Learning Objectives: After reading this article, the physician should be able to:
- Discuss the physical findings associated with Baker's cyst.
- Describe the conditions that mimic Baker's cyst.
- Outline the diagnosis and treatment for this condition.
Pain in the posterior knee or a swollen and painful calf are common presentations to the emergency department. The emergency physician's differential diagnosis in these cases is usually thrombophlebitis or some sort of muscular sprain or strain.
A number of other problems, however, are fair game for diagnosis in the ED by the sagacious clinician, and can cause symptoms that mimic deep vein thrombosis (DVT) or simple soft tissue injuries. One of these conditions is a Baker's cyst, also known as a popliteal cyst. Clinically and historically, one cannot differentiate this condition from the more serious DVT.
A ruptured Baker's cyst cannot be unequivocally differentiated from DVT on clinical grounds
The patient often thinks that pain in the calf has to be a blood clot that is going to kill him, and he is unhappy unless he has an alternative explanation or a firm diagnosis to the contrary. Popliteal cysts can be diagnosed in the ED if they are considered. Because the physical exam is not confirmatory enough to clinch the diagnosis, DVT has to be ruled out by some sort of additional investigation. Usually, when ultrasound is used to look for DVT, the cyst is apparent, especially if you're sharp enough to put “R/O Baker's cyst” on the radiology request form.
A Man with a Swollen Calf and Discoloration of the Foot Berkum Y, et al Postgrad Med J 2002;78:300
This is a section of the self-assessment questions of the Postgraduate Medical Journal that deals with a Baker's cyst. It is well done and rather complete. I suggest you read it. This is a question/answer exercise that describes a 48-year old man who presented to the ED with painful swelling of the left calf. The condition had developed gradually over the previous week. There was no cough, dyspnea, or chest pain. There was no past history of thromboembolic events, local trauma, or other risk factors for DVT.
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Figure: A ruptured Baker's cyst, also called pseudothrombophlebitis, cannot be unequivocally differentiated from DVT on clinical grounds. A few findings can suggest the presence of a cyst rather than venous thrombosis. A 68-year-old woman, left, had rheumatoid arthritis, a predisposing condition for the development of a Baker's cyst. She noted pain and swelling in the calf over a few weeks, associated with waxing and waning swelling of the knee. Note the swollen knee and swollen calf. This indicates a communication between the cyst and the knee joint. An arthrogram was the old way to diagnose this condition, but ultrasound is currently the initial investigation of choice. Because a ruptured Baker's cyst is an inflammatory condition, a red, warm, swollen, and tender calf guides one's diagnosis away from DVT and suggests an inflammatory etiology for the symptoms, right. Note that after rupture, any fullness or swelling in the popliteal fossa has disappeared. Another tip-off, not shown, is hemorrhage under the ankle malleoli from bleeding occasionally associated with a ruptured cyst.
Physical examination revealed an afebrile patient without respiratory complaints. The lung exam was normal. The left knee was mildly swollen, but exhibited full range of motion. There was no clear joint effusion. There was no palpable mass. The calf was tender, swollen, and erythematous. There was a positive Homan's sign and pitting edema of the foot. A purple discoloration of the ankle was noted, just below the malleoli. A venous duplex ultrasound examination in the ED excluded DVT.
The authors state that the initial differential diagnosis in this scenario includes DVT, a popliteal cyst, popliteal varices, a popliteal aneurysm, ganglia, neural tumors, sarcoma, and haemangioma. Also included, especially if there is associated tenderness, is a tear of the medial head of the gastrocnemius or plantaris muscle, cellulitis, fasciitis, compartment syndrome, and simple venous insufficiency. It is emphasized that the most common and most clinically important diagnosis that cannot be missed in the ED is DVT. If this were a ruptured Baker's cyst, anticoagulation could cause significant bleeding and even a posterior compartment syndrome.
The physical examination simply cannot differentiate DVT from a ruptured Baker's cyst. Although an uncommon finding, a crescent-shaped hemorrhagic sign of the malleolar region of the foot has been described. This would not be associated with DVT, but this hemorrhagic sign is often absent.
The authors believe that evaluation of the posterior calf and knee is best accomplished by a venous duplex ultrasound scan. They point out that once a Baker's cyst ruptures, it may be difficult to diagnose or find a definite fluid-filled mass with ultrasound because the characteristic fluid-filled mass has dissipated. An MRI or CT can be diagnostic if the ultrasound is negative. The prior gold standard for diagnosing a ruptured cyst was an arthrogram. With this simple study that can be done in the ED with standard contrast material and a plain x-ray, dye is injected into the knee joint. The radiographic dye outlines the cyst and extravasate into the surrounding popliteal and posterior calf tissue if the cyst has ruptured. This test works because the cyst communicates with the knee joint.
There is no immediate specific treatment of a Baker's cyst except symptomatic care that would include elevation, local heat, and analgesics. Some authors suggest an intra-articular cortisone injection or aspiration of the cyst if it has not already ruptured. The symptoms should resolve spontaneously, but it may take a few weeks`.
A popliteal cyst is often caused by a tear of the knee joint capsule that allows communication between the joint space and the gastrocnemius or semimembranous bursa. The cyst was first described by a British surgeon, Dr. William Baker. Usually these cysts are seen in older individuals and are accompanied by degenerative disease, such as rheumatoid arthritis. It would be uncommon in a young individual.
Inflammation results in increased synovial fluid production, filling the cyst. Rupture can be spontaneous. Because this is an inflammatory and degenerative process, knee extension during exercise or simply stepping on an elevated platform may cause rupture. Sometimes a history of sudden onset of posterior knee pain can clue the physician into this diagnosis, but a gradual onset of symptoms is just as common. The synovial fluid can be quite inflammatory, leading to inflammation of the entire surrounding calf. A Baker's cyst can be painful, and the ensuing inflammation may produce a red, hot, swollen, and tender indurated area. One giveaway to the diagnosis is a concomitant joint effusion in the knee.
Comment: Although a Baker's cyst is not a serious problem, it should be diagnosed in the ED. The astute clinician should know that an ultrasound examination of the popliteal area may miss an isolated calf DVT, but will identify DVT of the popliteal area if present. If a clot is present, it should be picked by the ultrasound. While assessing the popliteal area for a DVT, the examination often identifies a Baker's cyst, especially if you are smart enough to alert the tech that you also are looking for the cyst as an alternative diagnosis to DVT. The Baker's cysts I have seen were usually found when I was sure the patient had DVT, complete with a swollen foot and a positive Homan's sign.
A ruptured cyst will leave inflammatory fluid in the calf, and occasionally fluid can be seen between the muscle layers. Because the patient is walking around and the rupture may be hemorrhagic, blood will migrate all the way to the foot and produce the characteristic crescent sign around the ankle. Ruptured Baker's cysts are not, however, always hemorrhagic. I have always been impressed by the fact that although this is a cystic structure, you can rarely palpate a distinct mass in the posterior knee. Pain should be isolated to the posterior knee if a cyst has not ruptured, but once it ruptures, the entire calf is tender, hence a positive Homan's sign. Providing anticoagulation for ruptured cyst administered before a diagnosis is known could cause significant bleeding, even enough to cause a posterior compartment syndrome. I have never seen this syndrome, but it has been described (see following article).
Ruptured Baker's Cyst Causing Posterior Compartment Syndrome Dunlop D, et al Injury 1997 28(8):561
This is a single case report demonstrating an unusual complication of a ruptured Baker's cyst that was misdiagnosed and mistreated. The authors present the case of a 58-year-old man who had two days of left calf pain and swelling. The patient had noted some swelling behind the knee for a number of weeks previously, but the swelling disappeared as the calf became swollen. There was no history of trauma. The calf circumference was 7 centimeters greater than the uninvolved leg. A DVT was strongly suspected, and the patient was admitted and anticoagulated with heparin.
Figure: Despite being a fluid-filled bursa, a Baker's cyst cannot always be palpated as a distinct mass. This patient has tenderness in the popliteal fossa and the suggestion of a mass.
Figure: A Baker's cyst is classically an enlargement of the semimembranous bursa that lies between the semimembranous muscle and the head of the gastrocnemius. The enlarged bursa may be palpated as a soft mass in the midline or medial to the midline, depending on how it bulges. Often a distinct structure cannot be palpated, especially if it ruptures. This drawing shows how a ruptured Baker's cysts can mimic calf venous thrombosis. Other bursae also occur around the posterior knee, such as the gastrocnemius bursa, further confusing the specific anatomical diagnosis. A Baker's cyst can also be the result of a synovial hernia off of a tendon sheath.
A venogram, not performed until the next day, was negative. Although the heparin was then stopped, the calf pain and swelling increased. Pressures in the superficial posterior compartment of the calf were measured and were significantly elevated at 66 mm Hg. A four-compartment fasciotomy was performed, reveling an edematous and necrotic gastrocnemius muscle and a 200 ml clot of blood. The dead muscle was excised, the wound was left open, but was able to be closed two days later. The patient eventually made a good recovery, but it required a number of weeks for him to regain function and for the pain to dissipate.
The authors note that a DVT and a ruptured Baker's cyst cannot be distinguished on clinical grounds. Obviously in this case. the excessive bleeding and resultant need for fasciotomy were secondary to inappropriate heparin administration. The authors did not say why heparin was administered prior to confirming the diagnosis. I assume it was because the naive clinician was sure of the diagnosis of DVT, and there was concern that the clot would extend or migrate to the lungs. Or as is often the case, the proper study was not available after hours.
If the cyst does not re-accumulate, rupture, or cause hemorrhage, the condition will never be diagnosed until it flares up again
Comment: This article is a perfect reason why an ultrasound should be available 24 hours a day for ED patients. It is often tempting, especially in hospitals that have limited ultrasound resources, to admit patients with likely DVT, start heparin, and obtain the ultrasound in the morning. In this case, it was obviously a bad thing to do, and it resulted in significant morbidity. A fasciotomy is a very big deal, and results in prolonged recovery. This case would be a slam-dunk malpractice award in Philadelphia.
Because most ruptured Baker's cysts have a component of hemorrhage, heparin will likely be detrimental in some cases, although you may get away with it for a few hours in most cases. In the acute phase of a ruptured cyst, the physician will not see ecchymosis or the characteristic crescent sign around the ankle, which takes a few days to develop. The condition will present like a classic DVT. Interestingly, in this case, the patient gave a history of swelling in the posterior knee that went away when the calf pain developed. This should have alerted the clinician to a possible ruptured Baker's cyst.
This case also exemplifies why a ruptured Baker's cyst is called pseudothrombophlebitis. It just cannot be differentiated from DVT by clinical means. Although this patient did not have a history of rheumatoid arthritis or other degenerative arthritis, he was in the right age group.
There is some confusion in the literature concerning a variety of synovial herniations and various types of bursitis in the posterior aspect of the knee. Several nonspecific conditions are loosely termed Baker's cyst, but the classic one is bursitis of the semimembranous or gastrocnemious bursa. This bursa lies between the medial head (origin) of the gastrocnemious and semimembranous tendon.
In this area, there is no particular obstruction to the expansion of a bursa due to trauma or inflammation. Often a nonspecific aching type pain in back of the knee will precede development of a palpable mass. In my experience, the mass is rarely palpated in the ED because the patient presents too soon or the bursa has already ruptured. In addition to a bursal origin, a Baker's cyst can be a result of synovial herniation of one of the tendon sheaths. The term Baker's cyst may not be specific for any specific anatomical abnormality.
Swelling behind the knee may come and go, occasionally being associated with intermittent knee swelling, because the bursa frequently communicates with the knee allowing to and fro migration of the fluid. A history of a swollen knee followed by a swollen posterior aspect of the knee coming and going would be a good tip-off if this information can be obtained. In addition to differentiating a popliteal cyst from DVT, one also must consider tumors, aneurysms, AV malformations, and occasionally cellulitis. This would be an unusual place for cellulitis to begin. On physical examination, a cyst may be confused with a posterior tear of the medial meniscus. Other traumatic conditions simulating a Baker's cyst would be partial rupture of gastrocnemious insertion, a rupture of the plantaris tendon, or a simple calf strain.
Most patients should be treated conservatively with analgesia and reassurance. I could find no firm recommendations for steroid injections or aspiration. It may take a number of weeks for the condition to resolve. Surgery is rarely required. Some patients remain convinced that the ultrasound was incorrect and seek another opinion because they are sure they have a venous thrombosis. There are a number of surgical approaches to removing or manipulating the bursa, but these usually are not required or results may be disappointing. Anti-inflammatory medications are usually given, but there are no data supporting their use. (See also the excellent discussion: Am J Emerg Med 1997;15[7]683).
A Baker's cyst is actually a relatively common entity. One study demonstrated a 37 percent incidence in patients clinically suspected of having DVT, but most studies report the incidence to be less than five percent (Circulation 1981;64:622). I suspect many patients are seen in the ED with pain behind the knee, and the diagnosis is missed. The DVT study is negative, and the diagnosis is a strain or sprain.
If the cyst does not re-accumulate, rupture, or cause hemorrhage, the condition will never be known or diagnosed until it flares up again. Although it is not mandatory to establish a diagnosis of a uncomplicated Baker's cyst once DVT is ruled out, it does make you look like a pro and may keep your patient from obtaining a second ultrasound or another ED visit when the extravasant fluid now becomes inflammatory. Most importantly, even though it is tempting, one should never institute heparin therapy for even classic DVT of the posterior calf without a confirmatory ultrasound evaluation. Get the radiologist or technician out of bed. You are awake suspecting the diagnosis; they need to be awake to confirm it and avoid mistreatment.
Conditions Producing Pain and Swelling of the Posterior Knee and Calf
- ▪ Thrombophlebitis
- ▪ Baker's (popliteal) cyst
- ▪ Muscle tear/strain
- ▪ Plantaris tendon rupture
- ▪ Cellulitis
- ▪ Fasciitis
- ▪ Compartment syndrome
- ▪ Popliteal aneurysm
- ▪ Gangla
- ▪ Neural tumors
- ▪ Sarcoma
- ▪ Hemangioma
Source: James R. Roberts, MD, September 2003.
