Investigating the Visual System with MEM Retinoscopy9:00 AM
We have discussed the symptoms that clue you in to an investigation for visual system inefficiencies, but what tests during your examination yield the most useful information? In the coming posts we will be diving in to Dr. S’s normal and preferred battery of testing for a visual system evaluation. Today’s topic: near retinoscopy and what it can reveal about the patient’s focusing system strength and ability.
When: All patients under age 18, adults that are in school or are complaining of fatigue with near work (under age 40 when that would be atypical).
Why: MEM stands for Monocular Estimate Method and is a dynamic retinoscopy, meaning the patient is doing an active visual process while being observed. Unlike standard retinoscopy that we use to find a distance prescription, MEM retinoscopy reveals the focusing system’s work or effort (accommodation). Ideally the patient will under-accommodate for a target; +0.50D to +0.75 “lag” of accommodation at near is considered normal. A higher than normal lag of accommodation means that the focusing system is under-accommodating at an abnormal level, and a “lead” of accommodation means that the system is over focusing. The lead or the lag is not the real problem, but an adaptation of the visual system to deal with excessive visual strain. It is quite common for people with binocular visual issues to develop accommodative lag or lead to compensate for their focusing inefficiency. For example, a patient with esophoria (an over-convergence of the eyes when focusing together) may have a high lead of accommodation at first because their accommodative and vergence system are over-acting due to the stress on the system. When this finally takes a toll on the eyes, a high lag forms in an effort to reduce the action and reduce the stress. Dr. S sums it up nicely when she says “it's not the lag or lead that causes the stress - it's the lag or lead that occurs because of stress.” Don’t view a lag or a lead of accommodation as the problem, but evidence that a problem exists that the patient’s focusing system is fighting to overcome.
How: We are measuring the person’s near focusing system response, so we need to get as true a measurement as possible of what real life is like for them. That means the patient should wear their glasses prescription, and the near target should be held at the patient’s working distance (called the Harmon distance in the optometry world) . A great way to do this is to hand the patient a book or magazine, and not say anything leading about where to hold things. Just see where they hold things naturally, and take a measurement of this distance. This is where you will hold your retinoscope with you are ready to start the test. On average we would expect a 40 cm working distance for adults, but depending on the person's arm length or the age of the child, it might be closer or farther.
The ideal target stimulus is the age appropriate MEM retinoscopy cards that usually come with your scope set when you purchased them at school. They are arranged with varying degrees of difficulty that you can select based on the patient’s grade level. In very young children the picture card would be most appropriate. Remember, you want the patient actively engaged, so if they seem bored or reading too quickly it could mean that your target is too easy. Stop and go up to the next difficulty level. With the picture cards you can even make a matching game by telling the child to “find the bicycle” or “find the cake”. You need to measure your patient’s response during the “finding” part and not when they have found the target to get meaningful data, so you have to be on your toes with this method.
|MEM Retinoscopy Cards via|
With your retinoscope in one hand, held at your patient's working distance, your other hand is now going to be ready to “dip” trial lenses in place to neutralize the reflex. You only want to keep a lens in place for about a second at a time to not disrupt the patient’s response, so again this is an act-fast skill. Dr. S has developed a great way of quickly collecting data, just by using a single +1.00D trial lens:
STREAMLINED MEM METHOD WITH JUST 1 TRIAL LENS
- 1) Dip +1.00 in front of the patient while they are reading. Did you see with or against?
- If you saw with, then you know they have a higher than normal lag (at least +1.25 or higher)
- If you saw against, then do a quick retinopscopy sweep of the eye without any lens in place
- If you saw with, then you know they are normal
- If you saw no motion or against, you know they have a lead in accommodation
Since your patient is actively reading or playing a “finding” game during the process, your results are tied to the patient’s behavior. If you find the lens required for neutralization fluctuates, this could be a hallmark of accommodative spasm. Fluctuation can also be a product of the patient’s lack of attention though. The target you use is very important due to this – make sure it is not too hard and not too easy because the effort involved to focus can be swayed by this difficulty level.
What Did You Learn?
Expected results for a normal accommodative response is +0.50 to +0.75 lag of accommodation at a 40 cm working distance. In presbyopes, you would expect the response to be whatever their near add is for that person (it doesn't make much sense to do this for presbyopes, honestly, unless you want to find an add in a more creative way haha). If your results do not reveal this normal lag range, then there are a number of possibilities that could be on your differential:
High Lag ( >+1.00):
-Presbyopia or pre-presbyopia (if around 40)
-Uncorrected or under corrected hyperopia
-Over minused patient
-Esophoria with insufficient vergence (patient under-accommodates to keep fusion)
-Patient wasn’t paying attention during the test or the stimulus was too easy
Lead of accommodation (<+0.25D or if you neutralized with no lens in place or a minus lens)
-Spasm of accommodation --the system is locked up creating “pseudo-myopia”
-Exophoria with insufficient compensating vergence (patient over-accommodates to keep fusion)
-Uncorrected or under corrected myopia
Unequal Results Between the Eyes
-Monocular Amblyopia – Dr. S sees this quite a bit in her vision therapy practice where the amblyopic eye shows a much higher than normal lag response
-Anisometropia (unequal prescription) resulting in unequal accommodative demand
-Incorrect binocular balance during the distance Rx
-Adie’s Tonic Pupil (1 eye has loss of accommodation due to pupil abnormality)
Fluctuation of Response