What Is the Costomediastinal Recess?

The costomediastinal recess is a narrow, slit-like potential space within the pleural cavity, tucked behind the sternum where the costal pleura (lining the inner surface of the ribs) reflects to become the mediastinal pleura (draping over the structures in the center of the chest). In a healthy person breathing normally, the lungs do not fully expand into this space, so it remains collapsed, its two pleural layers pressed together with only a thin film of fluid between them. The recess becomes clinically interesting when something fills it: air, fluid, or tissue that has no business being there. On the left side of the chest, where the recess sits directly in front of the heart, that distinction carries real consequences for diagnosis and for procedures like pericardiocentesis.

Where It Sits and How It Differs Left to Right

The pleural cavity is not a simple sac wrapped around each lung. It has several recesses, pockets where the pleural lining folds back on itself and the lung does not reach during quiet breathing. The costomediastinal recess runs vertically behind the sternum on each side, but the left and right versions are not mirror images. On the right, the anterior edge of the lung closely follows the midline, so the recess is quite narrow. On the left, the heart bulges into the chest cavity, pushing the lung’s anterior border laterally and creating a wider gap known as the cardiac notch. That notch means the left costomediastinal recess is substantially broader, especially at its lower end.

Cadaver dissections have confirmed this asymmetry in measurable terms. In two fresh cadavers examined in a study on the left recess, the space widened as it descended, measuring 75 mm and 55 mm across at the level of the sixth intercostal space.

1Journal of Computer Assisted Tomography. Left Costomediastinal Recess of the Pleura, the Potential Pleural Space Anterior to the Heart

That width places a layer of pleural space directly between the chest wall and the pericardium, a detail that matters for anyone inserting a needle in that region.

Why the Lung Normally Does Not Fill the Recess

During a relaxed breath in, the expanding lung fills most of the pleural cavity but stops short of the recesses. The costomediastinal recess and the larger costodiaphragmatic recess (at the lung base, above the diaphragm) serve as reserve space. During a deep inspiration, the lung edges can slide further into these pockets, partially occupying them, but they rarely fill completely. The recesses exist because the pleural cavity is slightly larger than the lung it contains, and because the rigid sternum and the beating heart set physical limits on how far the lung’s thin anterior edge can extend.

Pressure measurements across the pleural space show that the costomediastinal region behaves differently from the rest of the cavity during breathing. During inspiration, the pressure difference between the costal and mediastinal sides of the pleural space increases, meaning fluid within the space experiences a stronger pull toward the mediastinum as you breathe in.

2PubMed. Pleural liquid pressure gradients and intrapleural distribution of injected bolus

That pressure gradient helps explain why small amounts of free fluid tend to migrate toward the mediastinal surfaces rather than pooling uniformly throughout the cavity.

Fluid Dynamics in the Pleural Space

The thin film of fluid that normally separates the two pleural layers is constantly being produced and reabsorbed. Keeping this balance intact is critical: too much fluid (a pleural effusion) compresses the lung, and too little increases friction during breathing. The costomediastinal recess, like the rest of the pleural cavity, is governed by this balance, but the pressures that drive fluid movement are not uniform across the space.

Comparative studies across mammals of different sizes have found that pleural liquid pressure at a given chest height is lower on the mediastinal side than on the costal side, and this difference becomes more pronounced in larger animals. The result is a horizontal pressure gradient running from the rib surface toward the midline structures.

3PubMed. Comparative features of Starling-lymphatic interaction at the pleural level in mammals

In practical terms, this gradient gently pushes pleural fluid inward, toward the mediastinal pleura where much of the reabsorption takes place. The costomediastinal recess, sitting at the junction of these two pleural surfaces, is a transition zone where these pressure dynamics converge.

How the Pleura Clears Fluid and Particles

Fluid does not leave the pleural space through a single route. Some is reabsorbed directly through the mesothelial cells lining the pleura, driven by the balance of pressures across the pleural membrane. But the pleura also has a dedicated drainage system: lymphatic stomata, tiny openings between mesothelial cells on the costal (rib-side) and diaphragmatic surfaces that connect to the lymphatic network beneath.

The drainage apparatus on the costal pleura consists of these stomata, the initial lymphatic vessels they feed into, and a layer of connective tissue riddled with small openings. Together these form the main route for clearing fluid, particles, and cells from the pleural space.

4PubMed. Ultrastructural study of pleural lymphatic drainage unit and effect of nitric oxide on the drainage capacity of pleural lymphatic stomata in the rat

However, the contribution of stomatal drainage varies with how much fluid is present. Under normal conditions, when pleural fluid volume is small, lymphatic drainage through the stomata accounts for roughly 39% of protein removal from the space. When fluid volume rises, as in a developing effusion, stomatal drainage ramps up dramatically and handles about 64% of the clearance.

5PubMed. Contribution of lymphatic drainage through stomata to albumin removal from pleural space

This means the lymphatic stomata act as a kind of surge-protection system. They are not the dominant drainage pathway in a healthy chest, but they become the primary one when fluid begins to accumulate. When pathology overwhelms even this escalated capacity, fluid builds up in the pleural recesses first, because those are the lowest-pressure regions where the lung does not push back.

What the Recess Looks Like on Imaging

One of the more surprising findings about the left costomediastinal recess is that, in healthy people, you typically cannot see it at all on a CT scan. The two pleural surfaces are pressed together so tightly that no distinct space registers. A study examining CT scans from 68 individuals with healthy or near-healthy chests found the recess was invisible in every single case.

6PubMed. Left costomediastinal recess of the pleura, the potential pleural space anterior to the heart

The recess only becomes visible when something pathological separates those surfaces.

When fluid does collect in the left costomediastinal recess, its position directly in front of the heart creates a distinctive imaging appearance. Fluid there will obscure the ventral (front) border of the cardiac silhouette on a chest radiograph, a sign that can mimic pericardial effusion or other anterior mediastinal pathology if the anatomy is not kept in mind.

7BSAVA Library. The pleural space

Similarly, air in this recess (as in pneumothorax) can produce a radiolucent strip anterior to the heart that might be confused with pneumomediastinum. Recognizing that the costomediastinal recess is a pleural space, not a mediastinal one, is key to interpreting these images correctly.

The same CT study that found no visible recess in healthy scans identified 20 cases of recess lesions in patients with pleural disease. In 19 of those cases, the abnormality in the recess was continuous with pleural disease elsewhere around the left lung, confirming that fluid or tissue was tracking into the recess from adjacent affected pleural space. Only one lesion appeared isolated within the recess without an obvious connection to surrounding disease.

8Journal of Computer Assisted Tomography. Left Costomediastinal Recess of the Pleura, the Potential Pleural Space Anterior to the Heart

Why It Matters for Pericardiocentesis

Pericardiocentesis, the procedure of draining fluid from the sac around the heart, is commonly performed through a left paracardiac approach: a needle is inserted near the lower left sternal border or just below the xiphoid process and aimed toward the pericardial space. The left costomediastinal recess sits directly in the path of that needle. In a healthy chest, the recess is collapsed and functionally absent, so the needle passes through pleural layers without consequence. But when the recess contains fluid or is distended by disease, the needle may enter or traverse it before reaching the pericardium.

The clinical significance of this was documented in the same imaging study: among seven cases of successful left paracardiac pericardiocentesis, three were complicated by an ipsilateral pleural effusion.

9Journal of Computer Assisted Tomography. Left Costomediastinal Recess of the Pleura, the Potential Pleural Space Anterior to the Heart

The implication is that the needle crossed the pleural space on its way to the pericardium, creating a communication that allowed fluid to leak into the pleural cavity. This is not a common complication, but it is an anatomically predictable one that clinicians performing the procedure should be aware of, particularly when imaging before the procedure suggests the recess may be distended.

Common Misconceptions About Pleural Recesses

Because the costomediastinal recess and the costodiaphragmatic recess are both “potential spaces,” they are sometimes treated interchangeably in casual descriptions. In practice, the two are quite different in clinical behavior. The costodiaphragmatic recess, located between the diaphragm and the lower rib cage, is the lowest point of the pleural cavity when a person is upright and is where free-flowing effusions collect first. It is large, easy to access with a needle, and the standard target for thoracentesis. The costomediastinal recess, by contrast, is anterior and narrow, fills only when effusion is more extensive, and is not a routine target for drainage.

Another misconception involves the relationship between the costomediastinal recess and the cardiac notch of the left lung. The cardiac notch is the indentation in the lung’s anterior border caused by the heart’s position. The costomediastinal recess is the pleural space that exists because of that indentation. They are related but not identical: the notch is a feature of the lung, while the recess is a feature of the pleural cavity. You can have a cardiac notch without any pathologically visible recess (as all 68 healthy CT scans showed), and the degree to which the recess widens depends on factors beyond the lung’s shape, including pleural fluid volume and mediastinal shift.

The Recess in Veterinary Medicine

The costomediastinal recess is not unique to humans. It is a recognized anatomical feature in other mammals and carries diagnostic importance in veterinary radiology, particularly in dogs and cats. In small animals, the ventral (front) portion of the chest is where the costomediastinal recess lies, and it is one of the first places gas accumulates in pneumothorax because air rises to the highest point when the animal is lying on its side for imaging.

Veterinary radiologists watch for gas opacity without lung markings in the costomediastinal recess ventral to the heart as a sign of pneumothorax. When fluid rather than air fills the space, it obscures the ventral cardiac silhouette, mirroring the diagnostic challenge seen in human imaging.

10BSAVA Library. The pleural space

The parallel across species reinforces that the recess is not an incidental anatomical quirk but a consistent feature of mammalian thoracic anatomy, shaped by the same mechanical relationship between the lung, the heart, and the chest wall.

Emerging Imaging Approaches

Traditional assessment of the pleura, including its recesses, has relied on chest radiography and CT. More recently, endoscopic ultrasound has been explored as a way to visualize pleural anatomy from inside the esophagus, which sits in the mediastinum adjacent to the pleural surfaces. This technique can image different aspects of the pleura in both normal anatomy and in the presence of effusion, and has been used to guide sampling of pleural nodules and fluid collections.

11PubMed Central. Endoscopic ultrasound-guided evaluation of the pleura and cases of pleural effusion

While endoscopic ultrasound is not yet a routine tool for evaluating the costomediastinal recess specifically, its ability to reach mediastinal structures without external puncture makes it a potentially useful complement for cases where the anterior pleural space is involved but hard to access by conventional means. For now, CT remains the workhorse for identifying recess abnormalities, and knowing that the recess should be invisible on a healthy scan is one of the most useful diagnostic reference points a radiologist can carry.