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Diffuse Idiopathic Skeletal
Hyperostosis: Differentiation
From Ankylosing Spondylitis
Ignazio Olivieri, MD, Salvatore D’Angelo, MD,
Carlo Palazzi, MD, Angela Padula, MD, Reuven Mader, MD,
and Muhammad A. Khan, MD, FRCP, MACP
Corresponding author
Ignazio Olivieri, MD
Rheumatology Department of Lucania, Ospedale San
Carlo, Contrada Macchia Romana, 85100 Potenza, Italy.
E-mail: ignazioolivieri@tiscalinet.it
Current Rheumatology Reports 2009, 11:321–328
Current Medicine Group LLC ISSN 1523-3774
Copyright © 2009 by Current Medicine Group LLC
Diffuse idiopathic skeletal hyperostosis (DISH) and
ankylosing spondylitis (AS) share involvement of the
axial skeleton and peripheral entheses. Both diseases
produce bone proliferations in the later phases of their
course. Although the aspect of these bone prolifera-
tions is dissimilar, confusion of radiologic differential
diagnosis between the two diseases exists mostly as a
consequence of a lack of awareness of their character-
istic clinical and radiographic features. The confusion
may extend to the clinical eld because both advanced
DISH and advanced AS may cause the same limita-
tions of spinal mobility and postural abnormalities.
However, the radiologic spinal ndings are so differ-
ent that changes due to each disease can be recognized
even in patients in whom both diseases occur. This
article reviews the clinical and radiologic characteris-
tics that should help clinicians differentiate between
the two diseases without much dif culty.
Introduction
Diffuse idiopathic skeletal hyperostosis (DISH), also
known as ankylosing hyperostosis, is a totally differ-
ent disease from ankylosing spondylitis (AS), although
they share the involvement of the axial skeleton and the
peripheral entheses [1,2]. Both diseases produce bone pro-
liferation in the spine and at the extraspinal entheseal sites
in the later phases of their disease course. Although the
aspect of these bone proliferations is dissimilar, confusion
of radiographic differential diagnosis between the two
diseases exists mostly as a consequence of a lack of aware-
ness of their characteristic features [2,3]. In fact, Khan
[4] pointed out the error when a patient was reported
suffering from AS when the published radiographs in a
prestigious medical journal strongly supported the diag-
nosis of ankylosing hyperostosis.
In the past few years, it has been noted that the differ-
ential diagnosis between DISH and long-standing advanced
AS is not limited to the radiologic ndings and can extend
to the clinical aspects because DISH patients can occa-
sionally have severe limitation of spinal mobility, along
with postural abnormalities that resemble long-standing
advanced AS [5••]. Before examining the radiographic dif-
ferential diagnosis of AS and DISH, it would be proper to
brie y review the features of the two conditions.
Diffuse Idiopathic Skeletal Hyperostosis
DISH is characterized by calci cation and ossi cation of
soft tissues, mainly ligaments and entheses. This condi-
tion, described by Forestier and Rotes-Querol [6] more
than 50 years ago, was termed senile ankylosing hyperos-
tosis. The axial skeleton is often involved, particularly the
thoracic spine, but involvement of peripheral entheses led
to the term DISH [1,7]. The prevalence rates of the disease
range from 2.9% in Koreans to 27.3% in Caucasian men
in a European population [8–10,11•,12]. The disease is
observed mostly in the elderly with a male preponderance
[8–10,11•,12]. In a recent population-based study, 27.3%
of men and 12.8% of women older than 50 years old
were found to show evidence of DISH [12]. Its etiology is
unknown, but it is associated with various metabolic dis-
orders, especially obesity and insulin-dependent diabetes
mellitus [13–16].
Spinal involvement of DISH is characterized by “ ow-
ing mantles” of ossi cation occurring in the anterior
longitudinal ligament and to a lesser extent in the para-
vertebral connective tissue and the peripheral part of the
annulus brosus (Fig. 1) [1,17,18]. The ossi cation may be
limited to one section of the spine (ie, cervical, thoracic,
or lumbar), or it may affect more than one section. The
322 I Seronegative Arthritis
thoracic spine, especially in the middle and lower part, is
the most frequently involved section, followed by the lum-
bosacral spine and cervical spine [1,17,18]. Because the
anterior longitudinal ligament covers the anterior, as well
as the anterolateral aspects of the spine, its ossi cation
on anteroposterior radiograph of the spine may appear
as lateral ossi cations and bony bridging. The lateral
ossi cations or bridging are usually bilateral, but in the
thoracic region, they are either con ned to, or are more
prominent on, the right side of the spine [1,17,18]. The
predominant involvement of the left side of the spine in
individuals with situs inversus viscerum suggests that pul-
sation of the descending thoracic aorta may in uence the
location of ossi cation [19–21]. Frequently, radiolucency
is noted between the new bone and the anterior aspect of
the vertebral bodies on lateral view [1,6,17,18]. Radiolo-
lucent areas in the ossi ed bone mass may be noted at the
level of the intervertebral disk space, due to displacement
of disk material.
The ossi cation of the anterior longitudinal ligament
may be associated with ossi cation of the syndesmoses
connecting the laminae, spines, and transverse processes
(ie, ligamenta ava and supraspinous and interspinous lig-
aments). These ossi cations together with enthesopathies
of the zygapophyseal joints can produce stenosis of the
spinal canal [22]. DISH may also be associated with ossi-
cation of the posterior longitudinal ligament, especially
of the cervical spine, which can further aggravate the spi-
nal canal stenosis [23]. Some degree of ossi cation of the
syndesmoses of the vertebral arches and of the posterior
longitudinal ligament can occasionally also be observed
in patients with AS without any signs of DISH [24–27].
DISH may also affect the sacroiliac joints, which
can further result in being mistaken for sacroiliitis of
AS. The upper (ligamentous) portion of the joint may
show changes, such as vacuum phenomenon, narrowing,
sclerosis, and even partial or complete bony ankylosis
[2,3,28–31]. The lower two-third (synovial) part of the
joint is spared. However, ossi cation of the joint capsule
on the anterior surface of the joint may occur, resembling
the obliteration of the sacroiliac joints on anteroposterior
pelvic radiograph that may erroneously be interpreted as
postin ammatory ankylosis of the joint [2,3,28–32]. CT
can be useful in these cases by showing the normal aspect
of joint space and bony margins, and presence of the ante-
rior capsular ossi cations [30,31].
Extraspinal manifestations of DISH are frequent and
so characteristic to allow the recognition of the disease
even in the absence of proper spinal radiographs [32,33].
Even though any entheses can be involved, features are
especially common and distinctive at certain sites. On
pelvis radiographs, bone proliferation is seen on the
iliac crests, the ischial tuberosities, the pubis, lateral
acetabulum, and the greater and lesser trochanters (Fig.
2). Ossi cation of the sacrotuberous and iliolumbar
ligaments are further typical ndings. In the foot, the cal-
caneal insertions of the plantar fascia, the long plantar
ligament, and the Achilles tendon, and the insertions on
the navicular bone, medial cuneiform, and the base of the
fth metatarsal bones are frequently involved. Additional
frequent sites of bone proliferation include the attachment
of quadriceps femoris to the base of the patella, the inser-
tions of the ligamentum patellae on the patellar apex and
the tubercle of the tibia, and the insertions of the humeral
medial and lateral epicondyles.
In terms of clinical aspects, spinal involvement of DISH
has long been considered a radiographic entity with minor
and nonsigni cant clinical manifestations compared with
other spinal diseases. Generally, this is true. However,
patients with DISH can have marked limitations of spinal
mobility, and occasionally may have some spinal pain.
Mata et al. [34] found more frequent reduction in spinal
Figure 1. Lateral view of the lumbar spine showing fl owing mantles
of ossifi cation in the anterior longitudinal ligament extending from
the fi rst to the fourth vertebrae. A radiolucency (arrow) is visible
between the anterior aspect of the fourth vertebrae and the adjacent
bone proliferation.
DISH: Differentiation From AS I Olivieri et al. I 323
mobility and greater physical disability in patients with
DISH compared with healthy subjects. A recent study from
Italy and the United States reported that patients suffering
from DISH may have severe limitation of spine mobility
together with the typical postural abnormalities typical of
long-lasting advanced AS [5••]. This report emphasized
that the differential diagnosis between DISH and AS is
not restricted to radiologic ndings and extends to the
clinical aspects. For example, peripheral enthesopathy of
DISH is usually not as painful as peripheral enthesitis of
spondyloarthritis. The two conditions are also often easily
distinguished by painful and warm soft-tissue swelling in
peripheral enthesitis of spondyloarthritis. Recently, a man
presented with DISH, showing the postural abnormalities
of long-standing AS together with a diffuse swelling at
the insertion of the Achilles tendon resembling the typi-
cal fusiform soft-tissue swelling of Achilles enthesitis of
spondyloarthritis [35]. However, palpation of the region
did not reveal in ammatory ndings of enthesitis, but
a bony consistency of large spurs, also seen on radio-
graphs. This observation emphasized that the clinical
differential diagnosis between the two diseases extends to
peripheral enthesopathy. Other clinical manifestations of
DISH include dysphagia, hoarseness, stridor, myelopathy,
aspiration pneumonia, sleep apnea, atlantoaxial compli-
cations, and spine fractures [36].
The diagnosis of DISH is currently based on classi -
cation criteria that require the involvement of the spinal
thoracic segment. To differentiate DISH from AS and
degenerative disease, Resnick et al. [1] proposed the fol-
lowing criteria: 1) “ owing” ossi cation extending over
four contiguous vertebrae; 2) relative preservation of inter-
vertebral disc height in relation to age; and 3) absence of
apophyseal joint ankylosis or sacroiliac changes [7]. With
regard to the last criterion, the apophyseal joints may
show some narrowing, as well as hypertrophic alterations
and capsular ossi cation on conventional radiographs,
although there is no ankylosis of the joint per se in DISH.
Incidentally, similar changes can also be seen in costover-
tebral and costotransverse joints, resulting in limitation of
chest expansion in patients with advanced DISH [37]. We
have already discussed the sacroiliac joint changes seen in
patients with DISH.
According to Arlet and Mazières [18], the involvement
of three contiguous vertebral bodies at the lower thoracic
level is suf cient for diagnosing DISH. Julkunen et al.
[37] suggested that DISH can also be diagnosed when
bridging connects two vertebral bodies in at least two
sites of the thoracic spine. None of these criteria consid-
ers the involvement of peripheral entheses. Another set
of criteria suggested by Utsinger [38] for the diagnosis of
probable DISH lowered the threshold for spinal involve-
ment to three contiguous vertebral bodies and added the
presence of peripheral enthesopathy. Currently, a new set
of diagnostic criteria is needed to recognize milder forms
of the disease in the spine, those sparing the thoracic seg-
ments, and those beginning with peripheral enthesopathy
[39••,40]. The early recognition of the disease could allow
the management of the associated metabolic diseases in an
attempt to slow the progression of the disease to a more
advanced state [41].
Ankylosing Spondylitis
AS is an in ammatory disorder of the axial skeleton
involving the sacroiliac joints, the diskovertebral junction,
the apophyseal joints, and the costovertebral and costo-
transverse joints [2,42]. It occurs worldwide generally in
proportion to the prevalence of the HLA-B27 antigen [43].
The prevalence of AS varies between 0.10% to 0.87%,
and is even higher in some countries, such as China and
Turkey [44]. Symptoms usually start in the second and
third decades of life and rarely after age 40. The axial
distinctive radiographic ndings of disease evolve over
many years, with the earliest, most characteristic nd-
ings seen in the sacroiliac joints. The pathologic process
within these joints evolves over months or years and the
diagnostic ndings on conventional plain pelvic radio-
Figure 2. Anteroposterior view of the pelvis
showing bone proliferations of diffuse idio-
pathic skeletal hyperostosis (DISH) at the left
lesser trochanter and around the acetabuli,
more prominent in the left hip joint. A
bone bridge typical of DISH is visible at the
inferior part of both sacroiliac joints.
324 I Seronegative Arthritis
graphs emerge only several years after the onset of the
disease. In the preradiographic phase, in ammation can
now be demonstrated by MRI as cartilage abnormalities
and bone marrow edema [45]. The process consists of an
in ammatory chondritis and subchondral osteitis involv-
ing the iliac and sacral surfaces of the synovial (inferior
two-thirds) part of the joint. In ammation also involves
the ligamentous (superior one-third) of the joint. The
radiographic features of sacroiliitis in the synovial por-
tion are usually symmetric, consisting of subchondral
bony erosions and sclerosis, typically more evident and
severe on the iliac side because cartilage on the iliac sur-
face of the joint is thinner than that on the sacral side.
With progression of the erosions, the pelvic radiograph
may show pseudo-widening of the joint space. The early
in ammatory abnormalities in the upper third of the
joint are not as clearly demonstrated because of absence
of cartilage. With passage of time, the chondritis and
ligamentous in ammation in the sacroiliac joints results
in brosis, calci cation, and ossi cation, nally leading
to bony ankylosis of the joints. Radiographic sacroiliitis
is scored in clinical practice according to the New York
criteria [46].
In the spine, the in ammation at the diskovertebral
junction usually occurs at the attachment of the annulus
brosus. The subsequent adjacent subchondral osteitis
is radiologically characterized by a destructive vertebral
lesion and sclerosis con ned to the anterior corners of
the vertebral bodies (“Romanus lesion”). With the heal-
ing of these lesions, there is bony remodelling, together
with adjacent periosteal reaction, that results in “squar-
ing” of the vertebral bodies on lateral view of the spine.
At the same time, the healing process continues resulting
in gradual ossi cation in the periphery of the annulus
brous, as well as in the formation of syndesmophytes,
which are vertical bony bridges joining adjacent vertebral
bodies anteriorly and laterally to form a “bamboo-spine.”
The in ammatory process also involves the zygapophy-
seal, costovertebral, and costotransverse joints, slowly
resulting in their fusion and severe impairment of chest
expansion and spinal mobility. The ossi cation of the
interspinous and supraspinous ligaments can result in the
formation of a vertical radio-opaque stripe in the midline
on anteroposterior view of spinal radiograph, the so-
called “tram-track” and “dagger” signs [47]. Presence of
concomitant osteoporosis adds to the risk of development
of progressive spinal kyphosis.
McEwen et al. [48] described two different forms of
spondylitis. Primary AS and spondylitis associated with
in ammatory bowel diseases showed bilateral sacroiliitis,
symmetrical and marginal syndesmophytes, ligamentous
ossi cation, and progression of syndesmophytes from the
lumbar to the cervical spine. The spondylitis associated
with psoriasis and reactive arthritis was characterized more
often by asymmetrical ndings both in the sacroiliac joints
and the syndesmophytes, and, moreover, the syndesmoph-
ytes were mostly paramarginal rather than marginal.
Many radiologically detected alterations can also be
found at sites other than the sacroiliac joints and the spine.
These sites comprise sclerosis, erosion, and ankylosis of
the cartilaginous joints (ie, symphysis pubis, manubri-
osternal joint, and costosternal joints); erosion, joint space
narrowing, and bony ankylosis in the hip and shoulder
joints (peripheral joints are less frequently involved, espe-
cially in primary AS in developed countries); ossi cation
of different ligaments, including coracoclavicular, iliolum-
bar, sacrospinous, and sacrotuberous; and erosion and
new bone proliferation at different entheseal sites, more
often in the lower extremities, especially the heels [49••].
In general, the bone proliferative changes of enthesitis of
spondyloarthritis are ill-de ned, nely speculated, and
differ from the coarse and well-marginated nonin amma-
tory bony outgrowths of DISH [50]. The in ammation of
the entheses can be shown by MRI and ultrasonography
combined with power Doppler, and they can demonstrate
response to therapy [49••].
Differentiating Features
AS and DISH are two different diseases that could usually
be differentiated for several clinical features. Symptoms
of AS begin at a young age, usually late adolescence and
early adulthood, and consist of in ammatory spinal pain
and stiffness and decreasing range of spinal motion [42].
After many years, the illness can result in characteristic
postural abnormalities (eg, Bechterew stoop). In contrast,
DISH affects middle-aged and elderly persons and is often
asymptomatic, or is associated with mild dorsolumbar
pain and some restriction of spinal mobility [34].
From a radiologic point of view, the differential diagno-
sis between DISH and AS occurs when the two diseases are
in their later phases of evolution. Radiographs can demon-
strate erosive sacroiliitis, together with Romanus lesion and
squaring of vertebral bodies characteristic of patients with
AS. Differentiation between the two diseases has become
more important recently because of the marked ef cacy of
anti–tumor necrosis factor–α (TNF-α) therapy in patients
with symptomatic AS, even in an advanced state of the dis-
ease. Patients with DISH, especially those with impaired
spinal mobility and postural abnormalities, and ligamen-
tous ossi cation resembling bamboo spine, together with
narrowing and sclerosis in the upper ligamentous portion
of the sacroiliac joint and the capsular bridging obscuring
the joint space of the synovial part, can be erroneously
diagnosed as AS. Conversely, some AS patients with syn-
desmophytes, mainly of paramarginal type, and some
ossi cation of the anterior longitudinal ligament, may
be misdiagnosed as suffering from DISH if the sacroiliac
joint abnormalities are not characteristically evident [2].
However, additional analysis of the clinical and radiologic
characteristics helps differentiate the two diseases without
much dif culty (Table 1).
In the later phases of AS, both the ligamentous and syno-
vial parts of the sacroiliac joints show sclerosis, joint space
DISH: Differentiation From AS I Olivieri et al. I 325
narrowing, erosion, or fusion. In contrast, in DISH, only the
obliteration of the ligamentous portion and a mild narrow-
ing of the synovial part can occur, but erosions and bony
ankylosis are not observed. CT can easily demonstrate the
apparent obliteration of the synovial part on pelvic radio-
graph due to capsular ossi cation in DISH [30,31].
Syndesmophytes of AS represent ossi cation of the
peripheral layers of the annulus brosus. They connect
the angles of adjacent vertebral bodies and are usually
ne and marginal. In psoriatic spondylitis and spondylitis
associated with reactive arthritis, syndesmophytes can be
paramarginal. In contrast, spinal outgrowths of DISH
result from an ossi cation process involving the anterior
longitudinal ligament (Fig. 1). They are large and run
along the edges of the vertebral bodies and disc spaces.
A radiolucent line usually separates the ossi ed anterior
longitudinal ligament from the anterior aspect of the adja-
cent vertebral bodies.
Several other spinal radiographic changes are useful
in the differentiation of the two diseases. Although ossi-
cation of the posterior longitudinal ligament has been
described in AS [24–27], it is much more frequent in
DISH [23]. Apophyseal joint alteration—involving partial
or complete ankylosis of the zygapophyseal, costoverte-
bral, and costotransverse joints—is typically present in
advanced AS , whereas only some narrowing with hypert ro-
phic alterations and capsular ossi cation can be observed
in these joints in DISH. The ossi cation of interspinous
ligaments is also frequently seen in advanced AS, whereas
it only occasionally appears in DISH. Involvement of
symphysis pubis, marked by erosion, sclerosis, and bony
ankylosis similar to that observed in the sacroiliac joints,
can be seen in AS, whereas usually only bony bridging
occurs in DISH. Pelvic enthesopathy appearance can also
differentiate between the two diseases, with hypertrophic
whiskerings in DISH, whereas in AS the bone proliferation
is less evident and associated with bony erosion and scle-
rosis. These differences also extend to peripheral enthesis
involvement. The hip joints are also frequently involved in
AS, with concentric joint space narrowing, erosion, and
bony ankylosis. In DISH, only prominent enthesophytes,
often limiting joint mobility, are observed.
Coexisting Diffuse Idiopathic Skeletal
Hyperostosis and Ankylosing Spondylitis
As DISH and AS are not rare diseases, they can occur in
the same individual by chance [8–10,11•,12,44]. So far,
only 16 patients with the coexistence of the two diseases
have been reported in the English literature [51–62]. The
radiologic ndings of axial involvement of AS and DISH
are so different that, in patients with this coexistence, it is
possible for experts to recognize changes caused by each
of the two diseases at any level. For example, one of the
patients reported by Rillo et al. [54] and one by Olivieri
et al. [58] showed concomitant ndings typical of AS
Table 1. Distinguishing features of DISH and AS
DISH AS
Usual age of onset > 50 y < 40 y
Dorsal kyphosis Frequent Very frequent
Limitation of spinal mobility Frequent Very frequent
Pain Unusual Very frequent
Limitation of chest expansion Frequent Very frequent
Roentgenography
Hyperostosis Very frequent Frequent
SI joint erosion Absent Very frequent
SI joint (synovial) obliteration Unusual Very frequent
SI joint (ligamentous) obliteration Frequent Very frequent
Apophyseal joint obliteration Absent Very frequent
ALL ossifi cation Very frequent Unusual
PLL ossifi cation Very frequent Frequent
Syndesmophytes Absent Unusual
Enthesopathies (whiskering) with erosions Absent Very frequent
Enthesopathies (whiskering) without erosions Very frequent Frequent
HLA-B27 (European “whites”) [63,64] About 8% About 90%
HLA-B27 (African Americans) [63,64] About 2% About 50%
ALL—anterior longitudinal ligament; AS—ankylosing spondylitis; DISH—diffuse idiopathic skeletal hyperostosis; PLL—posterior longitudi-
nal ligament; SI—sacroiliac.
326 I Seronegative Arthritis
(sacroiliac joint erosions, joint space narrowing, sclerosis,
and ankylosis) and DISH (anterior capsular bridging) on
CT scans of the sacroiliac joints. Lastly, it is important
to point out that, unlike in AS, there is no association of
HLA-B27 with DISH (Table 1) [2,63,64].
Conclusions
DISH and AS are two completely different diseases that
happen to share the involvement of the axial skeleton and
the peripheral entheses, resulting in bone proliferations
in the spine and at the extraspinal entheseal sites in the
later phases of their course. Although the aspect of these
bone proliferations is dissimilar, confusion of radiologic
differential diagnosis between the two diseases exists
mostly as a consequence of a lack of awareness of their
characteristic clinical and radiographic features. The con-
fusion may extend to the clinical eld because advanced
DISH can present the same limitation of spinal mobility
and postural abnormalities as those seen in patients with
advanced AS. However, the radiologic ndings of axial
involvement of the two diseases are so different that it is
possible to recognize changes caused by each disease at
any level, even in patients in whom they may co-occur.
Correct diagnosing of a disease is the primary goal in
medicine. It is a conditio sine qua non for properly treat-
ing patients, and avoiding ineffective, unnecessary, and
risky therapies. The management of AS is vastly differ-
ent from that of DISH. The differentiation between the
two diseases has become even more important these days
because of the marked ef cacy of anti–TNF-α therapy in
patients with symptomatic AS, even in an advanced state
of the disease. Conversely, anti–TNF-α therapy has no
role in the management of patients with DISH. Neverthe-
less, some of us have been referred patients diagnosed as
having AS because they failed to respond to anti–TNF-α
therapy, when in fact they were suffering from DISH.
DISH, especially in those patients with impaired spi-
nal mobility and postural abnormalities, and ligamentous
ossi cation resembling bamboo spine, together with nar-
rowing and sclerosis in the upper ligamentous portion
of the sacroiliac joint and capsular bridging obscuring
the joint space of the synovial part, can be erroneously
diagnosed as AS. Conversely, some cases of AS in patients
with syndesmophytes, mostly of paramarginal type seen
mainly in psoriatic spondylitis, with some ossi cation of
the anterior longitudinal ligament, may be misdiagnosed
as DISH if the sacroiliac joint abnormalities are not char-
acteristically evident [2]. However, additional analysis of
the clinical and radiologic characteristics, as discussed in
this review, should help clinicians differentiate the two
diseases without much dif culty.
Disclosure
No potential con icts of interest relevant to this article
were reported.
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DISH: Differentiation From AS I Olivieri et al. I 327
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