CSU stands for chronic spontaneous urticaria, a condition where hives (raised, itchy welts on the skin) keep appearing for six weeks or longer with no identifiable trigger. Unlike a one-time allergic reaction to food or medication, CSU hives come and go unpredictably, sometimes for months or years. About half of people with the condition see their symptoms resolve within five years, but many need ongoing treatment during that time.
How CSU Differs From Regular Hives
Everyone gets hives at some point. A bee sting, a new medication, a food allergy: the cause is usually obvious, and the hives clear up within days. That’s acute urticaria. CSU is diagnosed when recurring hives persist for six weeks or more without an identifiable external cause. The “spontaneous” part of the name is key. It separates CSU from chronic inducible urticaria, where hives are triggered by something specific like cold temperatures, pressure on the skin, or sweating.
Inducible hives tend to be shorter-lived per episode and appear in a predictable pattern tied to the trigger. CSU welts last longer, show up without warning, and are associated with more systemic inflammation throughout the body. Some people have both types at the same time, which can make sorting out what’s happening more complicated.
What CSU Looks and Feels Like
CSU produces raised, red or skin-colored welts that are intensely itchy. Individual welts typically fade within 24 hours, but new ones keep forming, so it can feel like a nonstop cycle. The welts can appear anywhere on the body and vary in size from small dots to large patches that merge together.
About half of people with CSU experience hives alone. Roughly 40% also develop angioedema, which is deeper swelling beneath the skin, most often around the eyes, lips, hands, or feet. Around 10% deal mainly with angioedema and relatively few surface hives. Angioedema can be more alarming because the swelling is pronounced, but it follows the same underlying process as the hives themselves.
What’s Happening Under the Skin
CSU is driven by mast cells, immune cells embedded in the skin that act like tiny alarm systems. In a healthy immune response, mast cells release histamine and other inflammatory chemicals when they detect a genuine threat. In CSU, those cells fire off without a real trigger, flooding nearby tissue with histamine. This makes blood vessels leak fluid into the surrounding skin, producing the characteristic raised, itchy welts.
Researchers believe up to 50% of CSU cases may have an autoimmune component. In these patients, the immune system produces antibodies that mistakenly target mast cells or the receptors on their surface, essentially tricking them into activating. Two main types of antibodies have been identified: one that directly stimulates mast cells and another that binds to the antibody already sitting on the mast cell surface, causing it to release histamine. The other half of cases may involve internal signaling errors within the mast cells themselves, where the molecular “off switches” that normally keep mast cells quiet stop working properly.
How Doctors Diagnose CSU
There’s no single blood test that confirms CSU. Diagnosis is based on the pattern: recurring hives or angioedema lasting six weeks or longer, with no clear external cause and no underlying systemic disease to explain them. Doctors will typically run blood work to rule out thyroid disorders, infections, and other conditions that can mimic or accompany chronic hives.
To track how severe the condition is and whether treatment is working, doctors often use a tool called the Urticaria Activity Score (UAS7). You rate two things each day: itch intensity (0 for none, up to 3 for severe) and hive count (0 for none, up to 3 for more than 12 hives). Those daily scores are added up over a week, producing a number from 0 to 42. Higher scores mean more severe disease. This scoring system gives both you and your doctor an objective way to measure progress rather than relying on memory alone.
First-Line Treatment: Antihistamines
Standard over-the-counter antihistamines like cetirizine, loratadine, or fexofenadine are the starting point. These block histamine from reaching receptors in the skin, reducing itch and welt formation. The typical starting dose is one tablet daily, the same dose you’d take for seasonal allergies.
Here’s where CSU treatment diverges from allergy treatment: if the standard dose doesn’t control symptoms, guidelines recommend increasing to up to four times the normal daily dose. This is a well-established approach, not an off-label experiment. In studies, about 78% of CSU patients needed this higher dose. Of those, roughly 23% achieved adequate relief. That means antihistamines, even at higher doses, leave a significant number of people still struggling. Older, sedating antihistamines like diphenhydramine are generally avoided for daily CSU management because the drowsiness becomes unsustainable over weeks and months.
When Antihistamines Aren’t Enough
For people who don’t respond to high-dose antihistamines, the next step is a biologic medication called omalizumab, given as an injection under the skin every four weeks. It works by binding to the antibody (IgE) that sits on the surface of mast cells, reducing their tendency to activate. The dose for CSU is standardized and doesn’t depend on body weight or blood levels, which simplifies treatment compared to its use in asthma.
In clinical trials, 36% of patients receiving the higher dose were completely free of both itch and hives after 12 weeks, compared to just 9% on placebo. Those who didn’t become completely symptom-free still often saw meaningful improvement. Itch severity scores dropped nearly three times more than with placebo, and hive counts followed a similar pattern. Injections are typically given in a doctor’s office, at least initially.
For the smaller group that doesn’t respond to either approach, an immunosuppressant called cyclosporine is sometimes used. It dampens the broader immune response that drives mast cell activation. It works, but side effects are dose-dependent: more than half of patients on moderate doses experience at least one adverse effect, so it requires regular monitoring and is generally reserved for more severe, treatment-resistant cases.
Living With CSU
CSU is not an allergy in the traditional sense, and the fruitless search for a trigger can be one of the most frustrating parts of the experience. Many people cycle through elimination diets, allergy panels, and environmental changes before learning that “spontaneous” genuinely means no external cause will be found. That realization, while initially discouraging, can actually reduce the anxiety of constantly trying to identify and avoid a trigger that doesn’t exist.
The condition follows an unpredictable course. Some people have continuous daily symptoms. Others experience flares that last weeks, separated by quiet stretches. Stress, heat, tight clothing, and alcohol can worsen flares in some individuals without being the actual cause. About half of patients reach spontaneous remission within five years, meaning the condition simply stops on its own. For others, it persists longer and requires sustained treatment to manage. The good news is that CSU doesn’t damage the skin permanently, and the welts leave no scarring once they fade.