Baker's cyst

Around the knee joint is a capsule, and its lining produces the fluid that lubricates the joint. When a small tear opens in that capsule at the back of the knee, the lining can push through it and fill with joint fluid, and the pouch that forms is the cyst.

That can happen by itself, but it is far more likely in a knee that is already producing extra fluid, which is why a Baker's cyst so often turns up alongside another knee problem: osteoarthritis most commonly, but also inflammatory arthritis, a crystal flare such as gout or pseudogout, or a cartilage injury. The cyst is usually a sign of a knee problem: what matters is whatever inside the knee is making the fluid. Those causes are set out in our knee pain guide.

Most Baker's cysts are silent and come to light by chance. When they do make themselves felt, the usual picture is:

• Behind the knee, a lump or a swelling, often plainest when the leg is out straight
• A feeling of tightness, stiffness or aching there, which activity or a period on your feet can sometimes make worse
• The knee not bending fully, or a sense of pressure behind it
• An occasional click or catch

The calf below the knee suddenly becoming painful and swollen is quite another matter: that is the picture of a burst cyst, and equally of a blood clot, and "When to seek help" below deals with it.

Often the history and the examination identify a Baker's cyst by themselves, and where a typical cyst sits in a knee already known to have arthritis, no scan may be needed at all. Examination is only the first step, though: other swellings can sit in exactly the same place, so a lump behind the knee is not taken on trust as a cyst. Most swellings behind the knee do turn out to be a cyst or a normal scan, so where the picture is already clear a scan may add little.

When a scan is worth having, ultrasound answers more than one question at once. A lump that is not obviously a cyst can be shown for what it is and told apart from those other causes. The rest of the knee comes into view at the same time: how much extra fluid there is, whether the joint lining is inflamed, and what the likely cause is, which is often the more important question. And where drainage or an injection is worthwhile, it can guide the treatment in the same sitting. Point-of-care ultrasound is part of the consultation with Dr Borukhson, so one visit is usually enough to assess the lump and the knee behind it, and to treat both; the ultrasound clinic page describes the approach. An MRI comes next when the knee's deeper structures need closer inspection, as with a suspected meniscus tear.

Many Baker's cysts need no treatment at all in their own right. Some shrink by themselves and others subside as the knee condition is treated; once the diagnosis is confirmed, a cyst that neither hurts nor gets in your way can simply be left alone.

If the cyst or the knee is causing trouble, the choices are these:

• Simple measures: something for the pain, and a physiotherapy programme built around the knee
• Draining the cyst: a needle, guided by ultrasound, draws the fluid off and eases the pressure and tightness; the knee joint itself can be drained the same way, as our joint aspiration guide explains
• A steroid injection: where inflammation is driving the extra fluid, an ultrasound-guided injection into the knee can calm it
• Treating the cause: what makes any improvement last is dealing with the arthritis or knee problem underneath, since a cyst refills for as long as the knee keeps producing fluid

An operation on the cyst itself is a rarity; where surgery is suggested at all, its usual purpose is to mend damage within the knee rather than to remove the cyst.

Day to day there is little to do for the cyst as such; the knee behind it is where self-care counts:

• Once any acute soreness has settled, use the knee as far as comfort allows rather than protecting it, and keep the thigh muscles strong, building up gradually in the way our arthritis and exercise guide describes
• An ice pack wrapped in a tea towel, for around 10 to 20 minutes at a time, repeated every few hours, can ease an ache behind the knee at the end of a busy day
• Losing some excess weight, if you carry any, lightens the load on the knee at every step
• When the underlying knee problem flares, act early rather than soldiering on, because a knee that is settled makes less fluid

One event is worth knowing about in advance: the cyst bursting. Fluid that escapes into the calf produces a picture almost identical to a blood clot in the leg (a DVT), and a clot calls for treatment that a burst cyst does not, so this is a question to have answered the same day, by your GP or NHS 111, not one to watch.

Treat any of these as a 999 or A&E emergency: call 999 and do not drive yourself to A&E.

• Severe illness with suspected sepsis: a high fever with shaking, confusion, rapid breathing, or feeling dramatically worse over hours
• With a fever or feeling unwell, a rash that does not fade when you press it, a stiff neck with eyes that cannot bear bright light, or skin, lips or tongue turning blue, grey, pale or blotchy, since these can be signs of meningitis or sepsis, as can a very bad headache, confusion or a first seizure with a fever or feeling unwell; on black or brown skin the rash and the change of colour may be easier to see on the palms of the hands or the soles of the feet
• Breathlessness that is severe, with gasping, choking or no breath to get words out; coughing up blood; or chest pain that catches as you breathe in, since any of these can mean a clot has moved to the lungs; with a swollen or painful leg, any breathlessness or chest pain belongs here, and so do a racing heart and feeling faint
• Chest pain that is crushing, tight or like a pressure, that spreads to an arm, the jaw, the neck, the stomach or the back, that comes with breathlessness, sweating, feeling sick or feeling light-headed, or that came on suddenly and does not go away
• Palpitations happening now that do not stop, or that come with chest pain, breathlessness, feeling faint or fainting, since a heart whose muscle or rhythm is affected has to be checked at once
• Lips, mouth, throat or tongue that suddenly swell, a throat that feels tight, very fast breathing or a struggle to breathe or to swallow, skin, tongue or lips turning blue, grey or pale (on black or brown skin this may be easier to see on the palms of the hands or the soles of the feet), becoming suddenly very confused, drowsy or dizzy, or fainting and not being able to be woken, since this can be a severe allergic reaction, anaphylaxis, which can follow a dose of any medicine

Any of these needs an assessment that is urgent, on the same day, from your GP or NHS 111:

• A calf that is swollen, hot or painful, or a limb that is painful, swollen or discoloured, which can mean a clot; skin over the calf that has turned red or darker, which brown or black skin can make harder to spot, or a sudden worsening of pain or swelling in the leg, is part of that picture; with breathlessness, chest pain, palpitations or feeling faint, this is a 999 or A&E emergency: call 999 and do not drive yourself to A&E
• A joint that is hot and swollen, with a fever or feeling unwell, since an infected joint has to be ruled out at once, at A&E or through NHS 111
• New chest pain, new palpitations, or breathlessness that is new or getting worse; palpitations happening now that do not stop, or that come with chest pain, breathlessness, feeling faint or fainting, belong to the first list

The last sign of the second list, new chest pain, palpitations or breathlessness, is for a leg that is neither swollen nor painful, and for palpitations short of those the first list holds: with the leg signs, breathlessness, chest pain, palpitations or faintness of any degree belong to the first list. The hot, swollen knee with a fever or feeling unwell is an emergency too, one for A&E or NHS 111, and with shaking, confusion, rapid breathing, or feeling dramatically worse over hours a 999 or A&E emergency, as the first list says, and a knee that has turned hot and swollen out of the blue needs the same-day answer even if you have no fever and feel well, from an urgent GP appointment, NHS 111 or a specialist clinic that can see you that day, for the reasons our septic arthritis guide gives. A knee that will take no weight at all, or that cannot be moved, is a 999 or A&E emergency, whether or not there is a fever, as that guide also says. A lump behind the knee that has not yet been diagnosed should be confirmed rather than assumed to be a cyst: one that is simply there can wait for a routine appointment, but one that is gradually growing, hardening or becoming painful should be looked at promptly, by a GP or a specialist clinic, and one that turns painful, harder or softer suddenly needs an assessment the same day, from your GP or NHS 111. If you cannot tell how urgent something is, asking is never a mistake.

A lump behind the knee causes more worry than it usually deserves, and that worry is worth resolving quickly. With point-of-care ultrasound in the room, a specialist assessment can usually confirm the cyst at the first visit, find the knee condition producing it and, where appropriate, deal with both at that visit, draining the cyst or injecting the knee where that helps. Once it is looked at, a lump behind the knee usually has a clear answer quickly, and a specialist review is the way to get it.

Should I be worried about a Baker's cyst?

Usually not. It is a pocket of fluid, not a tumour, and it may settle by itself or once the underlying knee condition is treated. For many people the whole of it is an ache or a feeling of tightness behind the knee, and for many others there is nothing to feel at all. Breathlessness, chest pain, palpitations or feeling faint with a painful or swollen calf is a 999 or A&E emergency. Call 999 and do not drive yourself to A&E. Short of that, the main situation that needs an answer the same day is a burst cyst, because the fluid that leaks into the calf can cause pain, swelling and redness that look very like a blood clot, and telling the two apart takes a proper examination that day, from an urgent GP appointment or NHS 111.

How do you diagnose a Baker's cyst?

Frequently from the examination alone: a Baker's cyst can often be felt, and a typical one in a knee with known arthritis may need no scan at all. If it is less obvious, ultrasound does three things at once: it shows the cyst itself, it distinguishes it from the other things that swell behind the knee, and it looks into the joint for the surplus fluid and for whatever is producing it. Point-of-care ultrasound is part of the consultation with Dr Borukhson, so all of this can usually be done at one appointment. Occasionally an MRI scan is needed to look more closely at the deeper structures of the knee.

Is draining the cyst an option?

Yes. If a cyst is large or uncomfortable, a needle can empty it, and the inflammation producing the fluid can be calmed with a steroid injection into the knee. Both are done with ultrasound to guide the needle. Drainage on its own is not always a lasting fix, because a knee that keeps producing fluid can refill the cyst; the cause needs as much attention as the cyst does.

What happens when a cyst ruptures?

When the fluid tracks down into the calf, the lower leg can become suddenly and sharply painful, swollen, and red or darker in colour. In itself a burst cyst is not dangerous, but a blood clot in the leg (a DVT) looks very much the same and does need treatment, so sudden calf pain or swelling should be assessed the same day: ask for an urgent GP appointment or call NHS 111. Chest pain that catches as you breathe in, coughing up blood, or severe difficulty breathing, when you gasp, choke or have too little breath to talk, is a 999 or A&E emergency, because any of them can mean a clot has reached the lungs. So is breathlessness or chest pain of any degree, palpitations or feeling faint, when it comes with a painful or swollen leg. Without the leg signs, new chest pain, new palpitations, or breathlessness that is new or getting worse, short of what the first list under When to seek help holds at 999, still needs advice the same day, from a GP or NHS 111.