Ruptured Popliteal Cyst

More swelling
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Diagnosis and reasoning

The elderly patient who presents with acute calf swelling is a very common (and important) presentation; deep venous thrombosis (DVT) and cellulitis are two diagnoses which immediately spring to mind. Other possibilities include rupture of a popliteal cyst (Baker's cyst), rupture or dissection of a popliteal arterial or venous aneurysm, spontaneous rupture of the medial head of the gastrocnemius or plantaris muscles, or rarely, soft tissue tumors. According to Well's (DVT) score, this patient classifies as 'low risk', while his D-Dimer test is negative. Given the extremely high negative predictive value of D-Dimers (96%), this is sufficient to exclude DVT. It is extremely tricky to differentiate between the remaning diagnoses via clinical signs alone - further evaluation via imaging is necessary. Both ultrasonograpy and magnetic resonance imaging (MRI) can be utilized here; however, note that ultrasonography is both cheaper and more readily available, making it a better initial investigation. A musculoskeletal ultrasound scan will cover most of the above conditions; note that it also practical to perform a vascular ultrasound simultaneously, as this will exclude aneurysms. The vascular ultrasound scan reveals no features suggestive of arterial or venous aneurysms and additionally reconfirms the presence of a pristine deep venous system. Musculoskeletal ultrasound reveals no evidence of muscular injury or soft tissue tumors; more importantly though, a large popliteal cyst with evidence of rupture is noted, suggesting the diagnosis. Rupture of a popliteal cyst may be secondarily complicated by a DVT (although this is excluded here), and also by compartment syndrome. Fortunately, this patient shows no clinical features suggestive of the latter. Further evaluation via an MRI is not indicated, given the unambiguous ultrasound appearance. Uncomplicated rupture of a popliteal cyst is managed conservatively. Anticoagulation should be avoided, as this may result in bleeding from the ruptured cyst into the calf, resulting in compartment syndrome. Fasciotomy is not required in the absence of compartment syndrome, while antibiotics are not indicated either.


Discussion

Popliteal or Baker's cysts are not true cysts, but rather, a distension of the gastrocnemius-semimembranosus bursa. The underlying mechanism is believed to be leak of joint fluid via a weakened posteromedial joint capsule, into the gastrocnemius-semimembranosus bursa. The condition is associated with both degenerative joint disease (osteoarthritis) as well as inflammatory arthritis (such as rheumatoid arthritis and the seronegative arthritides); however, it can occur in otherwise normal individuals. In general, popliteal cysts have a benign natural history, with minimal symptoms. However, rupture of the cyst may occur; this is often secondary to powerful knee extension (which increases the intra-articular pressure). Rupture of a popliteal cyst results in an acute and florid presentation: prominent swelling of the posterior calf associated with warmth, erythema and marked tenderness; this is due to mechanical distension of the calf tissues by the synovial fluid, as well as due to the inflammatory enzymes present. As might be observed, rupture of a popliteal cyst effectively mimics a DVT; the only clinical finding differentiating these two conditions is bruising in the malleolar region, which is believed to be due to the synovial fluid dissecting the calf structures up to the foot. Note however that this is rare, and may take several days to appear. An important point of note is that the rupture may cause a secondary DVT (due to venous stasis); thus, the two diagnoses are not mutually exclusive. In addition, rupture may rarely result in compartment syndrome (which may occur hours to days after the initial presentation). It is essential to be vigilant for suggestive signs and symptoms in this regard. Ultrasound is the diagnostic investigation of choice in such patients; modern equipment can simultaneously confirm the presence of a popliteal cyst and exclude a DVT. In the rare instances where ultrasound is equivocal, or if a complication is suspected, alternative imaging modalities include magnetic resonance imaging (MRI) and computerized tomography (CT) of the affected extremity. Note that arthrography of the knee joint may also be used to diagnose rupture; this will show extravasation of contrast from the cyst into the surrounding calf structures. Rupture of a popliteal cyst, which is uncomplicated by compartment syndrome or DVT should be managed conservatively; key elements include pain relief, elevation of the affected extremity, heat therapy, and intra-articular corticosteroid injection following joint aspiration. Note that if compartment syndrome or a DVT are present, these should be given priority, and treated in the standard manner.


Take home messages

  1. The only clinical sign differentiating rupture of a popliteal cyst from a DVT is bruising below the medial malleolus; however, this is rare.
  2. The diagnosis of a ruptured popliteal cyst does not exclude a DVT; both conditions can exist simultaneously.
  3. Uncomplicated rupture of a popliteal cyst is managed conservatively.
  4. Very rarely, rupture of a popliteal cyst may result in compartment syndrome - it is essential to maintain a high index of suspicion for this.

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