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.
MEM Retinoscopy
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.
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| 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):
-Accommodative dysfunction
-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
-Accommodative spasm
-Loss of
attention
-Streff
Syndrome – a condition where the patient has normal prescription but has
difficulty seeing at all ranges and even tunnel vision
due to stress
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| Dr. S is a residency trained Vision Therapist and Pediatric Optometrist. |
Take Home: MEM retinoscopy is really a quick test to get set up once you
have practiced a few times, and can tell you if there are focusing system
problems in a short and efficient fashion.
Fused Cross Cylinder (FCC) is another test in the phoropter that can
yield the same accommodative lag or lead results, but especially in young children
this test can be difficult to get accurate or meaningful data. With FCC there
is no way for you to actively tell if the patient is paying attention to the
stimulus, and being behind the phoropter can really mess with a patient’s
“normal” behavior pattern. With MEM
retinoscopy you can actively visualize the patient’s behavior, making it a more
ideal method for gauging response. Even if the patient struggles to read the
card and stay focused long enough for you to perform a quick MEM, it
can be telling. If the child can’t stay
focused long enough for MEM, then it could mean that they actively avoid near work because reading and near activities have been so
difficult for them. The patient could even be
asymptomatic when you are discussing visual issues with them and their parent
in the room because they have been avoiding all difficult activities (like
reading)!
Stay tuned for more in office testing methods to come, enhanced
by the insights of Dr. S.
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