The curious case of the protruding muscle: A case report of muscle hernia

Abstract

Muscle hernias are an uncommon clinical entity, characterized by the protrusion of a muscle through the overlying weakened fascia. The most affected muscle is the tibialis anterior muscle, although other locations such as the upper limb are possible. Patients typically present with a palpable, soft-tissue mass that may be reducible upon physical examination. Imaging plays a critical role in accurately diagnosing muscle hernias, distinguishing them from other causes of palpable soft-tissue masses. Magnetic resonance imaging (MRI) is particularly valuable, as it allows detailed visualization of the muscle contour and the superficial fascial layers at the site of the abnormality. We report the case of a 32 years-old patient with a palpable mass of the thigh, presenting to our structure for an MRI, in which we concluded to a hernia of the vastus lateralis muscle.

Introduction

Muscle hernia is an uncommon condition characterized by the focal protrusion of a muscle through a defect or weakened area in the surrounding muscular fascia. Although it can occur in various locations, the lower limb is most frequently involved, with the tibialis anterior muscle being the most commonly reported site. In many cases, muscle hernias may be asymptomatic and discovered incidentally. However, when symptomatic, they often present as a visible or palpable mass that may enlarge with muscle contraction, sometimes accompanied by localized pain or discomfort. Because of their rarity and nonspecific clinical presentation, they can mimic more concerning entities such as soft tissue tumors or hematomas, thereby posing a diagnostic challenge. Imaging studies—particularly ultrasound and magnetic resonance imaging (MRI)—are instrumental in confirming the presence of a fascial defect and ruling out other pathologies.

While muscle hernias most frequently involve the anterior compartment of the leg, other muscle groups, including the quadriceps or even upper-limb musculature, can also be affected. Here, we present the case of a young athlete who developed an acute hernia of the vastus lateralis muscle in the thigh, initially resembling a soft tissue mass. This case highlights the importance of considering muscle hernia in the differential diagnosis of thigh swellings and underscores the value of appropriate imaging in guiding clinical management.

Case report

A 32-year-old male runner presented with acute swelling in the left outer mid-thigh that appeared during a high-intensity interval training (HIIT) session while preparing for an upcoming marathon. Over the following week, he noted progressively worsening pain, especially during running or other weight-bearing exercises. He had no significant prior medical or surgical history and denied any history of similar episodes. His family history was unremarkable for musculoskeletal disorders or neoplasm.

On examination, a soft, mobile protrusion was palpable in the lateral aspect of the left thigh, with no overlying skin changes or erythema. The differential diagnoses at this stage included: Muscle strain or partial tear of the quadriceps, fascial defect or muscle herniation, soft tissue neoplasm (eg, lipoma, sarcoma), and hematoma (possibly from a minor trauma not clearly recalled by the patient).

Given the visible bulge and pain, a laboratory workup was conducted (including CBC, basic metabolic panel, and creatine kinase) was within normal limits, with CK measured at 85 U/L indicating no acute muscle necrosis or substantial injury.

An ultrasound examination had been performed at another facility, but it was inconclusive: no sonographic abnormality was detected in the region of the palpable mass. Given the persistent suspicion of a fascial defect or muscle herniation and the need for a more detailed soft tissue assessment, an MRI was undertaken as the next imaging modality.

An MRI of the left thigh was performed on a 1.5 Tesla system, including STIR, DP FS, and T1-weighted sequences without fat suppression in axial, coronal, and sagittal planes. No contrast enhancement was used. These sequences revealed a focal protrusion of the vastus lateralis muscle through a thinning of the fascia into the subcutaneous tissue (Fig. 1, Fig. 2). Muscle striations and signal intensity were preserved (Fig. 3), and no circumscribed mass was identified. These findings were consistent with a diagnosis of muscle hernia of the vastus lateralis.

Fig. 1.

Axial STIR showing vastus lateralis muscle hernia through the weakened overlaying fascia (arrows).

Fig. 2.

Coronal T1 imaging of thighs showing left muscle hernia (arrow) allowing comparison with the controlateral muscle.

Fig. 3.

Coronal DP FS imaging of muscle hernia showing a thin overlaying fascia without defect.

Following a multidisciplinary discussion involving radiology, orthopedics, and sports medicine, conservative management was recommended. The patient was advised to rest from high-intensity and high-impact activities, use compression stockings to reduce the muscle's outward bulging and alleviate discomfort, and postpone participation in the marathon to prevent aggravating the injury.

Over the next 6 weeks, the patient followed a gentle rehabilitation program emphasizing low-impact exercises such as swimming and cycling at low resistance, and gradual reintroduction of running drills.

At his follow-up appointment 2 months after the initial presentation, he reported near-complete resolution of symptoms and had returned to moderate-intensity running, with instructions to continue using compression stockings during intense workouts, while clinical examination showed minimal residual swelling and no pain on palpation. Surgical intervention was not deemed necessary given the patient's favorable response to conservative measures.

Discussion

Muscle hernia is defined as the protrusion of muscle tissue through a defect or thinning in the overlying fascia. Although it is relatively uncommon, it is most frequently reported in the lower extremities, particularly involving the tibialis anterior muscle [1]. The true incidence is likely underreported, as many cases remain asymptomatic or misdiagnosed due to its subtle clinical presentation [2].

Muscle hernias result from an imbalance between intramuscular pressure and the strength of the fascia which may become weakened, allowing muscle fibers to herniate [3], and therefore can be classified as constitutional and acquired [1]. Constitutional or congenital factors can include a generalized weakness in the muscular fascia due to mesodermal insufficiency, or may arise in areas where nerves and blood vessels penetrate the fascia. Whereas acquired causes are secondary to trauma, including direct trauma such as penetrating traumas or closed fracture leading to fascial tears, or indirect trauma resulting in an acute rupture of the fascia caused by the application of force to a contracted muscle. Herniation risk is exacerbated by elevated intracompartmental pressures, which can result from conditions such as muscle hypertrophy or chronic exertional compartment syndrome (CECS). CECS is defined as a reversible condition involving abnormally high intramuscular pressure during physical activity or exercise, caused by the inability of osteofascial tissues to accommodate the increase in muscle volume induced by exercise [4].

The patients most commonly affected comprises athletic men, including military personnel, professional athletes, mountaineers, skiers, and individuals engaged in comparable high-intensity occupational or recreational activities.

Clinically, muscle hernias present as a localized, soft, and often reducible swelling, which may become more pronounced during muscle contraction or activity. Patients may report intermittent symptoms, including discomfort, muscle cramps, or a visible bulge. But more frequently, the patients are asymptomatic, and the hernia is incidentally diagnosed [5].

Excluding differential diagnoses, including hematomas, varicosities, arteriovenous aneurysms and tumors, can be a real challenge. Ruptured muscle and central neuropathy are other diagnoses to be considered [1].

Accurate diagnosis of muscle hernia relies heavily on imaging, with ultrasound and MRI being the modalities of choice:

Ultrasound - with a high frequency linear probe, and performed comparatively with the contralateral side - can directly visualize the fascial defect as a discontinuity or focal thinning of the fascia. Plus, ultrasound allows dynamic maneuvers such as contraction, where the herniated muscle can be observed protruding through the fascial defect [6].

While highly effective, the diagnostic utility of ultrasound is operator-dependent, and its ability to assess deeper structures or subtle changes remains limited. Plus, performing excessive may reduce the hernia and result in a false negative exam.

MRI provides superior soft tissue contrast and a more comprehensive evaluation of the hernia and its surrounding structures.

Muscle hernias are identified as an abnormal bulge in the muscle's contour or as protrusion of the muscle into the subcutaneous fat. The herniated muscle typically maintains normal T1 and T2 signals and architecture; however, increased T2 signal, suggestive of edema, may be observed and is often associated with muscle tearing, impingement, or altered perfusion. While a fascial defect may not always be directly visualized, it can often be inferred from the abnormal contour of the muscle [7].

The fascial defect is better detected on axial images, whereas coronal and/or sagittal images provide better characterization of the longitudinal location and extent.

Diagnosing and the early management of muscle hernia, allows reducing the risk of complications, including muscle ischemia, acute or chronic compartment syndrome, and nerve entrapments by the protruding muscle, causing peripheral neuropathy [8].

There are no standardized guidelines for the management of muscle hernia [9]. Conservative treatment is the first-line approach for symptomatic patients. It includes rest, activity restriction and elastic compression stocking. Failure of conservative management may indicate surgery: A variety of techniques have been described with no consensus. In constitutional muscle hernia, longitudinal fasciotomy by enlarging the fascial defect should be considered the surgical choice to prevent complications. In post-traumatic muscle hernia, a small fascial defect can be treated with the direct anatomical repair, consisting in suturing the edges of the fascial defect. While fascial reconstruction using mesh repair and autologous graft from fascia lata can be regarded as the most suitable techniques to avoid severe complications such as compartment syndrome [9].

Conclusion

Muscle hernias are a rare but important entity to evoke in patients presenting with localized swelling and discomfort appearing during exercise, especially in the lower extremities. Imaging, particularly dynamic ultrasound and MRI, plays a crucial role in confirming the diagnosis by identifying fascial defects and muscle protrusion. Early recognition and appropriate management, whether conservative or surgical, can significantly improve patient outcomes and quality of life.

Patient consent

The author certifies that all appropriate consent forms were obtained from the patient for the publication of the case report. The consent covers images and other informations being reported.