Lacertus Syndrome

I saw a patient whose symptoms started after he began weight training about 2 years ago. The patient reported experiencing sudden muscular weakness in his right elbow, forearm, and wrist while lifting weights. This weakness nearly caused him to drop the weights he was holding.

During the physical exam, the patient demonstrated normal and strong strength in both upper extremities, along with normal active range of motion in all joints of the upper extremities. Surprisingly (although this should not be surprising), all the electrodiagnostic tests I conducted yielded perfectly normal results. In fact, bilateral musculocutaneous motor nerve studies indicated even larger compound muscle action potential (CMAP) amplitude on the side presumed to be affected.

As I was unable to reproduce the patient’s symptoms during the physical exam and couldn’t identify any clues related to his reported muscular weakness, I had no other option but to interpret the data I collected from the patient as it was presented – normal, with no clinical recommendations.

The patient, then, gradually faded away from my memory.

A few weeks later, when I met the patient’s referring physician, the doctor informed me that they had made a “clinical diagnosis” of Lacertus syndrome for the patient. Subsequently, they performed a Lacertus fibrosis release procedure and were astonished to observe an immediate improvement in the grip strength.

Lacertus syndrome is a chronic exertional compartment syndrome of forearm.1 The pain initially manifests as an achy pain at the medial elbow and can encompass symptoms of compressive median neuropathy.2 These symptoms tend to arise during prolonged exertional activity and typically subside after a few hours of rest.1 The combination of delayed onset and symptom resolution with rest sets this syndrome apart. Although it clinically behaves similarly to exertional compartment syndrome in the legs, the unique pathophysiology stems from the anatomy of the lacertus fibrosis over the pronator teres.1


The median nerve travels alongside the brachial artery.3 In the level of antecubital fossa, it courses beneath the ligament of Struthers, if present, located medially to the biceps tendon. From there, the median nerve passes between the two heads of the pronator teres muscle and continues distally beneath the pronator teres, which is covered by the lacertus fibrosis (bicipital aponeurosis).4 The lacertus fibrosis is not the primary muscle fascia for the superficial volar compartment of forearm5, but the muscle fascia originated from the biceps brachii inserted to the deep fascia of forearm protecting the brachial artery and median nerve that run underneath.6 However, it does play a role as a muscle constrictor to the pronator teres during muscle expansion in various exertional activities, such as weightlifting or throwing a ball.5


Lacertus syndrome and its symptoms in baseball pitchers were first described by Bennett in 1959.5 There are various etiologies and mechanisms of injury associated with lacertus syndrome, which is characterized by compression of the median nerve in the proximal forearm, either directly by the lacertus fibrosus or indirectly by the pronator teres muscle. In cases where the compression is caused by the lacertus fibrosus, it can be detected through NCS/EMG studies if the compression persists for a prolonged period and is severe enough to induce pathological processes such as demyelination or axonal loss due to static compression.

However, lacertus syndrome can also present with intermittent symptoms during upper extremity activity. Researchers have conducted a study examining the intracompartment pressure of the forearm before and after exercises, using MRI scans. The study revealed an increase in swelling within the pronator teres muscle following exercises. This particular variation of lacertus syndrome involves transient, exertional compression of the median nerve caused by increased pressure beneath the lacertus fibrosus, exerted by the pronator teres muscle and the surrounding tissues.


Electrodiagnostic testing, such as NCS/EMG, is a useful diagnostic measure that enables clinicians to localize lesions and specify the type of lesion.7 Important differential diagnoses for lacertus syndrome include carpal tunnel syndrome, anterior interosseous nerve syndrome, thoracic outlet syndrome, and cervical radiculopathy. Nerve Conduction Studies (NCS) and Electromyography (EMG) tests can help rule out these conditions. As mentioned above, if patients experience static compressions from the lacertus fibrosis, the NCS/EMG would yield positive results. In cases where median nerve demyelination is caused by the lacertus fibrosis, the NCS can detect a slowing of nerve conduction or conduction block at or across the forearm. In cases where median nerve axonal damage is caused by the lacertus fibrosis, the EMG can demonstrate signs of denervation at or distal to pronator teres muscle.8

However, when patients do not experience static compression on the median nerve, and instead present with transient or episodic symptoms that improve after rest, NCS/EMG studies may not be as effective in providing conclusive results compared to cases with static compression.9

Physical Examinations (Special tests) with Lacertus Syndrome

Lacertus syndrome may be suspected in patients presenting with aching pain over proximal forearm and sensory symptoms over the distributions of the median palmar cutaneous nerve branch, as well as other median sensory nerve distributions. A Tinel sign can be observed when tapping on the nerve compression site, such as the cubital fossa or pronator teres. Though it is rare, pronator syndrome can be clinically differentiated by applying resistive pronator teres contraction. The scratch collapse test is another option used to test for nerve entrapment, with an overall sensitivity of 38% and specificity of 94%.10


Making a diagnosis solely based on a clinical test or a history/physical exam is a perilous clinical judgment. Electrodiagnostic or other imaging evidence should align with patients’ history and the results of the physical exam. However, negative findings of higher accuracy testing such as NCS/EMG or other imaging do not necessarily mean that patients’ symptoms do not exist or that patients are normal11, considering the dynamic nature of etiology and the various types of pathology patients may experience for some conditions.

For an electromyographer, who aids clinicians in making clinical judgments, it is essential to indicate the limitations of the NCS/EMG test for certain pathologic conditions. Additionally, they should conduct thorough history taking to delineate the mechanisms of injury or contributing/aggravating factors. If feasible, it is crucial for them to offer clinical recommendations that are strongly supported by other findings, particularly when NCS/EMG tests are normal. This approach aims to assist referring clinicians in gaining a deeper understanding of the patients’ conditions and enables accurate diagnosis and appropriate management.

Junghwan (Paul) Bang, PT, DPT, ECS


  1. A. Mehl et al. (2021). Lacertus syndrome: Use of pre- and post-exercise MRI to aid in diagnosis and treatment. Volume 16, Issue 5, Pages 1113-1117
  2. W. Seitz Jr. et al. (2007). Acute compression of the median nerve at the elbow by the lacertus fibrosus. J. of shoulder elbow surgery. 16(1):91-4. doi: 10.1016/j.jse.2006.04.005.
  3. S. Standringl. Gray’s anatomy. 41st Editiion. Elsevier.
  4. C. Rodner et al. (2013). Pronator syndrome and anterior interosseous nerve syndrome. Journal of the American Academy of Orthopaedic Surgeons 21(5):p 268-275, DOI: 10.5435/JAAOS-21-05-268
  5. S. Jordan. (2020). The Lacertus syndrome of the elbow in throwing athletes. Clinics in sports medicine. Volume 39, Issue 3, July 2020, Pages 589-596
  6. B. Morrey et al. Morrey’s the Elbow and its Disorders (Fifth Edition), Elsevier.
  7. R. Tubbs et al. (2015). Nerves and Nerve Injuries. Academic Press.
  8. P. Gross et al. (1992). Proximal median neuropathies: Electromyographic and clinical correlation. Muscle nerve 15(3):390-5. doi: 10.1002/mus.880150320.
  9. A. Özdemir et al. (2020). Clinical, Radiological, and Electrodiagnostic Diagnosis of Pronator Syndrome Concurrent With Carpal Tunnel Syndrome. The journal of hand surgery. volume 45, issue 12, P1141-1147.
  10. N. Jain et al. (2023). The Scratch-Collapse Test: A Systematic Review and Statistical Analysis. HAND Volume 0: Ahead of Print.
  11. P. Daley et al (2022). Use of Electroneuromyography in the Diagnosis of Neurogenic Thoracic Outlet Syndrome: A Systematic Review and Meta-Analysis. J. Clin. Med., 11(17)



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