Case Study 23 Craig Horbinski, M.D., Ph.D.

The patient is a 57 y/o female with a past medical history significant for acute intermittent porphyria. She recently experienced muscle cramps that began in her legs and progressed to her upper extremities. CPK is over 1000, LDH level of 128 (normal). On ROS, she states that she has had chronic back pain for several years, but that she started to have right leg pain unrelated to her spasms ever since her lumbar surgery for a ruptured disc in 2007, with residual foot drop and sciatic nerve problem from the knee up with the feeling of her leg being "hot". On inspection, her right calf was a ½ inch smaller in circumference than the left calf. The right gastrocnemius muscle was therefore biopsied. Motor nerve conduction studies showed low amplitude to left tibial nerve with prolonged distal latency to right tibial and right median nerves. All other findings normal. Concentric EMG performed to the bilateral lower limbs showed fibrillations, positive wave and complex repetitive discharges in distal and proximal muscles, right leg worse than left. Motor unit potentials demonstrate increased amplitude, polyphasity with mild decreased recruitment in right L4, L5, S1 and left L3, L5 myotomes.

Describe the biopsy and stains. Click on the following links to view slides: H&EH&E, NADH, ATPase 9.4, ATPase 4.3, EsteraseNADHATPase 9.4ATPase 4.3Esterase Question 1

Hematoxylin and eosin stained frozen sections show large clusters of angulated atrophic fibers scattered throughout the specimen. Nuclear clumps are present, as is tendon. The perimysial and endomysial space shows increased cellularity, but degenerating fibers or inflammatory infiltrates are not seen. Rare regenerating fibers are present. NADH reacted sections reveal abundant targetoid fibers and dark atrophic fibers. Esterase reactivity is increased in the angulated atrophic fibers. ATPase reacted sections reveal loss of the normal checkerboard pattern with a Type 1 to Type 2 fiber ratio of 3:1 and no selective myofiber atrophy. Answer

Question 2 What do these findings mean—what is the diagnosis, and how do these findings support such a diagnosis?

Answer This biopsy is showing classic neurogenic changes, both active and chronic. First off, there are a lot of atrophic fibers in clusters, many of which are angulated. Primary myopathic diseases, on the other hand, tend to produce atrophic rounded fibers scattered in between normally-sized myofibers. Second, the presence of nuclear clumps that appear to be outside any myofibers is a sign of severe myofiber atrophy (the myofibers are so atrophic, you can’t see their cytoplasm any more). This sort of thing is more suggestive of neurogenic rather than myopathic causes. Third, target fibers are seen on the NADH stain—the myofibers with dark rings in the center (they’re so prominent you can even see them on the H&E). This is unequivocal, specific proof that there’s nothing inherently wrong with the muscle, it’s a problem with the motor nerve. Seeing some dark atrophic fibers on the NADH stain helps, too. Fourth, the “active” part of the neurogenic changes comes from the increased esterase reactivity in many of the angulated atrophic fibers plus the darkly staining fibers on NADH. Finally, the “chronic” part comes from the fact that this muscle no longer shows a nice even distribution of type 1 (dark on ATPase pH 4.3) and type 2 fibers (dark on ATPase 9.4). This tissue has undergone denervation in the past with subsequent reinnervation. The myofibers will change their type in response to whatever nerve is reinnervating them—if it’s a type 1 nerve fiber, the myofibers will become type 1 fibers and vice versa. Since nerves tend to reinnervate multiple adjacent myofibers at a time, an ATPase stain will show large areas of only one fiber type. This is one of the few key facts you need to know for the general anatomic pathology board exams regarding nonneoplastic muscle and nerve pathology. This patient therefore has a serious active problem with her motor nerve that has been going on for awhile. It is important to note that the gastrocnemius muscle often shows mild “neurogenic changes” even in a healthy person. Also, that muscle is highly tendinous, and any muscle near a tendon insertion site might show some pseudo-myopathic changes like split fibers, rounded atrophy, or internalized nuclei.

Question 3 Do these findings fit with the clinical history?

Answer Absolutely. AIP itself can cause neuropathies, plus the woman had lumbar spine surgery for a ruptured disc with persistent sciatica and right foot drop. It’s conceivable that the motor fibers innervating the gastrocnemius have been affected by both the AIP and mechanical compression from the spine (if the ruptured disc affected both the dorsal and ventral roots).

Question 4 On the ATPase stains, what finding in many of the atrophic clusters is a little worrisome for a disease far worse than AIP or herniated disc? (It’s also hinted at in the EMG study report.)

Answer Many of the clusters of atrophic myofibers have an equal mix of type 1 and type 2 fibers. In typical neurogenic-related muscle conditions, the atrophic fiber clusters will be all of one type—i.e., within a specimen you will see both fiber types affected, but each individual cluster should be only one type, signifying reinnervation. In primary motor neuron disease, a.k.a. amyotrophic lateral sclerosis, the affected muscle will often contain both fiber types in the same cluster of atrophic myofibers. Furthermore, detecting muscle fibrillations on EMG is a sign that motor innervation is severely compromised, causing the sarcolemmal membranes to become irritable and spontaneously depolarize. It’s as if they’re getting bored with the lack of stimulation. Fibrillations are not specific for ALS, but that disease is one of the most common causes of muscle fibrillations. In this context it’s probably wiser to not mention this possibility, since there are already two other plausible causes for the neurogenic changes. If this unfortunate patient does indeed have ALS, the pathology report is still valid, and at any rate ALS is a clinical diagnosis.