Diagnosis and Management of Graves' Orbitopathy

with Standardized Echography

 

    Karl C. Ossoinig, MD

Iowa City

                Both the A-scan and B-scan techniques are employed to screen the straight and oblique extraocular muscles for lesions.  In this "Basic Examination of the e.o. muscles, the A-scan is particularly sensitive in indicating thickening or thinning of muscles and in differentiating the underlying pathologies, while the B-scan is very helpful in their topographic evaluation.

                The evaluation of extraocular muscles is one of the most important applications of Standardized Echography:  when the entire capacity of Standardized Echography, i.e., axial eye length measurements as well as diagnostic evaluations of the posterior and anterior eye segments, and of the orbital and periorbital regions, is available, more than 50% of all cases seen in an Echography Clinic are likely to be (peri)orbital in nature.  Among these patients with (peri)orbital  lesions, cases with disorders of the extraocular muscles (with or without optic nerve problems) are prevalent at a rate of more than 80% of these patients. Graves'orbitopathy is the underlying pathology in more than 80% of these.  

                        I.  Acoustic Properties of  Normal Muscles

                Normal extraocular muscles have a lower internal reflectivity than the surrounding orbital fat tissues (smaller average interface size).  Their internal structure is more regular (similar sizes and regular distribution of acoustic interfaces).  The very large outer surface of the extraocular muscles and the equally very large surfaces of their sheaths reflect very strongly (Fig. 2 ).   Only sound beams that reach these large enveloping surfaces of a muscle perpendicularly, allow the display of typical acoustic muscle patterns as characterized by the internal structure and reflectivity of the muscle. 

                The thickness, internal structure and reflectivity of the normal extraocular muscles vary greatly from person to person (and also depend on the race), but are very similar in the two orbits of the same person.  Children and youthful persons tend to have a lower reflectivity, older individuals tend to display a higher reflectivity of their normal extraocular muscles.

                Important:   The parallel A-scan beam is strongly refracted toward the extraocular muscle body at the outer layers of its sheaths when oblique sound beam incidence occurs.  This refraction makes it possible even in the posterior orbit, to aim the ultrasonic beam across the muscle producing displays and allowing precise measurements of true muscle cross-sections (the same situation as in optic nerve measurements).  The focussed B-scan beam is not sufficiently refracted to always allow the display of true cross-sectional displays of the extraocular muscles.

 

II.  EXAMINATION TECHNIQUES

1.  Basic Scanning and Measurements with Standardized A-scan

                For the screening of the 4 straight and the 2 superior oblique muscles, the A-scan probe is placed opposite the muscle on the bulbar conjunctive between limbus and fornix.   Probe and beam are first directed anteriorly toward the inserting tendon.  Minimal angling and shifting of the probe serve the purpose to detect the anterior muscle pattern.  While the eye is in straight gaze (if possible), the probe is angled posteriorly so that the beam scans the muscle from anterior to posterior.  If, during this basic scanning, the beam remaines aimed at the muscle body, the pattern stays displayed on the screen widening and shifting in a typical fashion.  If the examiner "loses" the muscle pattern, corrective probe movements must be undertaken to find it again and continue the screening.  This is like all examination techniques, a more time-consuming and difficult undertaking for the beginner, but becomes an easy, quick and automatic procedure with growing experience and improved skills.  The echographer develops automatic reflexes to scan and display optimal muscle patterns very much like anyone develops reflexes that guide walking and running or other activities of the body which for the beginner are impossible or difficult to achieve smoothly and promptly without thinking and analyzing what and how one is doing it. 

                                  

Figure 1A: Crossection of MR in A-scan                                                               Figure 1B: a muscle has much smaller interfaces than the surrounding orbital (fat) tissues

 

 

Figure 1 C: transverse (top) and longitudinal (bottom) B-scans of MR

                The probe location is maintained the same, whether the examiner needs to display the inserting tendon, the belly or any other portion of a  straight extraocular muscle.  The probe is only angled to achieve the complete scanning of the muscle from anterior to posterior and vice versa. The superior oblique muscle, in contrast, requires different probe positions for the display of its inserting tendon and anterior and posterior portions:

                                a) for the display of the anterior belly portion, the probe is place inferotemporally on the bulbar conjunctiva and the beam is angled and shifted in search for the superior oblique muscle and the trochlea ( the normal trochlea is recognized by the fact that the muscle pattern cannot be followed anterior to it - it stops existing when the beam reaches the trochlea during a forward scan along the muscle).  One needs to consider that the location of the trochlea  varies widely from person to person, but is always symmetric in the two orbits of the same individual.

                                b) for the display of the posterior belly portion, the medial rectus muscle which serves as the guiding structure, is first scanned into the posterior orbit.  There the beam is angled slightly superiorly

Figure 2 (above): Scleral Buckle with over-riding muscle tendon shown in B-scan demonstrating its usefulness for topographic documentation.  The A-scan is needed to diagnose the myositis (thickening and low reflectivity of the tendon)

 

 

Figure 3 (on right): B-scan cross-sections of a medial rectus muscle illustrating the "blooming phenomenon".

 

 

 

Figure 4: Examination technique for displaqying the measuring points I-V of the medial rectus muscle (left and above).

 

 
Figure 5: measurements of  right and (for comparison) left medial rectus muscles in a patient with myositis OD.

to display the pattern of the superior oblique muscle.  Forward and backward scanning will easily differentiate the two muscles:  the muscle pattern shifts away from the orbital bone signals when following the medial rectus anteriorly.  The muscle pattern stays with the orbital bone signals when following the superior oblique muscle forward until it stops to exist once the sound beam surpasses the trochlea .

                                c) for the display of the inserting tendon of the superior oblique muscle the probe is placed at the 6:00 meridian behind the limbus and the beam is directed toward the posterior portion of the 12:00 meridian between equator and optic nerve.  Slight angling and shifting of the beam serves the purpose of detecting the tendon as a lower reflective episcleral tissue layer with maximally high surface spikes on either side stemming from the large inferior (anterior) and superior (posterior) surfaces of the tendon.