becomes flattened or raised, the dye cannot adhere in the same way, and the physician can immediately recognize that the texture is abnormal. This method is mainly used to determine the extent and borders of a lesion.

The absorption method is based on ion channels on the cell membrane and pinocytosis. Commonly used agents include methylene blue, toluidine blue, and crystal violet.

Normal mature mucosal epithelial cells retain intact absorptive function and are able to take up the dye into the cytoplasm, thereby showing deep staining under endoscopy. In contrast, dysplastic or cancerous cells often exhibit impaired or absent dye uptake because of loss of cellular polarity and metabolic abnormalities.

This method commonly uses dyes such as methylene blue. The principle is straightforward: normal cells are active and able to "take up" the dye, so they appear blue; diseased cells, due to abnormal metabolism or reduced vitality, absorb little or none of the dye.

By observing which areas are stained and which are not, physicians can identify cells that have become functionally inactive or abnormal, allowing more accurate targeted biopsy and improving the diagnostic yield.

The reactive method is based on color-producing chemical reactions between specific substances and tissue components. The most representative agent is Lugol's solution.

Normal squamous epithelial cells of the esophagus are rich in glycogen. When exposed to iodine solution, glycogen forms a complex with iodine and produces a characteristic brown coloration. In contrast, early esophageal cancer and precancerous lesion cells have reduced or absent glycogen synthesis, so they do not produce this color reaction after iodine staining, resulting in a distinct unstained area. This mechanism not only helps determine the biopsy site, but also assists in assessing the extent of tumor infiltration by observing the clarity of the lesion margins. In addition, Congo red, as an acid-base indicator, can reflect gastric acid secretion through its color change from red to blue-black, and is used to evaluate the acid-secreting function of the gastric mucosa.

The most typical example is Lugol's solution. It is a very useful tool in esophageal examination. Normal esophageal mucosal cells contain a large amount of glycogen, which serves as an energy reserve, and they turn dark brown when exposed to iodine. Early cancer cells, however, consume energy rapidly and lose their glycogen, so they do not change color after iodine staining and remain yellowish-white.

This is similar to using a reagent to test pH: any area that does not change color becomes a key suspicious target. In addition, there is another dye called Congo red, which can be used to assess gastric acid secretion through its color change from red to black, helping determine whether the function of the gastric mucosa is normal.

The fluorescence method falls within the scope of photodynamic diagnosis. By intravenous injection or local spraying of exogenous fluorescent agents such as fluorescein sodium, imaging is performed based on differences between normal tissue and diseased tissue in vascular permeability and metabolic rate.

Tumor tissue is usually associated with neovascularization and incomplete basement membranes, which lead to increased leakage of fluorescein and the emission of strong fluorescent signals under specific excitation wavelengths. This method significantly improves the detection rate of tiny cancerous lesions and carcinoma in situ, and is especially valuable in the follow-up monitoring of Barrett's esophagus and atrophic gastritis.

Although chromoendoscopy has improved diagnostic sensitivity, it is relatively cumbersome, time-consuming, and highly influenced by subjective factors. At present, with the development of optical imaging technology, electronic chromoendoscopy techniques such as narrow-band imaging (NBI), blue light imaging (BLI), and linked color imaging (LCI) are gradually replacing some functions of chemical staining. By altering the wavelength of light, these technologies can highlight the microvascular architecture of the mucosa without the need to spray dyes, enabling “non-invasive” real-time pathological diagnosis.

In clinical practice, physicians need to evaluate the nature of the patient’s lesion and balance the diagnostic benefits of chemical staining against potential risks, such as allergy or DNA damage, in order to choose the most appropriate examination strategy.