Complex Regional Pain Syndrome (CRPS) is a rare chronic pain syndrome that may follow tissue injury. It is characterised by unbearable pain, out of proportion to the inciting event as well as -


  • painful responses to non-painful stimuli, such as light touch (allodynia)
  • particular sensitivity to cold
  • a distribution of pain that does not follow the normal nerve distribution for the affected region
  • limb swelling in the earlier stages
  • changes in skin colour, temperature, sweatiness
  • problems achieving full range of movement of the knee


Its presentation is variable, that is, the degree to which the patient may suffer from each of these features varies, and also the features may change over time.

There is no single test to confirm the diagnosis and, although there are several theories, there is no proven cause of the syndrome as a whole.

What is certain, though, is that when a patient who has had a knee problem develops pain that is excruciating together with one or more of these other features, then the doctor MUST give consideration to the possibility of it being CRPS, because early treatment offers the best chance of a good outcome, and procrastination may leave the patient with a serious and crippling disability.

One of the reasons why the condition is often missed is that over the years the condition has gone by many different names, and thus the medical literature has been confusing. Also, the condition is very uncommon, although it seems to be more common after wrist fractures and knee damage.

No-one really understands the neurological basis of the condition. It seems to involve a sort of ‘short-circuit’ between the two types of nerve system that involve the appreciation of pain. The one system moderates the discernment of ordinary sensations, such as touch, pressure, temperature and ‘normal’ pain. The other system is highly protective, and when it perceives one of these sensations as being of a magnitude that is highly threatening to the body, then it causes the release of chemicals that increase the heart rate and the blood pressure, increases anxiety, and readies the body for a ‘fight-or-flight’ response. The former is the ‘peripheral’ nervous system, and the latter is the ‘sympathetic’ nervous system. In CRPS even trivial stimuli to the affected skin area are ‘short-circuited’ and wrongly interpreted as ‘serious’, setting off this chemical reaction that makes the brain interpret the stimuli as severe and threatening.

The current nomenclature identifies two types of CRPS -

  • CRPS Type I

    This used to be called Reflex Sympathetic Dystrophy (RSD). It does not just trigger on its own but there is usually a history of an initiating noxious event, such as a meniscal tear.
  • CRPS Type II

    This used to be called Causalgia. The symptoms of CRPS II are very similar to CRPS I, except that the inciting event appears to be localised nerve damage, most commonly damage to the nerve close to a common portal that is generally used to pass the instruments during arthroscopy.

In both forms of CRPS, depending on the degree of ‘short-circuit’, the pain sensation is a mixture of -

  • sympathetically independent pain (SIP), that is, not involving the sympathetic nervous system
  • sympathetically maintained pain (SMP), which does directly involve the sympathetic nervous system.


CRPS and knee surgery

CRPS is rare but when it follows knee surgery it is frequently associated with arthroscopy, and to a lesser degree with knee replacement. In particular there seems to be an association with the nerve called the infrapatellar branch of the saphenous nerve, just below the joint line on the medial side. Here is can be directly injured by the surgeon’s scalpel when he/she makes the antero-medial portal there or by other instuments, such as a retractor. The nerve can become inflamed (saphenous neuritis) or, if cut, it may try to heal itself, but instead get tangled up into a ball called a neuroma, with some nerve endings raw and sensitive to any fight-or-flight chemicals (adrenaline and nor-adrenaline/epinephrine and nor-epinephrine) in the tissues.

An expert knee surgeon will take pains to avoid damaging this nerve by making a small incision in the area, identifying and retracting the nerve, and only then making the portal into the joint space.

It is the CRPS II type that seems to be triggered by such an injury. In CRPS II the initial painful area of skin maps with precision the normal area of sensory nerve supply. If not managed early and effectively, though, the painful region expands beyond this anatomical distribution and then the whole clinical picture becomes very much like CRPS I.

In CRPS I the pain is from the outset in this wider, non-anatomical, distribution. There is usually some inciting event – a ‘noxious stimulus’ but it seems to trigger from a quite different location than a local cut nerve.

What is common to both types of CRPS is that the pain experienced is way out of proportion to the inciting event. The doctor needs to have this condition in mind when presented with a patient who develops such disproportional pain and not dismiss the person as a neurotic or a potential opiate abuser. He/she needs to take care to diminish the patient’s anxiety by careful explanation, because the anxiety of seeing the doctor is sufficient to stimulate the production of the fight-or-flight chemicals and amplify the pain. The doctor needs to compare both knees, checking the temperature, colour and any boggy swelling. He/she needs to map out the area of sensitivity and this is best done with an ice cube In very early stages there may actually be a decreased sensitivity to the ice, but as the condition unfolds the ice may provoke an intolerable burning pain!


Evolution of the syndrome

CRPS evolves over time, and it may resolve or worsen and progress to a disabling condition. Early in the condition the affected limb may be red, warm and swollen, although the skin may feel dry. Later, when the condition has progressed, the limb may appear bluish, cool and paradoxically moist. Swelling may have lessened, but the skin may appear tight and shiny instead. The temperature difference between the two knees can be as much as three and a half degrees.

Three stages are identified, but patients may not progress through all the stages -

Stage 1 – acute stage

This usually lasts less than six months. This is the stage where the limb may be swollen, with boggy but dry skin and a hot, red knee – all accompanied by pain out of proportion to normal and a high intolerance to cold.

Stage 2 – dystrophic stage

Three to nine months after the acute stage settles, the limb may be blue and mottled, the nails of the foot may be thick and the leg hair coarse because of a decrease of blood supply to the limb.

Stage 3 – Atrophic stage

In this stage the skin becomes tight and shiny and the bones become osteoporotic. Contractures can form because the patient is avoiding bearing weight and holding the knee in a flexed position.

In addition to all the skin changes, there may be alterations of muscle tone and activity. In some patients this manifests as weakness or even a sort of paralysis, while in others the tone is increased and there may be muscle spasms or tremors and increased knee reflexes.

Associated with all of these problems may be depression and anxiety, and the doctor might feel tempted to dismiss the scenario as ‘a psychiatric problem’. Only by familiarising himself with this syndrome will the doctor be equipped to deal with it effectively in the important early stages, or be confident in referring the patient to a pain expert who can initiate effective treatment. As the condition resolves, the patient’s ‘emotional overload’ is likely to diminish.

Management needs to be a team effort – knee surgeon, pain specialist, physiotherapist and psychiatrist might all be involved.



Paravertebral Sympathetic Ganglion Block

There are no laboratory tests for CRPS, but there is a test to evaluate to some degree the amount of ‘short circuit’ between the peripheral and sympathetic nervous systems. Local anaesthetic agent is injected under expert control in the lower back next to the vertebrae to block the nerve ganglia there. The patient is observed for changes in the affected skin. The greater the ‘short circuit’ from peripheral to sympathetic systems the more improved the patient’s symptoms become while the anaesthetic agent is working. When the block is in full effect, any residual pain is deemed to be coming from a damaged nerve around the knee, or perhaps even from a meniscal tear.

Triple-phase Bone Scan

Another test that is used is a bone scan that may be helpful in the early stages when the limb is hot, but it may miss the underlying CRPS as the limb progresses into stage 2.

MRI and X-ray

MRI is unhelpful except to exclude other possible causes of the pain. X-ray is of little benefit in identifying the important early stage as it will only show the bony changes of osteoporosis in the late atrophic stage.



There is no well-established treatment algorithm for CRPS. However, what is established is that treatment should begin as soon as possible, preferably within three weeks of the onset of the distressing symptoms

Identify any painful focus

The team needs to identify and eliminate any painful focus, such as a neuroma


Physiotherapy is the core treatment, aimed at desensitising and mobilising the limb. Confidence must be established between patient and therapist – confrontation between them is likely to exacerbate the condition. Drug regimes are primarily aimed at reducing the pain sufficiently to allow full participation in physiotherapy. Patients with allodynia need to be retrained in their perception via vibration, massage, temperature contrast baths and electrical stimulation. As treatment progresses, the patient should be able to tolerate greater amounts of stimulation and limb movement. Forced passive range of movement (ROM) exercises are too aggressive, and the patient needs to be encouraged with active or assisted-active exercises.

As physiotherapy advances, the limb will need strengthening, stress loading and gait retraining, and warm-water hydrotherapy is very useful in this phase.

Finally the patient is taken through a regime to return to activities of daily living and work.


Many medications have been tried, and a few stand out as being more useful -

  • The doctor should avoid prescribing narcotics.
  • Oral cortisteroids may be useful early, to decrease the fight-and-flight chemical response
  • Alendronate to treat and prevent osteoporosis
  • Membrane stabilisers gabapentin and pregabalin affect the cellular chemistry of nerve cells, help the burning pain and improve sleep.
  • Ketamine, which is normally used as an anaesthetic agent, especially in animals, can be given in three treatments at a sub-anaesthetic dose to diminish the pain response.
  • Amitryptyline (an anti-depressant and mood stabiliser) can assist with both the burning pain and the anxiety associated with it.

Sympathetic blocks

Injecting long acting anaesthetic agent into the sympathetic ganglia adjacent to the spine (paravertebral sympathetic chain ganglion blockade) should allow resolution of symptoms with an increasing period of efficacy after each blockade.

Epidural block

If surgery needs to be performed in a patient with established CRPS, a continuous lumbar epidural with a long acting local anaesthetic agent is the anaesthetic of choice. The epidural can be continued after the end of the procedure to provide pain relief and start the process of physiotherapy.

Surgical Sympathectomy

This is a ‘last resort’ surgical procedure where the sympathetic ganglia are surgically destroyed.


Further reading

The information in this paper has been interpreted from the chapter – Diagnosis and Treatment of Complex Regional Pain Syndrome. by Saxton DL, Lindenfeld, T and Noyes, FR. in Noyes Knee Disorders: Surgery, Rehabilitation, Clinical Outcomes. Ed Frank R Noyes – and published by Saunders Elsevier 2010, pp 1116-1133.