Hinselmann (19.5) first described the basic colposcopic equipment and its use, establishing the foundation for the practice of colposcopy. A colposcope is a low-power, stereoscopic, binocular, field microscope with a powerful variable-intensity light source that illuminates the area being examined (Figure 4.1
The head of the colposcope, also called the ‘optics carrier’, contains the objective lens (at the end of the head positioned nearest to the woman being examined), two ocular lenses or eyepieces (used by the colposcopist to view the cervix), a light source, green and/or blue filters to be interposed between the light source and the objective lens, a knob to introduce the filter, a knob to change the magnification of the objective lens, if the colposcope has multiple magnification facility and a fine focusing handle. The filter is used to remove red light, to facilitate the visualization of blood vessels by making them appear dark. Using a knob, the head of the colposcope can be tilted up and down to facilitate examination of the cervix. The distance between the two ocular lenses can be adjusted to suit the interpupillary distance of the provider, to achieve stereoscopic vision. Each ocular lens has dioptre scales engraved on it to facilitate visual correction of individual colposcopists. The height of the head from the floor can be adjusted by using the height adjustment knob, so that colposcopy can be carried out with the colposcopist comfortably seated, without strain to the back.
Modern colposcopes usually permit adjustable magnification, commonly 6x to 40x usually in steps such as, for example, 9x, 15x, 22x. Some sophisticated and expensive equipment may have electrical zoom capability to alter the magnification. Most simple colposcopes have a single fixed magnification level such as 6x, 9x, 10x, 12x or 15x. Most of the work with a colposcope can be accomplished within the magnification range of 6x to 15x. Lower magnification yields a wider view and greater depth of field for examination of the cervix. More magnification is not necessarily better, since there are certain trade-offs as magnification increases: the field of view becomes smaller, the depth of focus dimishes, and the illumination requirement increases. However, higher magnifications may reveal finer features such as abnormal blood vessels.
The location of the light bulb in the colposcope should be easily accessible to facilitate changing them when necessary. Some colposcopes have bulbs mounted in the head of the instrument; in others, these are mounted elsewhere and the light is delivered via a fibre-optic cable to the head of the colposcope. The latter arrangement can use brighter bulbs, but less overall illumination may result if the cables are bent or twisted. A colposcope may be fitted with halogen, xenon, tungsten or incandescent bulbs. Halogen bulbs are usually preferred, as they produce strong white light. The intensity of the light source may be adjusted with a knob.
Focusing the colposcope is accomplished by adjusting the distance between the objective lens and the woman by positioning the instrument at the right working distance. Colposcopes usually have fine focus adjustments so that, if the distance between the base of the scope and the woman is kept fixed, the focus of the scope may be altered slightly using the fine focusing handle. The working distance (focal length) between the objective lens and the patient is quite important - if it is too long (greater than 300 mm) it is hard for the colposcopist’s arms to reach the woman, and if it is too short (less than 200 mm), it may be difficult to use instruments like biopsy forceps while visualizing the target with the scope. A focal distance of 250 to 300 mm is usually adequate. Changing the power of the objective lenses alters the magnification and working distance.
Colposcopes are quite heavy and are either mounted on floor pedestals with wheels, suspended from a fixed ceiling mount, or fixed to the examination table or to a wall, sometimes with a floating arm to allow for easier adjustment of position. In developing countries, it is preferable to use colposcopes mounted vertically on a floor pedestal with wheels, as they are easier to handle and can be moved within or between clinics.
Accessories such as a monocular teaching side tube, photographic camera (Figure 4.2
) and CCD video camera may be added to some colposcopes. However, these substantially increase the cost of the equipment. These accessories are added using a beam splitter in most colposcopes. The beam splitter splits the light beam in half and sends the same image to the viewing port and to the accessory port. Colpophotographic systems are useful for documentation of colposcopic findings and quality control. Teaching side tubes and videocolposcopy may be useful for real-time teaching and discussion of findings. With a modern CCD camera attached to a digitalizing port, it is possible to create high-resolution digital images of the colposcopic images.