What?! Have I lost my mind?! Well most optometrists would say that having an NPC of TTN is the ultimate convergence a patient can have; however I ask you to think differently.
Ok. Let’s take a step back and start from the beginning. As part of your routine entrance tests you take out an accommodative target (whether you use a pen tip, a column of letters or a ball on a stick is a discussion in and of itself) and tell the patient you will be bringing this object close to their nose. As it gets closer the patient must report as soon as it goes double. Sitting in front of your patient, you begin with the target ~12 inches away from the patient on their midline and slowly bring it in watching the patient’s eyes for any loss of fixation. If the patient reports diplopia, you would measure the distance from the outer canthus and then slowly pull the target back out until the patient reported a single target (this distance is also recorded). The same thing would be done if there were a loss of fixation. Other patients will not report diplopia and this would be recorded ‘to the nose’ or TTN.
Well that seems all well and good, but there is more to the test than just the ability to converge on a target. Many behavioral optometrists test the NPC by looking at the reach, grasp, release and then the re-grasp.
Here is what they are looking at:
Reach: The ability of the patient to find the target
Grasp: The ability to follow the target in towards the nose
Release: Once the patient reaches the near point (whether it be at the nose or where the patient is able to regain fusion) have them look away at a different target (I usually use my big Italian nose!). Be sure to watch the timing, accuracy and symmetry between the two eyes to release the convergence.
Re-grasp: Now that they are at the far target, have them look back at the original accommodative target. Ask again if the target it single. If not, bring the target back towards you until the patient can regain fusion. If it single, the patient has good re-grasp.
Repeat this process 2-3 more times.
Why do this? It is sort of like doing phorias and then ranges. You want to see how well the patient is functioning in the real world, under stressful situations and if they are able to compensate for any sort of abnormality.
Let’s say your patient is complaining of difficulty reading and maintaining single. If you just do a quick NPC you might miss a CI. Yes they may have the ability to converge, but by doing the reach, grasp, release and re-grasp you realize they are not able to maintain or quickly get it back it if they look away. This is a VERY important piece of information in understanding your patient.
I challenge you to do NPC this way in your next exam; I promise you’ll be surprised at what you’ll learn about your patients!!