This 11 year old male was in good health until 6 
days ago when he began to have right hip pain at rest, 
which worsened with walking.  The pain was described 
as intermittent in nature.  It did not radiate to the back 
or down the leg.  It was described initially as being of 
medium severity, worsening when moving from side to 
side and with walking.  The pain progressively 
increased in intensity.  Five days ago, the patient was 
seen by a pediatrician and an orthopedic surgeon.  
Pelvic radiographs were done (including hips).  These 
were interpreted as  normal.  WBC was elevated at 
25,600 and his ESR was elevated at 88.  A UA was 
normal.  The patient was observed and treated with 
acetaminophen for pain, which appeared to help 
somewhat.  Yesterday when the patient awoke, he was 
unable to bear any weight on his right leg and could not 
walk.  There was no past history of significant trauma, 
unusual physical activity, or definite fever.  He did have 
a history of a sore throat, with fever up to 39.5 degrees, 
vomiting x1 with diarrhea x1 three days prior to the 
onset of this hip pain.
     Exam:  VS T38, P102, RR 20, BP 126/89.  Weight 
54 kg (greater than 95th percentile), height 138 cm 
(25th percentile).  He is lying down eating a cookie in no 
distress.  There are no mouth lesions or palatal 
petechiae.  The posterior pharynx is non-erythematous; 
however, the tonsils are slightly enlarged, without any 
exudates.  Heart regular without murmurs.  Lungs clear.  
Abdomen is soft and somewhat obese.  There is 
definite right lower quadrant tenderness with palpation.  
There is no rebound tenderness.  Bowel sounds are 
active.  Definite right-sided pelvic pain is elicited with 
attempts to rotate the pelvis; however, there was no 
pain with either internal or external rotation of the hip 
joint.  Hip flexion and extension do not elicit any 
significant pain; however, straight leg raising elicits 
pain.  There appears to be some tenderness along the 
right lumbar area just lateral to the lumbar spine 
(paraspinal area) and over the posterior region of the 
iliac crest with palpation.  He is unable to bear weight 
on the right leg.  The left hip appears totally normal.  
His neurologic exam is unremarkable.
     A repeat sed rate is 62, and the following 
radiographs are ordered:  1) pelvis including hips, 2) 
abdomen, and 3) lumbar spine.

View radiographs.

     Only the flat plate of the abdomen is shown here.  
All of these radiographs are initially interpreted as 
normal (note that retrospectively, there is a scoliosis to 
the right in the abdominal films and one might consider 
that there is a right lower quadrant sentinel loop 
present).  
     Because of persistence of these physical findings, a 
CT scan of the abdomen is done with both oral and IV 
contrast administration.

View abdominal CT scan.

     The CT scan extends from the lower level of the 
kidneys down through the pelvis.  There is enlargement 
of the right psoas muscle (left side of the image).  
There is a septated 2.0 cm hypodense area with an 
enhancing margin in the medial aspect of the right 
psoas muscle (arrow).  This finding is consistent with a 
psoas abscess.  There is a small amount of edema of 
the adjacent retroperitoneal fat.  The appendix is filled 
with barium and appears normal.  No other 
abnormalities are seen.  
     Overall impression:  An abscess in the medial 
aspect of the right psoas muscle with diffuse 
enlargement of the muscle.  The abscess is situated 
just inferior to the lower pole of the right kidney.  
     Findings at surgery:  The psoas muscle was 
diffusely edematous, but only a small amount of fluid 
was obtained.  No well-established abscess was found.  
Culture of the fluid from the psoas muscle was positive 
for group A beta-hemolytic streptococcus.
     Post-operatively, the patient developed persistent 
fevers, with chills and a temperature greater than 40 
degrees.  Initially, he was treated with vancomyin; 
however, because of the above, switched to penicillin 
and clindamycin.  Clindamycin is of value in severe 
streptococcal and staphylococcal infections for two 
reasons:  1) It acts on bacterial cells at a different site 
than penicillin.  If the bacterial cells are not actively 
dividing, they will not be killed by penicillin.  In certain 
abscesses where organisms are present in high 
quantities, active division is not occurring for many of 
the organisms.  2) It has an effect of depressing 
harmful toxin production that may be produced by this 
invasive group A beta-hemolytic streptococcus.

Discussion of psoas muscle abscess in children:
     Although primary psoas abscess is very rare in 
children of "developed" countries, it is not rare in tropic 
and sub-tropical "third world" countries with poor 
socioeconomic conditions.  Staphylococcus aureus is 
the most frequent type of infection seen in these 
environments, with almost all children presenting with 
the triad of pyrexia, flank pain and hip symptoms. 
Psoas abscess can be a secondary problem associated 
with tuberculous spondylitis or in relation to 
inflammatory bowel disease (1).  More recently, in the 
United States, psoas abscesses have been seen 
secondary to transperitoneal low-velocity gunshot 
wounds to the spine (3), or gastrointestinal or 
genitourinary trauma (2).  Primary psoas abscess can 
be seen in patients with sickle cell disease, intravenous 
drug users, immunocompromised individuals or 
individuals positive for HIV.

     Bacterial infections of muscle also known as 
pyomyositis or tropical pyomyositis occur more 
commonly in tropical regions.  Pyomyositis should be 
considered in the differential diagnosis of 
septic-appearing children, as well as children 
complaining of joint pain or muscle aches (7).  Recent 
imaging techniques are important in diagnosis.  Plain 
radiographs occasionally show a blurring or 
indistinctness of the lateral margins of the psoas 
muscle but, in general, are not as helpful as other 
techniques.  Ultrasonography is useful in showing 
enlarged psoas muscle with hypoechogenic masses, 
however it is not as accurate as a CT scan in showing 
the abscess (5).  Drainage of the abscess by CT-guided 
percutaneous catheter has been recommended by 
some (4), while surgical drainage is recommended by 
others, especially when percutaneous catheter drainage 
is not successful.  MRI is advantageous because 
multiple processes can be evaluated (6).  If the patient 
does not respond quickly to antibiotics and surgical 
intervention, either there is a recurrence of the 
previously debrided abscess, or there is an 
unrecognized secondary abscess.  Multiple abscess 
sites should be considered prior to initial debridement.

References:
     1.     Sadat-Ali M, al-Habdan I, Ahlberg A.  
Retrofascial nontuberculous psoas abscess.  Int Orthop 
1995;19(5):323-6.
     2.  Santaella RO, Fishman EK, Lipsett PA.  Primary 
vs. secondary iliopsoas abscess.  Arch Surg 
1995;130(12):1309-13.
     3.  Lin SS, Vaccaro AR, Reich SM.  Low-velocity 
gunshot wounds to the spine with an associated 
transperitoneal injury.  J Spinal Disord 
1995;8(2):136-44.
     4.  Golli M, Hoeffel C, Belguith M.  Primary psoas 
abscess in children--6 cases.  Arch Pediatr 
1995;2(2):143-6.
     5.  Royston DD, Cremin BJ.  The ultrasonic 
evaluation of psoas abscess (tropical pyomyositis) in 
chilren.  Pediatr Radiol 1994;24(7):481-3.
     6.  Roe JB, Yalcin S.  Magnetic-resonance-imaging 
scans in discitis.  Sequential studies in a child who 
needed operative drainage: a case report.  J Bone Joint 
Surg Am 1995;77(2):329.
     7.  Renwick SE, Ritterbusch JF.  Pyomyositis in 
children.  J Pediatr Orthop 1993;13(6):769-72.