CLINICAL ORTHOPAEDICS AND RELATED RESEARCH
Number 391, pp. 258–265
© 2001 Lippincott Williams & Wilkins, Inc.
Differentiation of Psoas Muscle
Abscess From Septic Arthritis of the
Hip in Children
John Song, MD; Merv Letts, MD; and Ron Monson, MD
its demographic pattern.13–21,26,32,34 Nontuberculous pathogens currently are responsible for
most psoas abscesses and can be classified as
primary or secondary. Secondary abscesses
usually arise as a complication of other conditions, such as appendicitis or inflammatory
bowel disease. Primary psoas abscess, defined
as abscess formation in the absence of any detectable underlying focus of infection, has a
predisposition to children.10,20,26,33
Because of its rarity, primary psoas abscess
continues to be a diagnostic challenge for the
clinician. When psoas muscle abscess presents
in children, the disease is even more obscured
by the clinical resemblance to other diseases,
especially septic arthritis of the hip.4–7 The
authors’ experience with 11 children with psoas
muscle abscess at two children’s hospitals
forms the basis of this review, and a strategy is
proposed for diagnosing this disease and differentiating it from septic arthritis of the hip.
A 20-year review was conducted of children presenting with psoas abscess at two major pediatric hospitals. Eleven children with psoas abscesses were identified. The extreme variability
in the clinical presentation of this condition is
shown. Psoas abscess was most difficult to differentiate from septic arthritis of the hip in pediatric patients. This study also shows the often
circuitous investigative route traversed before
arriving at the diagnosis of psoas abscess. Atypical features, such as femoral nerve neurapraxia
or bladder irritability in association with hip
pain, should alert the clinician to consider psoas
abscess. Based on this study, a diagnostic algorithm to differentiate between psoas abscess and
septic hip was formulated.
Abscess of the psoas muscle, although rare, first
was described by Abeille in 1854.1 With the advent of successful treatment of tuberculosis
since that time, there has been clear evolution in
From the Division of Pediatric Orthopaedics, Children’s
Hospital of Eastern Ontario, University of Ottawa, and
the Winnipeg Children’s Hospital, University of Manitoba, Winnipeg, Manitoba, Canada.
Reprint requests to Merv Letts, MD, Division of Pediatric
Orthopaedics, Children’s Hospital of Eastern Ontario,
401 Smyth Road, Ottawa, Ontario, K1H 8L1, Canada.
Received: February 23, 1999.
Revised: August 21, 2000; December 8, 2000; March 13,
2001.
Accepted: April 10, 2001.
MATERIALS AND METHODS
All cases of psoas muscle abscess at two children’s
hospitals were identified, and the patients’ medical
records were reviewed. Of the 11 patients 18 years
of age or younger, two had abscesses that were related to appendicitis, two had spinal tuberculous
with psoas abscesses, three had psoas abscesses associated with osteomyelitis (one of the pelvis, and
two of the lumbar vertebrae), and the remaining
258
Number 391
October, 2001
TABLE 1.
259
Psoas Muscle Abscess in Children
Summary of Published Cases of Psoas Abscess in Children
Authors
Number of Cases
Tong et al30
Golli et al 10
Malhotra et al20
Parbhoo and Govender21
Average Age of
Patient (years)
Culture Positive for
Streptococcus Aureus
4.3
5.4
6.2
2
5
6
16
1 Beta Streptococcus
2
36/69
2
2
4
1 mixed
1 group Beta Streptococcus
3
6
9
24
Chaitow et al6
Bresee and Edwards*5
King et al16
Spiegel et al29
Song et al (current study)
2
104 (96 primary)
3
3
11 (7 primary)
10.6
8.2
9.6
12.6
9.3
*Includes eight cases related to abdominal infections.
four patients had no clear source of infection that
could be identified. These four children were considered to have primary, nontuberculous psoas abscesses (Table 1). This series was compared with
cases of psoas muscle abscess in children reported
in the literature to better define the typical symptomatology of this syndrome (Tables 2, 3).
CASE REPORT
A 17-month-old girl presented to the emergency room after a 2-week history of intermitTABLE 2.
tent fever and irritability. A limp had developed, and the patient had hip pain 3 days before admission. Apart from a slow healing
abrasion that was present 2 months previously
over the patient’s left knee, there was no history of prior illnesses or trauma.
At examination, the patient’s temperature
was 38C, and her right hip was held in flexion,
with extension, internal rotation, and abduction
causing severe pain. There were no masses palpable in her abdomen, flank, or groin, although
Clinical and Laboratory Results in Psoas Abscess
Physical Findings in Psoas Abscess
Psoas
Sign Fixed
Flexion
Deformity
Hip
Flank or
Abdominal
Pain
Palpable
Mass
Elevated
Erythrocyte
Elevated
Sedimentation Leukocyte
Rate
Count
Authors
Limp
Tong et al30
Golli et al10
Malhotra et al20
Parbhoo and
Govender21
Chaitow et al6
King et al16
Spiegel et al29
Bresee and
Edwards*5
Song et al
(current study)
3
4
9
24
3
8
10
0
1
11
0
3
3
0
9
11
6
9
24
2
3
3
38/72
0
0
9/69**
2
51/69
1
2
1
13/69
2
1
2
36/69
2
3
3
29/31
2
3
2
29/31
10
11
8
0
4
3
**Seventeen of 69 patients were described as having full, painless hip motion.
0
260
Clinical Orthopaedics
and Related Research
Song et al
TABLE 3.
Diagnosis and Treatment of Psoas Abscess
Diagnosis by
Authors
Tong et al30
Golli et al10
Malhotra et al20
Parbhoo and
Govender21
Chaitow et al6
Bresee and
Edwards*5
King et al16
Spiegel et al29
Song et al
(current study)
Treatment by
CT Scan
Ultrasound
Incision and
Drainage
Percutaneous
Drainage
Only Intravenous
Antibiotics
3/3
6/6
0
5/5
2/2
6/6
9/9
19/24
0
3
7
22
2
2
0
0
1
2
2
2
2/2
11/11
0/1
14/15
2
99/107
0
8/107
0
0
2
1
1
MRI (2)
2
1
1
2
0
0
9/9
1/1
6
2
3
the inguinal lymph nodes seemed to be enlarged. The erythrocyte sedimentation rate was
66 mm, and the leukocyte count was elevated at
29,300 per high power field. A radiograph of
the pelvis showed a shadow over the right iliac
fossa (Fig 1).
Intravenous cefazolin and gentamicin were
started, and the patient was admitted with a
presumptive diagnosis of septic arthritis of the
right hip. On the same day, with the patient under general anesthesia, aspiration of the hip
with fluoroscopic assistance yielded negative
Fig 1. The right sacroiliac joint is obscured by
the shadow of a psoas abscess in Patient 1.
results. A bone scan was negative for osteomyelitis; however, accumulation of the radionucleotide in the bladder revealed that it
was being displaced by a large space-occupying lesion in the right iliac fossa (Fig 2). This
was shown by computed tomography (CT)
scan to be a large abscess in the psoas muscle
Fig 2. The bone scan showed bladder displacement by a large psoas abscess in the right iliac
fossa. There is absence of activity in the bony
tissue.
Number 391
October, 2001
Psoas Muscle Abscess in Children
261
Fig 3. A large psoas abscess
can be seen in this CT scan (arrows).
(Fig 3). Surgical incision and drainage of a 6 8-cm abscess was done using a retroperitoneal approach, and methicillin-sensitive
Staphylococcus aureus grew from the culture
specimen. By the third day after surgery, the
patient was afebrile; the next day the patient
was discharged on a regimen of intravenous
cefazolin. After completing 2 weeks of antibiotics, the patient was clinically well and
walking without a limp.
RESULTS
Of 11 children (11 cases) with psoas abscess,
two cases were attributable to tuberculous abscess and two cases were from direct spread
from a primary appendicitis. Four of the remaining seven cases were primary psoas abscess. The remaining three cases were associated with osteomyelitis of the ilium and
vertebral bodies.
The average age of a child presenting with
psoas abscess was 8 years in patients in this series. A slight predisposition in boys was seen.
No predisposition to the infected side was seen.
The infecting agent was most commonly
Staphylococcus aureus. In the cases of psoas abscess with multiple bacteria cultured, Staphylococcus aureus also was isolated in each instance.
The most common clinical findings were
limp and pain in the hip. The wide variability
of symptoms and signs of a psoas abscess in a
child also was seen. This included abdominal
pain, thigh pain, back pain, and absence of any
hip symptoms reported in 17 of 69 published
cases5 (and in two of seven cases in the current
series). The psoas sign is attributed to inflammation causing spasm of the psoas muscle. To
perform this maneuver, the hip is flexed, relieving tension in the psoas muscle, which
then allows painless internal or external motion of the hip. In a positive test, the patient’s
pain will be aggravated with extension of the
hip and relieved in flexion. This test was not
useful in any of the current patients. More
commonly, the hip had pseudoparalysis and a
fixed flexion deformity. This may be inaccurate reporting, but in the current series, the authors were unable to show any passive motion
of the hip without causing severe discomfort
in four of the children.
The sedimentation rate and leukocyte count
were elevated in all of the patients in the current study and in more than 98% of patients reported in the literature. Preceding infection
within 1 month was reported in approximately
1
⁄3 of all patients in both the curent study and
those reported in the literature. (Table 2)
The most common misdiagnosis was septic
arthritis of the hip. Usually this resulted in an
262
Clinical Orthopaedics
and Related Research
Song et al
Fig 4. An algorithm for differentiation between psoas abscess and septic arthritis of
the hip is shown.
aspiration or arthrotomy of the hip. The most
effective imaging studies that were used most
frequently were ultrasound and CT (Fig 4),
which were equally effective in revealing the
diagnosis. Both have been used to guide percutaneous drainage. The other commonly used
study, the plain film radiograph, seldom was
useful although often informative in retrospect.
Treatment of the psoas abscess most frequently required incision and drainage and administration of intravenous antibiotics. With
the recent capability of interventional radiology, percutaneous drainage is becoming more
common with equally good results. One of the
patients with a large psoas abscess had
drainage of the abscess using ultrasound guidance. In two of the patients, only intravenous
antibiotics were used for treatment of small
psoas abscesses. The duration of intravenous
antibiotic coverage was variable. It varied
from 3 weeks to 6 months and was determined
by decreasing serial erythrocyte sedimentation rates and improving clinical responses.
Most of the patients were discharged from
followup 2 years after presentation without any
long-lasting effects of their illness. The only
exceptions were Patient 11, who continues to
be treated for tuberculous osteomyelitis and
spinal deformity, and Patient 7, who continues
to be observed for radiographic resolution of iliac sclerosis, although the infection resolved
after 9 weeks of intravenous antibiotics.
DISCUSSION
The exact pathophysiology of primary psoas
abscess remains unknown. The fact that most
cultures yield Staphylococcus aureus11 suggests abscesses likely arise by hematogenous
seeding from an occult cutaneous source.12,16,17
In this regard, primary psoas abscess resembles
tropical myositis, a well-known condition that
is seen more commonly in children who live in
tropical climates.16 Tropical myositis has been
reported to account for as much as 4% of all surgical admissions in some tropical countries6,14,28 and frequently yields Staphylococcus
aureus in cultures. Many of the cases of psoas
abscess are seen in patients from tropical climates, and one patient (Patient 7) in the current
series had immigrated to Canada from Jamaica
6 months before the diagnosis was made.
Number 391
October, 2001
Myositis in temperate climates is extremely
rare but has a propensity for the large muscles
around the hip.6,13,14 It is possible that psoas abscess is a variant of myositis but has become
recognized as an individual entity because of its
peculiar presentation.
The clinical presentation of psoas abscess
often causes confusion and delay in diagnosis. Most commonly, patients present with a
limp; however, the dysfunction of this joint is
extremely variable, ranging from complete
pseudoparalysis to completely free range of
motion. As Simons et al27 and Zadek34 described, the source of this confusion arises by
virtue of the multiple anatomic structures that
are related closely to the psoas muscle. An inflammatory process of the psoas muscle often
incites a sympathetic response involving these
surrounding structures. Thus, it is not unusual
for a child with a psoas abscess to present with
abdominal, genitourinary, spinal, or hip complaints. Because the psoas muscle is deep and
not examined easily, an abscess may become
quite large before it is clinically evident.17 This
difficulty in initial diagnosis is reflected in the
published case reports6,7,9,17,20,22,29 and in the
current series. The current study showed that
there are no unique features that might support
the diagnosis of psoas abscess. The presence of
a febrile illness may be acute or chronic and
may be accompanied by indolent symptoms of
several months’ duration,13,17 or fever may be
absent. A history of preceding trauma9,10 or infection20,27 is neither consistent nor reliable,
each occurring in only 35% of cases,5 which is
approximately the same frequency as in children who have septic hips. Physical examination often reveals what initially appears to be a
profoundly irritable hip, and understandably, a
diagnosis of septic arthritis usually is made.
Lam and Hodgson17 thought the presence of an
abdominal mass was a reliable indicator of
psoas abscess. This sign has not been shown
consistently in other series,11,16,21 including the
current series and especially in early psoas abscesses. Other clinicians have placed much emphasis on the psoas sign, which is done by flexing the hip and relieving tension in the psoas
Psoas Muscle Abscess in Children
263
muscle, thereby permitting painless passive
motion of the hip.9,18,22,23 For a child who has a
septic hip, it usually is impossible to move the
limb passively without pain, regardless of the
limb’s position. Although this may be a useful
way to differentiate the two, in practice, the relief of pain obtained by this maneuver, in the
authors’ experience, usually is minimal or
equivocal, especially in the younger child who
is in an overall general state of irritability and
apprehension. In other instances, the proximity
of the psoas abscess to the spine and peripheral
nerves has produced scoliosis, sciatica, and
femoral nerve neuropathy.12,18,22 The association of these symptoms with signs of septic arthritis is rare. The presence of these unusual
signs should favor the diagnosis of psoas abscess, rather than septic arthritis.
Blood test results in septic arthritis and psoas
abscess can be identical, with an elevated leukocyte count and elevated erythrocyte sedimentation rate.8,24,25 The use of C-reactive protein has
been shown to be helpful in following septic arthritis complicated with osteomyelitis,26,31 but it
is not helpful as a differentiating diagnostic tool.
The only laboratory tests that theoretically
might distinguish the two are muscle enzyme
creatinine kinase levels and urine myoglobin
analysis. Although it never has been reported in
cases of psoas abscess, there have been a few
cases of tropical myositis in which elevations
have been reported.2,28 Aspiration of the hip,
when positive, is helpful in confirming the diagnosis of septic arthritis but can be negative in
30% to 40% of the aspirations. None of these
initial examinations may differentiate significantly between psoas abscesses and septic arthritis. Occasionally, the plain film may show
an obscured sacroiliac joint, or the bone scan (as
in the authors’ second patient) may reveal compression of the bladder, which should alert the
clinician that this may not be a septic hip (Fig
2). The radiologic evidence of an effusion in the
hip, visible as joint space widening, does not
necessarily indicate septic arthritis of the hip because there often is a sympathetic sterile effusion associated with the local inflammation secondary to the psoas abscess.
264
Song et al
The diagnosis of psoas abscess usually can
be confirmed with ultrasound or CT. Ultrasound has been shown to be highly effective,18
not only in the psoas region, but for imaging
intramuscular abscesses in other parts of the
body.3 Involvement of bone, which also may
occur secondary to psoas abscess, would be
missed by ultrasound. In addition, if the ultrasonographer is not looking specifically for
retroperitoneal disease, an abscess may be
missed or obscured by gas or soft tissue.6,14
The authors’ preference has been a CT scan
with intravenous contrast (Fig 3), with ultrasound being used as a gross marker of progression, in guiding the needle aspiration. The
use of magnetic resonance imaging also has
been described7 and has a definite role in discriminating among abscess, hematoma, and
tumor if necessary.
Despite the wide variation in presentation,
abscess of the psoas muscle often resembles
septic arthritis of the hip. There are few reliable
historic, physical examination, or laboratory
clues that assist in this differential. Because a
delay in treatment is more serious in septic hips
than in psoas abscess, the safest approach
should be a hip aspiration to rule out joint sepsis. Treatment of psoas abscesses traditionally
combined a course of intravenous antibiotics
with extraperitoneal incision and drainage.
When clinically appropriate, less invasive regimens can be used with equal success. Percutaneous CT or ultrasound-guided needle aspiration with appropriate antibiotic coverage or
long-term intravenous therapy alone, with
close followup, also may provide good results,30 as was shown in the current series.
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