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The normal eye protrudes a distance of 16-20 mm as
measured from the lateral orbital rim to the corneal apex. This distance
is normally symmetrical with respect to the fellow eye.
Abnormal protrusion of one or both eyes is termed
exophthalmos, or proptosis. A difference of more than 2 mm between eyes
indicates unilateral exophthalmos. If an eye is thought to be shrinking
back into the socket, it is termed enophthalmos (sometimes termed
endophthalmos).
Exophthalmos can be caused by thyroid eye disease, an orbital
tumor, air or blood in the orbit, and other less common causes.
Congenitally abnormal orbits, lids, and/or globes
(axial myopia) can cause the appearance of exophthalmos without an
underlying disease process. This is sometimes referred to as
pseudoexophthalmos.
Exophthalmos is measured clinically using an
exophthalmometer, typically a Hertel type exophthalmometer. Some simply
refer to the instrument as "the Hertel" to avoid sounding like
Elmer Fudd when trying to pronounce "exophthalmometer".
In order to get an accurate measurement of proptosis,
we need a fixed point from which to measure. That fixed point is the
lateral bony rim of the orbit, at the outer canthus. We measure from
this point to the apex of the cornea.
The amount of proptosis may change over time, from
one office visit to the next. These changes may be an indication of a
change in the disease process. It is important to make sequential
measurements in a manner that avoids errors due to variations in
technique.
The instrument consists of two yokes mounted on a
horizontal bar. Each yoke has a notch for alignment with the bony
orbital rim, and a mirror, or mirrors, for observation of the corneal
apex. One of the yokes is adjustable along the bar to accommodate the
distance between the outer orbital rims of the two eyes.
Procedure
Sit face to face and at eye level with the patient.
Hold the instrument so that the bar is toward you,
and the mirrors are on top.
Hold the left side yoke in your left hand and the
right side yoke in your right hand.
If there is a previous measurement to refer to, set
the baseline indicator on the same mm mark. If you are using a B&L
instrument, tighten the locking screw.
The exophthalmometer can be a real torture device
if this step is not done properly. While holding the instrument in
front of the patient’s eyes, locate the lateral orbital rims with
your fingers before you place the notches on the bones. (Locate this
orbital rim on your own eyes with your fingers. Notice that there isn’t
much room between this bony rim and your eyeball. If you don’t make
a good placement of the instrument, you may be putting uncomfortable
pressure on the globe.) Keep your fingers on the orbital rims and
guide the notches into contact with the rims.
After properly placing the instrument on the bones,
have the patient look straight ahead. When measuring the patient’s left eye, close your
left eye and correctly align your right eye with the mirror of the
right yoke.
An image of the
patient's cornea is reflected in the mirror.
There is a mm scale on
the front of the mirror housing. It is your job to read the mm mark that is closest to the
image of the apex of the
patient’s cornea.
Your line of sight must
be perpendicular with the instrument in order for you to get an
accurate measurement. There are two red marks in the mirror housing of
the yoke that aid you in perpendicular alignment.
As you look into the
mirror, observe the positions of the two red marks. If you see
both red marks, as pictured below, then you are not in perpendicular
alignment. You must then move your line of sight so that the two
red marks superimpose.
In the photo below, the
red marks are close to being superimposed.
When you are properly
aligned, observe the image of the apex of the cornea and read the mm
mark that is closest to the apex.
Record the measurement for each eye along with the
baseline measurement. For example:
OD 22mm OS 23mm
base: 102 mm
Sources of error
There are three potential sources of error when using
the exophthalmometer.
The patient’s gaze: Make sure the patient is
looking level and straight ahead for proper alignment of the corneal
apex.
Instrument alignment: Make sure the notches are
touching the orbital rims at the outer canthus, and that the
instrument is level. If you have a baseline from a previous
measurement, use the same baseline for this measurement.
The operator’s perspective:
Make sure you are
viewing from the correct angle (perpendicular). With the B&L instrument the zero
mark should be just visible at the inside edge. With the Marco
instrument the red lines should be superimposed, as discussed above.
For the B&L
instrument, the correct alignment is pictured in the top graphic
below. Incorrect alignment is pictured in the bottom two
graphics:
With the Marco
instrument, the corneal image is superimposed onto the mm
scale.
With the B&L
instrument, the image of the cornea is below the image of the scale.
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