Module 4 Section 1
 

Home

Catalog

Go to Section 2
       
       
 

Module 4:

 Accommodation and Near Vision
 

Section 1:

 Controlling Accommodation
     
 

Introduction

Methods of control

Cycloplegic drops

Factors affecting  cycloplegia

Retinoscopy and refractometry

Fogging
   
 

Introduction

This article is about controlling accommodation, because if you don’t control accommodation, accommodation may very well control you when refracting. Of course you don’t have to control accommodation in every patient.  Aphakes and pseudophakes don’t have any accommodative ability, and 65 year olds have little left (although some may surprise you).

The patients you have to worry most about are those less than 20 year olds,  the adult low hyperopes,  and the anisometropes (unequal refractive error).  During the refraction process these patients may "over-use" their accommodative ability while responding to your instructions or trying to read a line of letters.  This may result in an over-correction of the myope, an under-correction of the hyperope,  or an unbalanced correction may result.

Methods of control

There are two ways for you to control accommodation: cycloplegia and fogging. Fogging is used to control accommodation during a manifest (no cycloplegia) refraction (MR). Cycloplegia is used to control accommodation during a cycloplegic refraction (CR). Fogging is also used to control accommodation during a cycloplegic refraction because cycloplegia may be incomplete, leaving the patient with a degree of accommodative control.

Why not give everyone a cycloplegic refraction and forget about the manifest refraction?  Actually, this approach works fairly well with myopes who are not presbyopes.  It doesn’t work with presbyopes because you have to measure their accommodative ability at near. It doesn’t work with hyperopes because these people like to maintain a degree of accommodative control in their distance vision.  A prescription for a hyperope written from a CR will be blurry for them in the distance because they don’t relax their accommodation all the way.

Cycloplegic drops

Cycloplegia, of course, is accomplished by instilling the same drops that dilate the eyes. These drops have a temporary paralyzing effect on the ciliary muscle.

You test-takers need to remember that there is a difference between a cycloplegic drop (tropicamide, cyclopentolate) and a mydriatic drop (phenylephrine).  Cycloplegics paralyze the sphincter muscle of the iris (causing dilation) and paralyze the ciliary muscle (suspending accommodation).  A mydriatic, such as phenylephrine (a poor dilator by itself), works with cycloplegics to boost dilation by stimulating the dilator muscle of the iris.

Learn to identify drops by their scientific name (tropicamide) instead of the trade name (Mydriacyl). Trade names can be misleading.  Mydriacyl is a cycloplegic,  not a mydriatic.

Tropicamide is a fairly good cycloplegic with a duration of 1 to 4 hours; but it is not good enough for the kiddies.  Cyclopentolate is a good cycloplegic; strong enough for most children, although it sometimes takes two applications.  Duration is 2 to 8 hours.

Homatropine and atropine are strong cycloplegics that are used therapeutically,  but are usually not necessary for refractive cycloplegia.

Factors affecting cycloplegia

Have you ever taken a call from a patient who is still dilated the next day after an eye exam? Keep in mind these factors before you load up a patient with cycloplegic drops:

  • The younger the patient is, the stronger the drops need to be, and/or the more applications that are needed.

  • Patients with lighter colored irises need less strength, or fewer applications.

  • Many patients are acutely aware of the effects of cycloplegia and can (and will) advise you as to the amount needed.

  • Be aware that patients can be allergic to any of the cycloplegics.

  • Dilation may not be appropriate for patients with certain iris-clip IOLs.

  • Patients who have never been dilated, or who are new to your office or clinic, should be examined for narrow angles prior to dilation.

Tropicamide and cyclopentolate are available in .5% strength as well as the standard 1% strength.  Phenylephrine should routinely be used in the 2.5% strength instead of the 10% strength because of the reduced risk of adverse reactions.  If you do not have the .5% strength available, exposure to the 1% strengths can be somewhat limited by having the patient immediately wipe the eye after instillation.

Be sure to give the drops enough time for the full cycloplegic effect;  at least 15 minutes for tropicamide,  and 20 to 30 minutes for cyclopentolate.

Retinoscopy and refractometry

Many refractionists believe that retinoscopy is easier on a dilated/cyclopleged eye.  I find it easier on an un-dilated eye, with the exceptions of patients with pupils less than 2mm, and young patients with very active accommodation. If you do retinoscopy on a dilated eye,  be sure to pay attention to the central reflex and ignore the sometimes confusing reflexes seen at the pupil edge.

The cycloplegic refraction is performed just as you would a manifest refraction; with one exception.  You will not be checking near vision because accommodation has been suspended.  If you are interested in the patient's near vision, it must be checked during a manifest refraction performed prior to cycloplegia.

When performing a cycloplegic refraction,  be aware that there is the occasional patient who will not refract to the same visual acuity that you arrived at during the manifest refraction. This may be because the dilated eye lacks the benefit of the pinhole effect of the smaller pupil.

Fogging

Accommodation can be controlled,  to a limited extent,  by fogging techniques. Fogging is accomplished by adding a limited amount of plus sphere power to plus corrections, or by reducing minus sphere power in minus corrections.

The goal is to move the focal point in front of the retina. The eye accommodates in order to see more clearly.  If an eye is optically fogged,  and the eye accommodates, the vision will get blurrier,  not clearer. Thus,  accommodation is discouraged.  Care must be taken to not fog the eye too much.  If the vision is blurred too much, accommodation may actually be stimulated in a effort to see better.

How fogging works

The eye in the diagram to the right has a glasses correction that has left it under-plused. When accommodation is relaxed,  light focuses behind the retina and the vision is slightly blurry.

 

The patient accommodates (the natural lens gets "fatter") to see better.  Accommodation moves the focal point onto the retina.

 

 

We fog the eye by adding enough plus power to move the focal point in front of the retina.

 

 

If the patient accommodates now,  his vision will get blurrier instead of clearer.   Accommodation now causes the focal point to move furthur forward. Thus, accommodation is discouraged.

 

Fogging must not be used when using the cross-cylinder to check the cylinder power and axis. Cross-cylinder techniques work best when the eye is able to accommodate to see better.

Fogging procedure

Fogging works best when used binocularly, as part of a binocular balancing procedure (see Module 3, Section 1). However, sometimes it is necessary to fog each eye individually.

  • Have both eyes un-occluded if working binocularly. Occlude the fellow eye if working monocularly.

  • Add .75 D plus power to the endpoint of the manifest refraction.  Have the patient look at a full screen of letters with 20/50 at top and 20/15 at the bottom.  Ask the patient if her vision is slightly blurry.  If it is not, add .25 D plus power until it is.

  • If you are performing a binocular balancing procedure,   you would balance at this point (Module 3, Section 1).

  • Start reducing the plus power (or add more minus) while you check the patient’s vision on the chart.  It is important not to ask the patient if his vision is getting sharper.  Do not ask him to compare one lens power to another.

  • Keep reducing plus power only if the change results in the patient’s ability to read more letters or smaller letters. In other words, the change must lead to better vision.

  • The endpoint is the most plus power, or least minus power, that gives the best vision.

You will not need to use fogging techniques on everyone you refract.  Experience will teach you which patients need it,  but don’t hesitate to fog if in doubt.

Top of page