WASHINGTON — Wake up to find your shoulder killing you but don’t recall an injury? It could be the start of frozen shoulder, a curse of middle-aged women and one of the most puzzling joint conditions.

The shoulder’s normally smooth lining becomes so inflamed it resembles cherry gelatin. That leads to scar tissue, making the shoulder too stiff to move.

Known medically as adhesive capsulitis, it’s a fairly common ailment, estimated to strike between 2 percent and 3 percent of the population, the vast majority women ages 40 to 60. Yet too few sufferers get diagnosed in time for a simple shot that could cut a year or more off recovery time.

In fact, doctors can easily confuse early symptoms with a rotator cuff injury, and the wrong physical therapy can make the condition worse by further irritating it.

So Dr. Jo Hannafin of New York’s Hospital for Special Surgery is excited when patients show up after only two to three months of shoulder pain. She injects cortisone deep into the joint and 15 minutes later lifts and twists it.

“If the range of motion is now full, I’ve hit a home run,” says Hannafin, a leading expert on the condition. “I’ve caught a patient in the first stage.”

That early treatment means they’ll be healed in about a month. But usually patients show up months later, and recovery can take two years or more.

Nobody knows just what triggers frozen shoulder. Diabetics are at higher risk; up to 20 percent get it. Having an underactive thyroid also is a risk factor.

Add the fact that 70 percent of patients are middle-aged women, and specialists say hormones clearly play some role but they don’t know what.

Key signs are pain at night and at rest, along with gradually increasing stiffness. Movement problems typically begin with reaching back and up, like into a back pocket or to unfasten a bra. An exam should include the doctor attempting to lift and rotate the arm; problems with this so-called passive movement are another tip-off. Surrounding the ball of the shoulder is a thin stretchy sac, or capsule. Inflammation in that lining is the start of frozen shoulder, and it causes immense pain.

When the pain starts to wane, that’s bad news. It means the capsule is thickening with excess collagen, a sort of scar tissue, that further stiffens the shoulder. Most late-stage patients find themselves undergoing painful physical therapy or even surgery to break apart the collagen.

Only in recent years have studies proved that a cortisone injection in the earliest stages can prevent collagen buildup and spur dramatically faster recovery.