Diagnosis and Management of Graves' Orbitopathy  

with Standardized Echography  Continued

2. Measuring Points:

A.   Medial, Superior and Lateral Rectus Muscles (Figs. 1,2,7 ):     

       These three muscles can be reliably measured and compared between the two orbits of a patient and over time at 5 specific measuring points:

1)  INSERTING TENDON as far foreward as possible (tendon pattern immediately following the scleral signal (outer scleral surface spike and anterior (inner) tendon surface spike are one and the same).

2)  TENDON HALFWAY BETWEEN INSERTION (1) AND MOST ANTERIOR      BELLY (3)

3)  MOST ANTERIOR MUSCLE BELLY:  as the beam travels posteriorly, the muscle pattern not only widens but also shifts away from the globe pattern toward the orbital bone signal.  The most posterior  point at which the maximized bone spike (muscle sheath surface spike next to bone) is still maximally high, is the measuring point # 3,  Any further angling of the beam in a posterior direction will result in a drop of this (bone) surface spike from its 100% high maximum.

4)  POINT OF MAXIMUM THICKNESS OF MUSCLE BELLY: this is the only relative measuring point which will change according to type of disease underlying a muscle thickening or thinning.  Still, its comparison over time and with the corresponding muscle in the fellow orbit is fully valid.

5)  APICAL (most posterior) SEGMENT OF MUSCLE.

Figure 7: Measurement of SR + Levator at 5 measuring points (left series) and of SR and Levator individually (left and right echograms in right series)

B.   Inferior Rectus Muscle (Fig. 8):

               This muscle offers usually only 3 reliable measuring points:

1)  MOST ANTERIOR BELLY (behind crossing of inferior oblique muscle).  There are two reasons why the inserting tendon of the inferior rectus muscle can not always be measured:  a) the superior orbital rim prevents the probe to be pushed posteriorly enough for the beam to reach the inserting tendon at a favorable angle.  Even raising of  the patient's chin, which clearly improves the examination condition for the inferior rectus muscle, does not ususally suffice to achieve above goal.  b) the over-lying belly  of the inferior oblique muscle is likely to interfere and confuse the examiner about the true maximum thickness of the inferior rectus inserting tendon.

2)  POINT OF MAXIMUM THICKNESS OF MUSCLE BELLY:  the reason why the #3  measuring point of the other straight muscles is usually not applicable to the inferior rectus muscle is that it often does not come to lie next to the bony orbital floor.  Rather, it remains separated from the orbital floor throughout the orbit by peripheral orbital fat tissue.                                                             .

3)  APICAL (most posterior) SEGMENT OF MUSCLE.

Important:  The inferior rectus muscle crosses over the inferior orbital fissure in an oblique fashion.  The inferior fissure, however,  also produces a pattern of decreased reflectivity simulated by the partial blockage of the ultrasound beam at the level of the orbital floor (only part of the beam enters the extraorbital tissues through the fissure and weakened echoes from these tissues result). 

Watch out, not to confuse the fissure pattern with the muscle pattern.  Such confusion can be avoided safely by (1) considering that the IR lies more nasally than 6:00 and (2) by dynamically identifying the inferior rectus muscle which can be followed continuously as the beam is angled anteriorly and posteriorly and then shows the shifting behavior so typical of straight extraocular muscles. In contrast, the fissure pattern shows up only in one specific beam position and direction; angling the beam anteriorly or posteriorly makes the pattern disappear abruptly.

Figure 8: measuring the inferior rectus muscle at the 5 measuring points ( I-V from top to bottom )

C.  Superior Oblique Muscle(Fig. 9):

This muscle has 3 specific measuring points:

1)  INSERTING TENDON:  this is the most important measuring point of this muscle for the diagnosis of superior oblique myositis and trochleitis.

2)  ANTERIOR BELLY OF MUSCLE (maximum thickness behind the trochlea).  Again, this measuring point is important especially for the diagnosis of myositis.

3)  POSTERIOR BELLY OF MUSCLE (maximum thickness in posterior orbit and apex).  This measuring point is particularly significant in the diagnosis and management of Graves' 

     orbitopathy.

Figure 9: measuring a thickened (myositis) right superior oblique muscle at 4 measuring points - I, III, IV, and V from top to bottom (left series) and the normal left SO of the same

  patient (right series)

D.  Inferior Oblique Muscle(Fig. 10):

This muscle has 2 specific measuring points:

1) MUSCLE BELLY (maximum thickness at or near 6:00). Again, this measuring point is important in Graves' orbitopathy

2)  INSERTING TENDON

Figure 10 (right above): measuring a (myositic) inferior oblique muscle at its two measuring points: I (top echoram) and IV (bottom)

3.  MEASURING ACCURACY:

                Using precise digital measurements  as offered by the MINI-A Scan Instrument, easy, quick and precise as well as  accurate measure-ments of the straight and oblique extraocular muscles can be performed by the trained echographer.  The measuring accuracy of Standardized Echography far surpasses that of the radiological imaging methods.  Both the dynamic approach and the high resolution of echography are responsible for this advantage of Standardized Echography.

                The actual measuring accuracy depends on the muscle measured, the measuring points used, and the disease process involved.  It is always better than +- 0.3 mm.  If a medial or lateral rectus muscle is measured, and the muscle has low reflectivity (e.g., in myositis) the measuring accuracy can be as good as +- 0.1 mm, especially when the inserting tendon is measured.  When, on the other hand, the involved muscle is (1) the superior rectus muscle, (2) has high reflectivity  (because of the disease process, e.g., Graves' orbitopathy), (3) cannot be differentiated from the levator muscle (in the posterior orbit where both are measured together), and (4) has irregular thickening (often the case in Graves' orbitopathy), the measuring accuracy may be as low as +- 0.3 mm.  In general, however, even under less favorable conditions, a difference of > 0.5 mm between the two corresponding muscle bellies or a difference of > 0.2 mm between the two correspon-ding inserting tendons in the two orbits of a patient, is always dia-gnostic for muscle pathology.

Important:

                (1)  The comparison between right and left orbits is much more meaningful and clinically significant than the absolute values (the normal range of muscle thicknesses is wide and overlaps greatly the abnormal range);  in contrast, the comparison between two correspon-ding muscles in the two orbits of a patient is a highly sensitive indicator of wheter a muscle is abnormal or normal.

                (2)  When the measurements differ only slightly between the two orbits, repeated measurements of the thinner muscle with the aim at getting a wider measurement of this muscle and thus disproving thickening of the previously wider measured muscle is an effective technique for avoiding mistakes.

                (3)  Always measure e.o. muscles with the patient's eyes in pri-mary gaze position.  If this is not possible, then compare the two orbits in strictly symmetric gaze directions. 

III.  Quantitative A-Scan for the Diagnosis of Graves'Orbitopathy

                About 60% of all cases of muscle disorders in the orbit are pa-tients suffering from Graves' disease.   While these patients have a variety of abnormal orbital findings, the thickening of the extraocular muscles is the key to the diagnosis.

A. Acoustic Criteria:

B. Acoustic Profile (Fig. 11):

The maximum thicknesses of all extraocular muscles together with the thicknesses of the optic nerve sheaths, the periorbi-tae, and the lacrimal glands are documented by mounting the echogra-phic pictures of these structures of both orbits of a patient.  The acoustic profile is arranged according to the topography of the measured struc-tures as one looks face to face at the patient. 

                The acoustic profile serves the purpose of  (1) grading the severity of the Graves' orbitopathy,  (2) selecting cases that need more frequent follow-up and may be endangered by compression of the optic nerve, and (3) following the natural course or the effectiveness of treatment in a quantitative fashion.

An important part of the acoustic profile is:

Figure 11: documentation of orbital profile of right (left group) and left (right group) orbital profile of a Graves' patient indicating also the MI and SNI of each orbit.

C. The Muscle Index (Fig. 12):

                The Muscle Index  (MI) of an orbit is the sum total of the maximum thicknesses of the 6 extraocular muscles, divided by 6.

Another important part of the orbital profile is the

D. Superonasal Index (Fig. 12):

The Superonasal Index (SNI) of an orbit is the sum total of the maximum (most posterior) thicknesses of the medial rectus, the superir oblique and the superior rectus muscles divided by 3.

Figure 12 (bottom left): documentation of left SNI in patient with Grade IV Graves' orbitopathy.