A collapsed lung, or pneumothorax, shows up as a visible gap between the lung and the chest wall where air has become trapped. On a chest X-ray, this appears as a dark, empty space with no blood vessel markings, bordered by a thin white line where the edge of the lung has pulled away from the ribs. In person, a collapsed lung can sometimes be spotted by uneven chest movement during breathing, though many of the signs are subtle enough that imaging is needed to confirm what’s happening.
What Happens Inside the Chest
Your lungs stay inflated because of a slight vacuum in the space between them and the chest wall, called the pleural space. When air leaks into that space, either from a hole in the lung or a puncture through the chest wall, it disrupts that vacuum. The trapped air presses against the lung tissue, causing it to shrink inward like a deflating balloon. A small leak might only compress one section of the lung. A large one can flatten the entire lung against the center of the chest.
In a tension pneumothorax, the most dangerous form, air enters the pleural space but can’t escape. This one-way valve effect causes pressure to build continuously, eventually pushing the heart and major blood vessels toward the opposite side of the chest.
What It Looks Like on a Chest X-Ray
The hallmark of a collapsed lung on X-ray is a thin white line running parallel to the chest wall, called the pleural line. This line marks the actual edge of the deflated lung. Beyond that line, toward the ribs, the image looks uniformly dark because the space contains only air, with no blood vessels or lung tissue visible. On a normal X-ray, you’d see a web of vessel markings extending all the way to the chest wall.
When you’re standing upright for the X-ray, trapped air rises to the top of the chest, so the gap between lung and chest wall is most obvious near the apex, the uppermost point of the lung. If you’re lying flat, as trauma patients often are, the air shifts forward and downward. That makes the collapse harder to spot because the gap hides behind the heart or along the lower edge of the lung rather than appearing at the top.
Radiologists have to be careful not to confuse the pleural line with artifacts like skin folds, clothing creases, or medical tubing, all of which can mimic the appearance of a lung edge on film. The key difference is that a true pleural line follows the curve of the chest wall and no vessel markings exist beyond it, while an artifact often extends past the boundaries of the rib cage or has faint vessels visible on both sides.
What It Looks Like on CT and Ultrasound
A CT scan shows a collapsed lung in cross-section and is far more sensitive than a standard X-ray. The trapped air appears as a black crescent or pocket sitting between the bright white chest wall and the denser gray lung tissue. CT can detect even tiny pockets of air that a chest X-ray would miss entirely, making it the gold standard when a collapse is suspected but not visible on plain film.
Bedside ultrasound has become a rapid alternative, especially in emergency rooms. On a normal lung, ultrasound shows a shimmering, sliding motion where the lung surface moves against the chest wall with each breath. When a pneumothorax is present, that sliding disappears. In a special recording mode called M-mode, a normal lung produces a grainy, sand-on-a-beach pattern (the “seashore sign”), while a collapsed lung produces a series of flat horizontal lines resembling a barcode. The exact point where normal sliding meets absent sliding, called the “lung point,” marks the border of the collapse.
Ultrasound picks up a collapsed lung with about 88% sensitivity, compared to just 52% for a standard chest X-ray. Both methods are nearly perfect at ruling one out when it isn’t there, but ultrasound catches significantly more cases that X-rays miss, particularly small collapses in patients lying flat.
Physical Signs You Can See and Hear
A collapsed lung doesn’t always produce dramatic visible changes on the outside of the body, but several physical signs can point to one. The affected side of the chest may not rise as much during breathing, creating a subtle asymmetry that’s easier to notice when looking along the chest from the foot of the bed than from straight on. In severe cases, the neck veins may bulge visibly due to increased pressure in the chest compressing the veins that return blood to the heart.
With a stethoscope, the affected side sounds quiet or completely silent compared to the other side, where normal breath sounds are present. Tapping on the chest over a pneumothorax produces a hollow, drum-like tone instead of the usual dull thud, though this finding is unreliable and can be absent even in large collapses. Tracheal deviation, where the windpipe visibly shifts away from the affected side, is often described as a classic sign of tension pneumothorax, but it’s actually a late finding that indicates the situation has already become critical.
Pneumothorax vs. Other Types of Collapse
Not every collapsed lung is a pneumothorax. Atelectasis, a collapse caused by a blocked airway rather than trapped air, looks quite different on imaging. Where a pneumothorax creates a dark, air-filled gap, atelectasis produces a white, dense area because the lung tissue has compressed without air replacing it. The two conditions can even occur simultaneously, which creates a confusing overlapping pattern on X-ray where the darkness of trapped air and the whiteness of compressed tissue partially cancel each other out, sometimes appearing as a vague haze that’s easy to misread.
On CT, the distinction is clearer. A pneumothorax shows a black air pocket separate from the lung, while atelectasis shows the lung itself becoming smaller and denser without any surrounding air collection. The structures around the collapse also behave differently: a pneumothorax pushes the heart and windpipe away from the affected side, while atelectasis pulls them toward it.
What Recovery Looks Like on Imaging
As a collapsed lung re-expands, whether on its own or after a chest tube drains the trapped air, follow-up X-rays show the pleural line gradually disappearing as the lung fills back out toward the chest wall. The vessel markings return to the areas that were previously dark and empty.
One complication to watch for during re-expansion is a pattern called re-expansion pulmonary edema, where the newly inflated lung fills with fluid. On imaging, this appears as a hazy, white cloud spreading through one side of the chest, typically within hours of the lung reinflating. It shows up in about 64% of cases within the first hour and in the rest within 24 hours. The cloudiness can worsen over one to two days and usually takes four to five days to clear. In many cases, the X-ray changes are the only sign, with no noticeable symptoms, though some patients experience significant breathing difficulty.