Acute chest syndrome (ACS) is a serious lung complication of sickle cell disease. It occurs when sickle-shaped red blood cells block blood flow in the lungs, causing inflammation, pain, and difficulty breathing. It is the leading cause of death among people with sickle cell disease, with mortality rates of 4.3% in adults and 1.1% in children. Understanding the warning signs and how it’s managed can make a significant difference in outcomes.
How It Happens in the Lungs
In sickle cell disease, red blood cells become rigid and crescent-shaped instead of round and flexible. These misshapen cells can clump together and get stuck in the tiny blood vessels of the lungs. When that happens, oxygen can’t reach the surrounding lung tissue, triggering inflammation and tissue damage. The result is something that looks and feels a lot like pneumonia, but the underlying cause is the blood vessel blockage rather than infection alone.
Several things can set off an episode. Lung infections are a common trigger, particularly in children. Fat from bone marrow can enter the bloodstream (often after a pain crisis damages bone tissue) and lodge in the lungs. Sometimes a pain crisis itself leads to shallow breathing, which causes parts of the lung to collapse and sets the stage for ACS. In many cases, no single cause is identified.
Symptoms to Recognize
ACS typically starts with fever, chest pain, and cough. You may also notice shortness of breath or a feeling that it’s harder to take a deep breath. In some cases, especially in children, fever is the first and most prominent symptom, with chest pain developing later. Adults tend to present with more severe respiratory symptoms from the start.
One important pattern: ACS often develops while a person is already hospitalized for a sickle cell pain crisis. The shift from pain crisis to ACS can happen over hours, which is why hospital teams monitor breathing closely during admissions for vaso-occlusive episodes. New or worsening shortness of breath, a drop in oxygen levels, or a new fever during a pain crisis should raise immediate concern.
Why Adults Face Higher Risk
Adults with sickle cell disease experience more severe ACS episodes than children. The mortality rate in adults is roughly four times higher than in children (4.3% versus 1.1%). Adults are more likely to develop widespread lung involvement and to need intensive care. They also tend to have more rapid progression of symptoms once an episode begins.
The Role of Asthma
Having asthma alongside sickle cell disease substantially increases the risk of ACS. Research published in the journal Blood found that children with both conditions had nearly twice as many ACS episodes per year compared to those without asthma (0.39 episodes per patient year versus 0.20). They also experienced their first episode earlier, at a median age of 2.4 years compared to 4.6 years for children without asthma. On top of that, these children had roughly three times as many painful episodes overall. Keeping asthma well controlled is one of the most practical ways to reduce the frequency of ACS in children who have both conditions.
How ACS Is Treated
Treatment focuses on three things: fighting any underlying infection, improving oxygen delivery, and preventing the episode from getting worse.
Because lung infections often trigger or accompany ACS, antibiotics are started right away. The standard approach covers both typical and atypical bacteria. Supplemental oxygen is given to keep blood oxygen levels in a safe range, and pain management is carefully balanced. Too little pain relief leads to shallow breathing that worsens lung collapse, but overly sedating medications can suppress breathing as well.
Blood transfusions play a central role. A simple transfusion adds healthy, flexible red blood cells to the bloodstream, improving oxygen delivery. This is considered when hemoglobin drops more than 1 g/dL below a person’s baseline. If the episode progresses rapidly, with oxygen levels falling below 90% despite supplemental oxygen, worsening lung involvement on chest X-ray, or continued hemoglobin decline, an exchange transfusion becomes urgent. In an exchange transfusion, a large portion of the sickle cells are removed and replaced with donor red blood cells, which can reverse the crisis more quickly.
Prevention During Hospital Stays
Since ACS frequently develops during hospitalizations for pain crises, prevention strategies focus on keeping the lungs fully expanded. Incentive spirometry, a simple breathing exercise using a handheld device, is one of the most effective tools. The protocol studied in a randomized trial involved taking 10 deep breaths through the device every two hours during waking hours (8 a.m. to 10 p.m.) until at least 24 hours after pain subsided.
This is straightforward but easy to neglect when someone is in significant pain. If you or a family member is hospitalized for a sickle cell pain crisis, using the spirometer consistently is one of the most important things you can do to prevent the situation from escalating. Getting out of bed and walking when possible also helps keep the lungs clear.
Long-Term Prevention
Hydroxyurea is the cornerstone medication for reducing ACS episodes over time. It works by increasing the production of fetal hemoglobin, a form of hemoglobin that prevents red blood cells from sickling as easily. People on hydroxyurea generally experience fewer pain crises, fewer ACS episodes, and fewer hospitalizations overall. For those who have had multiple ACS episodes, a chronic transfusion program (regular transfusions every few weeks) may also be recommended to keep the percentage of sickle cells low.
Staying up to date on vaccinations is also important, since respiratory infections are a common trigger. Pneumococcal vaccines, annual flu shots, and COVID-19 vaccines all reduce the chance of a lung infection that could spiral into ACS.
What Repeated Episodes Mean
Each episode of ACS causes some degree of lung damage. Over time, repeated episodes can lead to chronic lung disease, with scarring that reduces lung capacity and makes everyday activities more tiring. Pulmonary function tests may show a restrictive pattern, meaning the lungs can’t expand as fully as they should. This cumulative damage is one of the main reasons aggressive prevention with hydroxyurea or chronic transfusions is recommended after even one or two significant episodes. Protecting the lungs from repeated injury preserves quality of life over the long term.