Identification of the site of blockage requires one or more of the following tests.
Two or three drops of sodium fluorescein are instilled into the lateral fornix. Dye may drain completely (dye disappearance) and be collected by a swab at the inferior meatus (Jones I), when the drainage system is patent. No more tests are necessary at this stage. With compromised drainage, dye usually overflows medially onto the cheek. In the presence of lid malpositions it overflows medially, centrally or laterally, according to the lid position.
The ocular surface is examined simultaneously. Conjunctival and corneal staining should be noted to rule out ocular surface disease. On the whole dye tests are objective and not very reliable.
The lower puncti are gently dilated under topical anaesthesia. Next, one or two mls of local anaesthesia are injected using a lacrimal cannula. If there is regurgitation, the largest lacrimal probe which can be inserted without damaging the annulus is inserted. If it enters the sac without any resistance, the site of blockage is most probably NLD. If a site of resistance is noted, the probe is grasped with forceps at the punctum and withdrawn. The exposed end is measured to identify accurately the site of blockage. A smaller size probe is then inserted. Resistance at the same site reveals a complete canalicular obstruction. In the case of stenosis the smaller probe can be passed through and into the sac. Syringing of the NLD then follows. The same examination is repeated for the upper puncti. An experienced examiner can gather enough information at this stage to plan treatment.
These further investigations may be used to confirm the diagnosis. MDCG is particularly useful to reveal details of lacrimal sac anatomy and the site of naso-lacrimal duct obstruction8. MDCG with a delayed erect film 5 minutes after injection of contrast medium can detect functional NLDO by showing delayed clearance of the lacrimal sac9. Scintigraphy is mainly used to confirm a diagnosis of functional blockage when there is delayed or no outflow of radioactive media in the presence of a normal DCG.
More recent investigative tools are available such as the microcanalicular endoscope, which can demonstrate the site and type of blockage. However, experienced lacrimal surgeons can usually gather sufficient information by simply probing the canaliculi.
