Rectus Muscle for ID only
How do you examine rectus muscles?
Rectus muscles receive a gross description after viewing under the dissecting microscope. The high magnification under the dissecting microscope is of the same order of magnitude (usually greater) than the low magnification at the compound microscope. During the gross examination the resident should try to identify tendon which is white and usually quite abundant with striations that run parallel to the apparent short axis (yellow arrow 2 in the figure above ). Muscle appears tan and is usually most abundant at one edge (white arrows above and arrowhead 2 in the figures). The length of the muscle removed needs to be documented accurately. Since the extraocular muscle is quite wide in vivo (about 9 mm) but the length resected quite small (usually 2-4 mm), the width that the pathologist sees in the gross is really the length of the muscle. This is especially evident when one sees the striations running in the short axis of the specimen. In the figure striations are obvious both in muscle and tendon (arrows 1 and 2) and therefore the muscle length is actually the distance between the arrowheads marked 3. In the body the long axis of the muscle is oriented parallel to arrow 3. An accurate measurement has ramifications medico-legally if the patient is under or overcorrected. So be careful!
How do you write up the gross description of an extraocular muscle?
Specimen 1, in formalin, labeled "right lateral rectus muscle" consists of one rectangular red and white fragment measuring 9 x 2 x 2mm in greatest dimensions. Under the dissecting microscope the external surface has fibrous white strands which are parallel and longitudinally oriented, adipose tissue, and blood vessels.
FINAL DIAGNOSIS: Muscle and fibrous tissue, "right lateral rectus muscle" (resection) - consistent with muscle and tendon
Very often a short length of muscle tendon is removed at its scleral insertion. Knowledge of the lengths of the extraocular muscle tendons is essential to determining whether it is likely that there is any muscle in the specimen at all. The medial rectus tendon is only about 3.8 mm in length and the lateral rectus tendon can be about 8 mm in length (see link for others). Without any muscle tissue the diagnosis would read:
FINAL DIAGNOSIS: Fibrous tissue, "right lateral rectus muscle" (resection) - consistent with tendon
When should you send an extraocular muscle for microscopic sections? Extraocular muscle may be sent for sectioning whenever it is necessary for the diagnosis. For example if a "slipped" muscle is suspected the surgeon wants to muscle or tendon versus a fibrotic pseudotendon. In this case microscopic sections are mandatory and a trichrome stain will highlight muscle fibers in red and collagen fibers (fibrosis and tendon in blue). Tendon appears as a compact lamellar array of fibers oriented in parallel.
Another example of an extraocular muscle that should be sent for sectioning is in cases of suspected chronic progressive external ophthalmoplegia. Here frozen sections for muscle enzymes, immunohistochemistry stains done in panels for specific proteins, and electron microscopy are necessary for the diagnosis. The surgeon needs to be wary of the needs of the pathologist to make the diagnosis, lest information will be lost without the proper fixation. The infamous ragged red fiber of CPEO is often not diagnosable in a specimen largely composed of tendon. A notable lawsuit was awarded against the surgeon that dumped a muscle biopsy into formalin in a patient with a suspected muscle dystrophy.
There are many other example when microscopy may be needed. Graves ophthalmopathy is associated with muscle enlargement, chronic inflammation, and Alcian blue mucopolysaccharide deposition.