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Israel Journal of Veterinary Medicine  Vol. 69 (3)  September 2014 151 Hypertrophic Osteopathy in a Horse
INTRODUCTION
Hypertrophic osteopathy has been reported in multiple spe-
cies, including humans, dogs, and horses, but is considered
uncommon in equine patients. Diagnosis of hypertrophic
osteopathy in horses is typically based on physical examina-
tion fndings of bony enlargement in multiple distal limbs
and radiographic evidence of diaphyseal and/or metaphy-
seal periosteal proliferation. Lesions are most commonly de-
scribed on the metacarpal and metatarsal bones, phalanges,
and distal radius and tibia. Tere have also been reports of
bony proliferation involving the mandible, maxilla, and nasal
bones, either alone or in combination with distal limb long
bones (1, 2). Te inciting cause for the new bone growth is
not always identifed, but hypertrophic osteopathy has been
associated with intrathoracic pathology, such as neoplasia or
granulomatous disease, and less commonly, lesions involving
other organ systems, such as ovarian granulosa cell tumors
and gastric squamous cell carcinoma (2-4).
CLINICAL CASE REPORT
A three-year-old female intact Quarter Horse was donated
to the Veterinary Health Center at Kansas State University
Hypertrophic Osteopathy in a Tree-Year-Old Quarter Horse Mare
Bayless, R., Almes, K., Choudhary, S., Beard, W., Garcia, E. and Biller, D.
Kansas State University College of Veterinary Medicine, Manhattan, KS.
Kansas State Veterinary Diagnostic Laboratory-Department of Diagnostic Medicine/Pathobiology (Almes, Choudhary) and
Department of Clinical Sciences (Beard, Biller, Garcia).
* Corresponding Author: Rosemary Bayless, 2150 Georgetown Road, Lexington, KY 40511. Email: RBaylessDVM@gmail.com
ABSTRACT
A three-year-old female intact Quarter Horse with a history of bony enlargement of the mandible and
both radii, third metacarpal bones, and third metatarsal bones was donated to a university teaching hospital
for euthanasia. Mild weight loss of several months duration and a previous wound near the site of the
mandibular swelling were also reported. Tere were no signifcant fndings on physical examination except
for the frm enlargement of the left mandible and distal limbs. She was non-reactive to palpation of the
swellings and did not appear lame at the walk, although a complete lameness examination was not performed.
Radiographs revealed bilaterally thickened cortices of the third metacarpal and metatarsal bones and
periosteal proliferation along the ventral portion of the left mandible immediately caudal to the mandibular
symphysis. Toracic radiographs were unremarkable. On post-mortem exam, the only gross abnormalities
noted were difuse fbrous adhesions between the pericardium and the epicardial surface of the heart and
bilaterally symmetrical frm bony proliferation hemi-circumferentially around the distal radii, along the
length of the third metacarpal and metatarsal bones, and on the rostral left mandible. Histopathology of
the heart revealed fbrous tissue deposition along the epicardial surface with small multifocal infltrates of
lymphocytes and plasma cells. Microscopic examination of the afected mandible and third metatarsal bone
were consistent with periosteal proliferation consisting of bony and fbrous tissues. Hypertrophic osteopathy
is uncommonly diagnosed in horses and has been most commonly described secondary to intrathoracic
lesions such as infectious or neoplastic processes. In this case, the development of periosteal proliferation
may have been a consequence of a previous episode of pericarditis.
Keywords: Hypertrophic Osteopathy, Pericarditis, Quarter Horse, Equine.
Israel Journal of Veterinary Medicine  Vol. 69 (3)  September 2014 Bayless, R. 152
(KSU-VHC) with a history of bony enlarge-
ment of the left side of the mandible and
both radii, third metacarpal bones, and third
metatarsal bones. A referring veterinarian
had examined the horse two months previ-
ously for an old wound on the left mandibular
body and noted the bilateral metacarpal and
metatarsal enlargement. Te owners reported
no recent changes in the appearance of the
horse’s limbs. Since the initial evaluation by
the referring veterinarian, the horse had lost
weight but the bony proliferation appeared
unchanged.
On presentation, the patient was quiet
and had normal vital parameters except
for mild tachypnea (Temperature: 36.7°C,
Pulse: 40 bpm, Respiration: 28 bpm). Both
metacarpal and metatarsal regions were
noticeably enlarged, as were the distal ra-
dii (Figures 1a, 1b) and the left side of the
mandible. Tere was no longer a visible
skin wound in the area of the mandibular
enlargement. She was not sensitive to pal-
pation of the bony swellings on her limbs
or mandible and did not appear to be lame,
although a full lameness examination was
not performed. No abnormalities were noted
during a rectal examination.
Radiographs by the referring veterinar-
ian showed thickened cortices with pali-
sading bony proliferation. Upon presen-
tation to KSU-VHC, skull and thoracic
radiographs were taken to evaluate the
mandibular osseous changes and check for
primary intrathoracic disease. Te dorso-
ventral view of the skull showed thickening
of the rostral left hemimandible just caudal
to the mandibular symphysis (Figure 2a).
Te borders of the new bone were smooth,
and there was minimal soft tissue swelling.
On the lateral view, there was a mineral opacity bulge
ventral to the mesial root of the mandibular third pre-
molar. A lucent tract appeared to extend from the alveolar
bone immediately adjacent to the tooth down to just be-
neath the skin surface (Figure 2b).
No abnormalities were noted on the thoracic flms, and
the lower respiratory tract appeared to be within normal lim-
its. Humane euthanasia was performed as was planned at
time of donation, and no further diagnostics were performed
prior to death. A post-mortem examination was performed
several hours after euthanasia.
Figure 1: Photographs upon presentation of a) distal front limbs and b) distal hind limbs
showing bilateral gross bony enlargement of metacarpal and metatarsal diaphyses and distal
radii (arrows).
Figure 2: Radiographs of the skull a) dorsoventral view (arrowheads indicate focal
thickening of left hemimandible) and b) lateral view (arrow indicates bony swelling
containing lucent tract)
Case Reports
Israel Journal of Veterinary Medicine  Vol. 69 (3)  September 2014 153 Hypertrophic Osteopathy in a Horse
GROSS LESIONS
Te pericardium was difusely adhered to the epicardial sur-
face of the heart, and the two layers could not be separated
from one another without tearing the fbrous tissue. Tere
was no signifcant fuid contained in the pericardial sac. After
removal of the pericardial tissue, plaques of fbrous tissue
remained on the surface of the heart (Figure 3). No exten-
sion of fbrous tissue or signs of infammation were observed
grossly in the myocardium or endocardium.
Tere was marked hemi-circumferential frm expansion
of both distal radii, both third metacarpal diaphyses and both
third metatarsal diaphyses. Te left hind limb was removed
immediately distal to the tarsometatarsal joint, and a cross-
section was taken from around the level of the middle one-
third of the metatarsal diaphysis. Tis cross-section dem-
onstrated extensive bony proliferation as well as exuberant
fbrous tissue which contained small islands of mineralized
bone. Te right hind limb was removed proximal to the tarsus
for radiographic evaluation.
Te rostral left hemimandible was signifcantly enlarged
when compared to the right hemimandible; this marked
asymmetry was emphasized when the skin and soft tissues
were removed (Figure 4a). On cross-section of the mandible
around the level of the second premolar, the extent of new
bony proliferation and partial obliteration of distinct corti-
comedullary architecture were evident (Figure 4b).
No abnormalities were noted on gross examination of
the trachea, lungs, great vessels, liver, spleen, kidneys, adrenal
glands, gastrointestinal tract, uterus, or ovaries.
MICROSCOPIC LESIONS
Histopathology of the epicardium and superfcial myocar-
dium revealed a layer of fbrous tissue tightly adhered to the
epicardial surface. Tis fbrous tissue varied from densely-to
loosely-arranged and contained multifocal infltrates com-
Figure 3: Heart with pericardial tissue removed demonstrating
difuse fbrous tissue adhered to the epicardium.
Figure 4: Bony proliferation of left hemimandible a) ventral surface of mandible with skin and soft tissues removed (arrowheads indicate
approximate margins of bony enlargement) and b) cross-section of mandible at approximately the level of the second premolar (arrows indicate
original periosteal margin)
Case Reports
Israel Journal of Veterinary Medicine  Vol. 69 (3)  September 2014 Bayless, R. 154
posed of small to moderate numbers of lymphocytes and
plasma cells. A diagnosis of lymphoplasmacytic epicardial
fbrosis was made based on these results along with the gross
fndings.
Microscopic examinations of the left hind metatarsal
cross-section and mandibular enlargement were performed
following decalcifcation. Portions of the new bone forma-
tion on the dorsal surface of the third metatarsal bone formed
projections of trabecular bone surrounded by dense fbrous
tissue. Tere were voids within the proliferative bone that
resembled medullary-like areas that contained adipose tissue
but no evidence of myeloproliferative tissue.
Te histopathology of the mandibular mass showed three
distinct layers composed of original compact cortical bone of
the mandible, proliferative trabecular bone, and dense fbrous
tissue. Te new trabecular bone on the mandible was similar
to the new metatarsal bone but appeared slightly more orga-
nized. Te histological appearance of tissue sections from the
distal limb and mandibular bony lesions were consistent with
a morphologic diagnosis of moderate, multifocal periosteal
hyperostosis.
FURTHER DIAGNOSTICS
Radiographs of the right hind distal limb and left hind meta-
tarsal cross-section were obtained post-mortem. Te meta-
tarsal cross-section radiograph (Figure 5a) closely resembled
the gross appearance of the cut section (Figure 5b) and was
characterized by signifcant smooth hemi-circumferential
periosteal new bone formation on the medial, dorsal, and
lateral aspects of the third metatarsal bone. Te dorsal sur-
face of the bony proliferation featured irregularly-marginated
mineral opacities separated from the main portion of bone
by thin curvilinear lucencies. Tere was also thickening of
the soft tissue surrounding the metatarsal bones, especially
on the medial side of the leg, and the soft tissue itself had
irregular edges. Te second and fourth metatarsal bones ap-
peared within normal limits.
Te irregular bony margins of the right hind metatarsi
can be visualized on the lateromedial (lateral) radiograph
(Figure 6a), and periosteal proliferation was also evident on
the dorsoplantar view (Figure 6b). Te smooth new bone
formation appeared to stretch from immediately distal to
the tarsometatarsal joint to the distal metaphyseal region
of the third metatarsal bone, but there was no radiographic
evidence of articular involvement. On the lateral and dor-
soplantar views, the dorsal, medial, and lateral cortices were
markedly thickened, and the medullary cavity appeared
subjectively narrowed in the mid-diaphyseal region. Te
majority of the overlying irregularly-marginated mineral
opacity was centered on the proximal two-thirds of the third
metatarsal.
Figure 5: Left hind metatarsal cross section a) radiograph and b) photograph (arrows indicate normal periosteal margin)
Case Reports
Israel Journal of Veterinary Medicine  Vol. 69 (3)  September 2014 155 Hypertrophic Osteopathy in a Horse
DISCUSSION
Tis report describes a case in which a presumptive ante-
mortem diagnosis of hypertrophic osteopathy was made in
a horse based on history, physical examination fndings, and
skull and distal limb radiographs. Te gross and histopathol-
ogy lesions from the post-mortem examination were also
consistent with hypertrophic osteopathy. Since hypertrophic
osteopathy is often secondary to an intrathoracic lesion, the
fbrous pericarditis appears to be the inciting cause in this
horse.
Te horse was originally examined by the referring vet-
erinarian for mandibular swelling, and the duration of distal
limb bony enlargement prior to presentation was not clear.
Te patient also had no reported history of clinical signs
consistent with pericarditis, such as pyrexia, depression, or
inappetence, so the chronological relationship to the devel-
opment of the bony proliferation, and thus the signifcance
of this lesion as a potential etiology for her hypertrophic
osteopathy, was unknown. Te weight loss observed by the
owners may have been associated with the hypertrophic os-
teopathy disease process or may have occurred independently
due to a number of diferent factors, including diet, exercise,
and gastrointestinal parasitism.
Te bilateral and symmetrical nature of the distal limb
periosteal proliferation was consistent with descriptions of
typical hypertrophic osteopathy lesions. Te asymmetry of
the mandibular lesion was uncharacteristic of hypertrophic
osteopathy, suggesting a separate pathologic process from
the periosteal proliferation observed in the distal limbs. Te
history of a wound on the mandible corresponding to the
area of bony swelling and radiographic evidence of a previ-
ously draining tract presented another possible etiology for
the mandibular new bone production: local infammatory
response to trauma.
Te pathophysiology of hypertrophic osteopathy is not
completely understood. Teories generally involve an initial
increase in blood fow to the distal limbs via vasodilation and
subsequent proliferation of connective tissue leading to bone
deposition (1, 5). Proposed mechanisms of increased circu-
lation include increased parasympathetic tone due to vagal
nerve stimulation, a hypothesis that has been supported by
the clinical improvement in some human patients after va-
gotomy or infusion of local anesthetic next to the vagal nerve
(5). Others suggest that pulmonary vascular shunting arising
from lung pathology results in increased amounts of vasoac-
tive substances, such as growth factors, that would otherwise
be inactivated or removed in pulmonary capillaries. Te pos-
sible role of hormonal factors has also been investigated, but
estrogen levels in horses have generally been within normal
limits in the limited cases in which they were measured (2).
Te results of some studies suggest that equine hyper-
trophic osteopathy tends to be diagnosed more commonly
in young adult males, but horses of varying age and gender
appear susceptible and small sample sizes of published data
pose risk for over-interpretation of possible signalment risk
factors. Afected horses commonly present with histories of
weight loss and may also show other signs of systemic dis-
ease, such as fever, coughing, or dyspnea. Te bony swellings,
which are typically symmetrical and present on all limbs,
can range from warm and sensitive to palpation, suggestive
of active infammation, to cool and apparently painless (2).
Lameness and reduced range of motion in afected limbs
have been reported but are not consistent in all cases (1, 6).
Radiographs of the afected limbs typically display pali-
Figure 6: Radiograph of right hind metatarsal; a) lateromedial view, b)
dorsoplantar view (asterisk shows cortical thickening, arrow indicates
irregularly marginated periosteal proliferation).
Case Reports
Israel Journal of Veterinary Medicine  Vol. 69 (3)  September 2014 Bayless, R. 156
sading new periosteal bone formation, but more chronic
lesions may have smooth margins. Te diaphyses and me-
taphyses of distal limb long bones are the most common
locations for bony proliferation, as was observed in this horse.
Although lesions often extend to the metaphyses, articular
surfaces are generally not involved. When bony swellings as-
sociated with the skull are present, they also may have either
smooth or spiculated edges (1). Te use of nuclear scintigra-
phy to confrm increased bone turnover in the areas around
palpable lesions has also been described (7).
Tough additional diagnostics beyond physical examina-
tion and radiographs may not be necessary for diagnosis of
hypertrophic osteopathy, they can provide information about
the possible etiology of the periosteal proliferations. Clinical
pathology results are often nonspecifc, but the most com-
monly reported fndings include a leukocytosis characterized
by mature neutrophilia and hyperfbrinogenemia. Toracic
radiographs, bronchoalveolar lavage, transtracheal wash, and
airway endoscopy may reveal a source of pulmonary disease
contributing to the bony proliferation. Abdominal ultra-
sound, abdominocentesis, and rectal palpation can be used
to assess the likelihood of a peritoneal lesion or an ovar-
ian tumor (1, 2). Te lack of signifcant fndings on thoracic
radiographs and rectal palpation in this horse was consis-
tent with the absence of gross lesions observed in the lungs,
peritoneal cavity, and retroperitoneal space during the post-
mortem examination.
Te prognosis of patients with hypertrophic osteopathy
often depends on the underlying condition, if any can be
identifed, responsible for the bony proliferation. If the pri-
mary cause can be addressed, the bony enlargements may re-
gress, as has occurred in some cases of hypertrophic osteopa-
thy thought to be associated with infammatory or infectious
disease (2, 8). Many horses are euthanized soon after diagno-
sis without attempting treatment, while other non-survivors
are euthanized several months later after a deterioration of
their condition. In horses for which a primary condition was
not determined, resolution was sometimes seen following
symptomatic therapy with non-steroidal anti-infammatory
drugs (2).
While there was no history suggestive of clinical pericar-
ditis in this horse, the fbrous tissue adhering the pericardium
to the epicardium is indicative of previous infammation. A
review of the literature reveals one other report of equine
hypertrophic osteopathy attributed to fbrous pericarditis and
epicarditis. A Saddlebred gelding had a history of intermit-
tent pyrexia prior to development of distal limb bony en-
largement but was afebrile with normal thoracic auscultation
when he was donated to another teaching hospital. As was
the case for the Quarter Horse mare described in this report,
no cardiac abnormalities were noted on thoracic radiographs
of the gelding prior to euthanasia, but gross evidence of f-
brous pericarditis and epicarditis was observed during the
post-mortem exam (7).
Although hypertrophic osteopathy is uncommon in
horses and generally associated with a primary condition,
it should be considered as a diferential for bilaterally sym-
metric bony enlargement afecting multiple limbs even in the
absence of clinical signs or ante-mortem diagnostic fndings
of clinical disease afecting other body systems.
ACKNOWLEDGEMENTS:
Special thanks for assistance and images: Dr. Kelli Almes, DVM,
DACVP; Dr. Shambhunath Choudhary, DVM; PhD., KSU-
VHC Radiology Service, Kansas State Veterinary Diagnostic
Laboratory-Histopathology Section; Dr. Sarah Keiser Czarnick,
DVM.
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Case Reports