Transient Synovitis of the hip (Irritable hip)

A common presentation in young children is the mysterious limp.  Transient synovitis (irritable hip) of the hip is the most common cause of an unexplained limp under the age of 6.  The current usual practice is to make the diagnosis of irritable hip on clinical grounds.  It is no longer routine practice to support this diagnosis with blood tests or imaging.  This development in practice opens the possibility for a young child with a mysterious limp to be managed outside of a hospital setting where appropriate.

What is transient synovitis of the hip (irritable hip)?
The cause of transient synovitis of the hip is unclear.  It is presumed that most cases are a reactive arthritis with a viral trigger.  Injury can also be implicated but it is unclear whether such associations are causal or simply bring the problem to someone’s attention.  It is also common to find typical features of transient synovitis of the hip in children who have no history of viral illness or injury.  There is therefore no need to rely on a history of a possible trigger to make the diagnosis.

As the name suggests, there is inflammation of the synovium of the hip.  Ultrasound may show or synovial thickening or increased fluid.  Inflammatory markers are not usually significantly raised.  Neither of these investigations is reliable and no longer done routinely since the gold standard is clinical diagnosis.

How is transient synovitis of the hip diagnosed?
Typical features of transient synovitis of the hip are:

• Unexplained onset of limp (no history of more significant injury consistent with fracture)
• Well and afebrile child with no signs or symptoms of other significant acute illness
• Unilateral hip signs – painful or reduced range of movement

If a child has these features and no signs of another cause, the diagnosis is almost certainly irritable hip.  Other important diagnoses to consider are:

• Septic arthritis - usually presents with fever and complete refusal to weight bear
• Osteomyelitis - usually febrile and there is localised tenderness or swelling in the bone
• Toddler fracture - undisplaced fracture of the tibia usually presents with complete refusal to weight bear and has localising signs in the tibia (tender or slightly warm to touch)
• Other significant fractures are usually accompanied by localised swelling or tenderness and are clinically obvious.
• Juvenile idiopathic arthritis (JIA) - This is very uncommon under the age of 6.  Obvious swelling of a single joint (without signs of infection) may indicate a transient arthritis of a joint other than the hip.  If the affected joint is the hip and the diagnosis is JIA, this will probably be clinically indistinguishable from irritable hip.  However the child would only need to be referred if the arthritis persisted for several weeks, so analgesia and watchful waiting is the initial management in any case.  If multiple joints are involved or symptoms cannot be managed easily, early referral is indicated.
• Perthe's disease - The cause of this disease of the hip is unknown.  The femoral head becomes avascular and breaks down.  The typical age is a school age child but there is some overlap with the age at which irritable hip presents.  The unexplained limp in a child over the age of six should raise suspicion of Perthes disease.  Under the age of six, progressive symptoms or symptoms that fail to improve after a few days are concerning.
• Non- musculoskeletal - Limp may be a sign of pathology unrelated to the lower limb.  Abdominal pain or scrotal pain can cause a child to limp.

One of the best discriminators between all of these problems is the course of the symptoms.  While symptoms can vary in any illness, a significant fluctuation in pain and limp is most indicative of transient synovitis of the hip.  While syptoms may be still present after three days, it is unusual not to see significant improvement in that time.

A typical presentation, absence of red flags and a classical course of the symptoms usually make it obvious when the problem is irritable hip.  If all of these things apply, management is watchful waiting with good safety netting advice.

Edward Snelson
@sailordoctor

Is this really an injury or something else?

This is a magical time of year for the British paediatric emergency physician as children and young people are getting far fewer illnesses.  Instead of wall to wall snot and vomit, the paediatric waiting area is filled with injuries of all varieties.  Although I assume that obvious fractures will present directly to the Emergency Department, many injured children will also present to primary care.  This tip applies equally to both settings.  Hidden amongst all of the injured children will be a ‘something else’ from time to time.  They are particularly hard to spot but there are a few things that can help.

In most cases there is no doubt that an injury actually is the case of the pain.  If the mechanism fits, go with it.  (Well doc, he cycled off the roof.  Do you think that’s why his leg hurts?) What we are talking about here are soft presentations.  These are  the things that present as injury but are in fact the manifestation of something else:

• A hip thing (Transient synovitis, Perthe’s or Slipped Upper Femoral Epiphysis)
• Juvenile Idiopathic Arthritis (JIA)
• Infection (septic arthritis and osteomyelitis)
• Apophysitis (the most common being Osgood-Schlatter’s)
• Malignancy (e.g. Osteosarcoma)

Even excluding the odd one out in that list (transient synovitis, or irritable hip to give it its other name), these conditions comprise a surprisingly large number of injury presentations.  So in order to avoid the pitfall of allowing the presentation to frame your diagnosis, ask the following questions:

Is it the hip?

This is a deceptive question but an important one as hip problems are rarely true injuries.  Very often the child either has referred pain to the knee or, if younger, does not localise the pain at all, choosing to blame their foot or another random body part.  So for any lower limb problem without a blatant cause the hip must be assessed.  If the hip is then found to be suspect, what you do next depends on your experience and where you work.  I would suggest that any of these factors mandates urgent referral of a suspected hip problem:

• Age over six
• Fever
• Unable to weight bear on that limb

How long has the pain really been there?

In any of the chronic causes of pain the symptoms may have been present for a considerable time.  Despite this the pain is often put in the context of a football game or other event.  In many cases there was no injury as such, simply a worsening of pain after a fall or an exertion.  Any pain that was there already and was made worse by anything should be assumed to have a chronic cause.  This may be something relatively benign such as Osgood Schlatter’s disease but it may also be something more in need of early detection such as Juvenile Idiopathic Arthritis or a malignancy such as osteosarcoma or leukaemia.

Is the child unwell and is the ‘injury’ hot to touch?

Septic arthritis and osteomyelitis are thankfully both rare in children.  They are also completely devastating and rather difficult to detect.  If infection is suspected, refer and do not allow the buck to be passed back in the form of advice to do some blood tests to rule out infection.  If the onset is acute or the child is very young, inflammatory markers may not yet be raised. (1)  Juvenile idiopathic arthritis can also cause hot, swollen joints and may even cause systemic symptoms.

Is the problem bilateral?

If there are two of the same body part affected (and no good mechanism to explain this) the possibilities are limited.  Thankfully you can essentially rule out infection and tumour.  What becomes much more likely is one of the other possibilities.  Although an apophysitis is more probable if the tibial tuberosity or the Achilles tendon insertion is involved, other bilateral pains make juvenile idiopathic arthritis much more likely.

Have there been other mysterious joint aches over the past months or years?

One of the laments of the paediatric rheumatologists is that children present late with juvenile idiopathic arthritis (JIA).  Recurrent pains in children are often put down to growing pains.  While it is true that young people often get unexplained pains that are not related to any of the above conditions, it is also important to look for the signs and symptoms of JIA.  One of the most important considerations is the recurrent nature of the pain.  Severity is often difficult to assess as children and young people are surprisingly likely to under-report chronic pain.  They tend to assume that it is nothing (after all they haven’t yet learned to worry about all the serious possibilities that occur to their adult counterparts) or simply alter their activity so that the pain has little impact. 

If JIA is suspected, examine the affected joint(s) for swelling and perform a PGALS screen which will detect other joints that are affected which may not have been volunteered in the history.

In summary, most children that present with minor injuries do indeed have minor injuries, with the exception of hip pain, which is usually something unrelated to an injury.  Hip pain aside, it is easy to miss the rare causes of bone and joint pain.  Asking about the mechanism of ‘injury’ and pre-existing or recurrent symptoms may help to uncover chronic causes.  Examination of the affected part and consideration of the possibility of infection are also important if there is any possibility that the history of injury is a distraction.  Any unremitting well localised pain should raise the possibility of malignancy(2) unless another good explanation can be found.  Finally juvenile idiopathic arthritis is uncommon but needs to be actively sought as it is easy to miss.

Edward Snelson

@sailordoctor

Disclaimer:          I’m talking rubbish:  I’ve just had a peek in the waiting room and it is wall to wall snot and vomit after all…

(1)          BRITISH SOCIETY FOR CHILDREN'S ORTHOPAEDIC SURGERY The management of acute bone and joint infection in childhood - A guide to good practice

(2)          Suspected cancer (part 1—children and young adults): visual overview of updated NICE guidance  BMJ 2015; 350 doi: http://dx.doi.org/10.1136/bmj.h3036
http://www.bmj.com/content/350/bmj.h3036