Negative powered diagnostic contact lenses in conjunction with the slit-lamp biomicroscope can be used to view the vitreous and retina and in particular the optic nerve head and the macula. For practitioners with some level of binocularity, a binocular direct image can be obtained.
The use of the slit-lamp biomicroscope allows a stereoscopic view of the retina, which is useful in assessing the elevation of interesting features. The diagnostic contact lenses provide high magnification with excellent resolution. Although several aspects of this type of procedure prevent it from becoming part of a routine examination, there is no other way to achieve a comparable sense of depth.
Slit-lamp direct ophthalmoscopy can be performed with several types of diagnostic contact lens. All have a high minus power which produces an upright virtual image that is not laterally reversed. Diagnostic contact lenses also have an advantage in that the examiner’s view of the retina is not interrupted by the patient’s blink reflex.
The use of diagnostic contact lenses with this technique is contraindicated in those situations were the minimal trauma associated with this technique would be harmful to the patient, such as:
- The period immediately following an eye operation.
- Active corneal disease.
- Penetrating or perforating injury
- Before other ocular procedures that depend on corneal clarity e.g. fundus photography.
Advantages of this technique are high magnification and a stereoscopic view. The main disadvantage is the small field of view.
- Click stop in
- Beam straight ahead
- 2 mm beam width
- Maximum beam height
- No filter
- Medium illumination
- 10 to 15X magnification
Scan the anterior eye with the biomicroscope to rule out conditions that preclude the use of this procedure.
Instil topical anaesthetic. Fill the concave surface of the lens with the coupling solution. Hold the lens with the thumb and first finger of your dominant hand such that it can easily be applied to the patient’s eye.
Move the slit lamp in front of the eye that is not being examined to allow easier access. Ask the patient to look down, and retract their upper lid with the thumb of your non-dominant hand. Ask the patient to look down and then place the lower rim of the lens into the patient’s lower conjunctival sac. Tilt the lens to contact the cornea and then slowly release the upper lid. Maintain light pressure on the lens to prevent the patient’s blinking from dislodging it.
Look around the side of the instrument and position the slit lamp beam in the centre of the lens
Looking through the oculars, you should be able to see a red reflex. Push the slit lamp gently towards the patient; any opacities in the lens or vitreous should become apparent. Once the retina is in focus, the area under observation can be altered by changing the patient’s fixation with the slit lamp fixation target
The slit lamp must be moved horizontally and vertically to keep the slit beam in line with the pupil. The magnification and beam width can be altered to improve the view, although be aware that this may become uncomfortable for the patient.
Remove the contact lens when the examination is finished. You may need to apply light pressure to the globe in order to release the suction from the lens, or alternatively ask the patient to look up and blink
Irrigate any excess coupling solution using sterile saline. Use fluorescein to check for corneal staining.
Hints and tips
When using a diagnostic contact lens make sure that there are no bubbles in the coupling gel as these will obscure the view. Artificial tears may need to be recommended for 12 to 24 hours following the procedure as it may induce superficial corneal staining.
Mirrored lens examination should be performed at the end of a routine eye examination; otherwise, results may be contaminated. Intraocular pressure is reduced because some aqueous humour is forced out through the drainage channels by the pressure exerted on the globe
The cornea is slightly hazy from the topical anaesthetic, viscous coupling gel and microtrauma from the lens. This blurs the patient’s vision rendering any subjective examination results invalid. The examiners view inward is also compromised affecting other types of ophthalmoscopy.
It is important to remember that the view through the mirrors is reversed; descriptions and illustrations should be anatomically correct. For retinal evaluation, it is necessary to indicate where the lesion is located on the retina, not where it appears to be through the mirror. The mirrored lens must be constantly held in contact with the patient’s eye. The lens can be rotated and angled to enhance the view. The slit beam should be kept perpendicular to the mirror as the lens is rotated.
The examination may be long and the arm supporting the lens may become fatigued. An elbow rest such as a tissue box placed on the slit lamp table can support the arm and make the examiner more comfortable
The image is upright and not laterally reversed which aids in interpretation and record keeping. The patient is allowed to blink and the view of the retina will not be interrupted, as the lids do not pass between the lens and the eye. It is acceptable to manoeuvre the lens by changing its angle in order to enhance the view. High magnification will reduce the field of view and any hand tremor may be exacerbated.
Because the posterior pole is under examination the patient may find it very uncomfortable if the high light intensity is used.