Computed tomography findings and surgical outcomes of dermoid sinuses: a case series

Abstract

Objective:
This case series describes the computed tomography (CT) and surgical findings of nine dogs with dermoid sinuses.

Methods:
Medical records were reviewed and summarised.

Results:
CT with intravenous contrast showed a superficial, peripherally contrast-enhancing, fluid-filled structure with a contrast-enhancing, soft tissue-attenuating tract that extended from the skin through the dorsal midline soft tissues, often to the level of the nuchal ligament. The tract often extended in an oblique direction to the nuchal ligament, but the tract ventral to the nuchal ligament, found at surgery, was not always discernible on CT. The dermoid sinuses were excised from all dogs. At surgery, the tract often extended ventral to the nuchal ligament, terminating on the vertebra. The most ventral aspect of the tracts involved a fibrous strand extension of the sinus, which was not discernible on CT.

Conclusion:
Preoperative CT facilitated the diagnosis of a dermoid sinus and directed surgical planning; however, careful dissection beyond the nuchal ligament is imperative for complete excision.

Full text

Computed tomography findings and surgical outcomes of dermoid
sinuses: a case series
C Appelgrein,* G Hosgood and SL Reese
Objective This case series describes the computed tomography
(CT) and surgical findings of nine dogs with dermoid sinuses.
Methods Medical records were reviewed and summarised.
Results CT with intravenous contrast showed a superficial,
peripherally contrast-enhancing, fluid-filled structure with a
contrast-enhancing, soft tissue-attenuating tract that extended
from the skin through the dorsal midline soft tissues, often to the
level of the nuchal ligament. The tract often extended in an
oblique direction to the nuchal ligament, but the tract ventral to
the nuchal ligament, found at surgery, was not always discernible
on CT. The dermoid sinuses were excised from all dogs. At sur-
gery, the tract often extended ventral to the nuchal ligament, ter-
minating on the vertebra. The most ventral aspect of the tracts
involved a fibrous strand extension of the sinus, which was not
discernible on CT.
Conclusion Preoperative CT facilitated the diagnosis of a der-
moid sinus and directed surgical planning; however, careful dis-
section beyond the nuchal ligament is imperative for complete
excision.
Keywords computed tomography; dermoid sinuses; dogs;
nuchal ligament
Abbreviations C1, first cervical vertebrae; C2, second cervical
vertebrae; C3, third cervical vertebrae; CT, computed tomography;
MRI, magnetic resonance imaging
Aust Vet J 2016;94:461–466 doi: 10.1111/avj.12521
Adermoid sinus is a congenital defect of neural tube
development that arises from abnormal separation of
the neural tube from the skin ectoderm during embryo-
genesis. The invaginated skin contains hair follicles and skin
glands, which may form an abscess and cause pain.
1,2
Dermoid
sinuses occur most commonly in Rhodesian Ridgebacks,
1
but
isolated cases have been reported in other breeds such as
Spaniels, Golden Retrievers, Yorkshire Terriers and Saint
Bernard’s.
2–4
The mode of inheritance has been investigated in the Rhodesian
Ridgeback. Dermoid sinuses are closely associated to the ridge
phenotype,
3–6
but the ridge gene is not exclusively responsible for
the presence of a dermoid sinus and other genetic factors also play a
role. The exact mode of inheritance is still unknown.
There are six types of dermoid sinuses categorised by the rela-
tionship between the sinus and supraspinous ligament/nuchal lig-
ament, and the presence of a skin orifice (Figure 1).
7
Three new
subtypes based on differences between the anatomical location
and the embryologic origin have also been classified. Subtype a
denotes a dermoid sinus on the dorsal midline in relation to the
vertebral column, subtype b denotes a dermoid sinus on the head
(excluding the nose) and subtype c denotes nasal dermoid
sinuses.
8
The presumptive diagnosis of a dermoid sinus is based on typical
clinical findings, particularly in a known affected breed, and includes
a palpable swelling over the dorsal midline with or without an asso-
ciated cutaneous fistula opening. The lesion can occur anywhere
from the nose
9
to the sacrum.
10
The term ‘fistula’is used because it
indicates any abnormal tube-like passage within body tissue; in this
context, the dermoid cyst and the skin. The term ‘sinus’only indi-
cates a recess cavity or channel.
11
Characterisation of the swelling via advanced imaging, including
ultrasound, computed tomography (CT) or magnetic resonance
imaging (MRI), is recommended. Fistulography with radiography
or CT is only possible in cases of a cutaneous opening, but it may
be incomplete or risk forcing contaminants into the spinal cord.
12
The diagnosis is confirmed by microscopic examination of excised
tissue.
*Corresponding author.
College of Veterinary Medicine, Murdoch University, Perth, Australia;
c.appelgrein@murdoch.edu.au
Figure 1. Schematic illustration of the six types (I–VI) of dermoid sinus
in dogs. s, skin; st, subcutaneous tissue; m, epaxial musculature; sl,
supraspinous ligament; v, vertebrae; sc, spinal cord and meninges into
the vertebral canal.(Reprinted with permission.
8
)
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Surgical planning is based on diagnostic imaging, but there are few
reports on the precision of imaging or how it relates to the surgical
findings The reporting of advanced imaging findings is limited to
single case reports, including three reports using MRI.
12–14
Although
CT has been performed in various cases reports, descriptions of the
imaging findings were not included nor how the CT findings com-
pared with the surgical findings.
7,12
The objective of this case series was to describe the CT findings and
corresponding surgical findings of nine dogs with a confirmed der-
moid sinus. Of interest was how definitively CT could describe the
depth of the sinus compared with that noted at surgery. Postopera-
tive outcomes were also reviewed.
Materials and methods
Medical records of dogs with suspected dermoid sinuses referred to
Murdoch University Veterinary Hospital between February 2011
and July 2015 were reviewed. Dogs were included if there was preop-
erative CT, a confirmed histological diagnosis of a dermoid sinus,
and postoperative follow-up of 4 weeks or more. Information
extracted from the records included signalment, treatment prior to
referral, CT findings, surgical findings and postoperative outcome.
Eight CT studies were acquired using a Siemens Somatom Emotion
16 (Erlangen, Germany) and one CT study was acquired with a Phi-
lips Brilliance 6 (New Delhi, India), with slice thickness ranging
between 0.75 and 2 mm with the dogs positioned in sternal recum-
bency under general anaesthesia. Images were obtained pre- and
post-intravenous contrast, after administration of 2 mL/kg Iohexol
300 mgI/mL (GE Healthcare, NSW, Aust). All images were re-
evaluated by a board-certified radiologist (S.L.R.) who was blinded to
the history and surgical findings, but aware that all cases were con-
firmed dermoid sinuses. Images were reviewed on a dedicated diag-
nostic imaging workstation (AGFA IMPAX, Mortse, Belgium) and
viewed in bone and soft tissue windows, with the ability to adjust
window level, window width and magnification as required. Imaging
findings recorded included how many tracts were present, the com-
munication of the tract at the level of the cutaneous margin, the
course of the tracts, the association of the tract with the nuchal liga-
ment, the termination of the tract and any evidence of spinal cord
involvement.
A board-certified surgeon performed all the surgical procedures. Sur-
gical findings gathered from the records included site of incision,
number of tracts and the course of the tracts, the appearance of the
tract as it reached and passed through the nuchal ligament, the most
ventral aspect of the tract and whether bone was excised at the ven-
tral aspect, closure with a closed suction drain, postoperative medi-
cations, postoperative complications (including seroma formation or
wound dehiscence) and histological assessment of the excised tissue.
The dermoid sinuses were classified based on the surgical findings.
Results
Of the nine animals included, there were seven Rhodesian Ridge-
backs, one Labrador Retriever and one Labrador cross (Table 1). The
dogs ranged in age from 4 months to 3 years (median, 7 months).
All dogs presented with a cutaneous fistula, located over the dorsum
in seven dogs and adjacent to the right pinna in two dogs. One dog
had four cutaneous openings, one dog had two and the remaining
four dogs had one cutaneous opening each.
Previous treatment prior to referral included antibiotics (6/9), previ-
ous drainage (3/9), non-steroidal anti-inflammatory drug (2/9) and
oral prednisolone (1/9).
CT findings
All the dermoid sinuses were evident on pre-contrast imaging but
contrast-enhanced delineation of the tract allowed the extent of the
tract to be followed further. Only the imaging findings on the post-
contrast CT are explained since these indicate the extent of the tract
that imaging is able to reveal.
Dog 1 had 4 tracts identified, but it could not be definitively estab-
lished if these tracts communicated (Figure 2). Dog 2 had 2 tracts that
clearly did not communicate (Figure 3). A total of 11 of the 13 tracts
found in seven dogs had the cutaneous opening located over the dor-
sal aspect of the cervical spine and two had the cutaneous opening
located immediately caudal to the right pinna. All tracts had a focal
concave defect in the cutaneous margin dorsal to the tract.
All tracts coursed in a ventral tangential direction, with 11 tracts fol-
lowing a caudoventral path and 2 tracts following a cranioventral
path. The 2 tracts in which the superficial aspect was caudal to the
right pinna coursed towards the midline in a caudoventral direction.
In seven dogs, the cutaneous opening was located over the dorsal
cervical neck. Of the 11 tracts, 7 were dilated dorsal to the nuchal lig-
ament and only 1 could be followed ventral to this dilated section,
passing through the nuchal ligament. The remaining 6 tracts (4 from
dog 1 and 1 each from dogs 2 and 6) had no evidence of passing
through the nuchal ligament. In dog 1, there was a focal area in the
nuchal ligament that did not have a central hypo-attenuating region.
The tract in dog 2 could be followed ventral to the nuchal ligament,
where it attached to the dorsal aspect of the second cervical vertebrae
(C2), cranial to the insertion of the nuchal ligament. In dog 6, it
appeared that the tract did not travel through the nuchal ligament
and had its ventral aspect at the caudodorsal margin of the C2 spi-
nous process, adjacent to the nuchal ligament insertion.
In the 4 tracts that were not dilated dorsal to the nuchal ligament,
there was a thickening of the nuchal ligament in 2 tracts on the left
and in 1 tract on the right lateral aspect of the nuchal ligament as
the tracts reached it; these tracts were described as being in direct
contact with the nuchal ligament. Dog 7 had no involvement of the
nuchal ligament.
The ventral aspects of the tracts were noted to be in various positions
along the midline for the sinuses over the dorsal cervical neck. The
most ventral aspect of the 4 tracts in dog 1 was dorsal to the nuchal
ligament. The most ventral aspect of the cranial tract in dogs 2 and
6 could be followed to C2. The tract in dog 7 was located within the
dorsal midline subcutaneous tissue and did not appear to extend to
the nuchal ligament. For the other 4 tracts, the most ventral aspect
was described as being close to the nuchal ligament: 2 were located
just ventral to the nuchal ligament and 2 were located lateral to the
nuchal ligament.
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Table 1. Signalment, physical examination findings, computed tomography (CT) and surgical findings of 9 dogs with dermoid sinuses
Dog Age
(months)
Breed Type &
number
of tracts
External
openings
Location
of
external
openings
CT findings Surgical findings Postoperative
complications
and drain
duration
1 36 Rhodesian
Ridgeback
IIa
4 tracts
4 Dorsal,
cervical
neck
Four tracts
extending to the
nuchal ligament
but not through.
Tracts did not
connect
Four tracts connected,
coursed through
the nuchal
ligament and
became one
dermoid sinus with
deepest aspect on
the caudal spinous
process of C2
Seroma/minor
wound
dehiscence
(7-day drain
duration),
drain
replaced
2 7 Rhodesian
Ridgeback
Ia/IIa
2 tracts
2 Dorsal
cervical
neck
Cranial tract
extends to C2
Caudal tract
extended
through the
nuchal ligament
and deepest
aspect ventral to
the nuchal
ligament
Cranial sinus:
consistent with
imaging
Caudal sinus:
extended through
the nuchal
ligament with
deepest aspect on
spinous process
of C3
Seroma
(4-day drain
duration)
3 4 Rhodesian
Ridgeback
IIa
1 tract
1 Dorsal
cervical
neck
Tract extended to
the nuchal
ligament
Extended through the
nuchal ligament
and deepest aspect
on dorsal spinous
process of C2
Seroma
(3-day drain
duration),
drain
replaced
4 6 Rhodesian
Ridgeback
IIIa
1 tract
1 Dorsal
cervical
neck
Tract extended to
the nuchal
ligament
Consistent with
imaging
Seroma/ minor
wound
dehiscence
(4-day drain
duration)
5 24 Rhodesian
Ridgeback cross
IIa
1 tract
1 Dorsal
cervical
neck
Tract extended to
the nuchal
ligament and
imaging
suspicion of
attachment to
C2
Dermoid sinus
extended through
the nuchal
ligament with
deepest aspect on
caudal spinous
process of C2
None reported
(6-day drain
duration)
6 30 Rhodesian
Ridgeback
IIa
1 tract
1 Dorsal
cervical
neck
Tract extended to
the nuchal
ligament
Tract extended
through the nuchal
ligament to insert
on C2
None reported
(5-day drain
duration)
7 8 Rhodesian
Ridgeback
IIIa
1 tract
1 Dorsal
cervical
neck
Tract extended to
cleidocervicalis
muscle
Tract extended to
nuchal ligament
Seroma
formation
(5-day drain
duration)
Drain replaced
8 7 Labrador ×Poodle IIa
1 tract
1 Caudal to
the
right
pinna
Tract extended to
the right dorsal
nuchal crest
along the
occipital bone
Sinus extended to the
dorsolateral margin
of C1
Seroma
(no drain)
9 22 Labrador Ib
1 tract
1 Caudal to
the
right
pinna
Tract extended to
the right dorsal
nuchal crest
along the
occipital bone
Consistent with
imaging
None reported
(no drain)
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The tracts with a cutaneous opening caudal to the right pinna in
dogs 8 and 9 were described as having the most ventral aspect
located at the nuchal crest. None of the 13 tracts appeared to have
any spinal cord involvement.
Surgical findings
All the surgeries were performed by a board-certified surgeon. In all
cases, an elliptical skin incision was made around the cutaneous
opening and the dermoid sinus was dissected to its most ventral
aspect. CT facilitated the course of the dissection to some degree and
gave an indication of the separation and direction of the tracts in
dogs with multiple tracts.
For the 11 tracts with a cutaneous opening over the dorsal neck,
9 coursed in a caudoventral direction to the level of the nuchal liga-
ment and 2 coursed in a cranial direction to the level of the nuchal
ligament. This was in agreement with the CT findings. In dog 1, the
tracts converged to form one dermoid sinus just dorsal the nuchal
ligament.
At the level of the nuchal ligament, 8 tracts remained, 6 of which
passed through the nuchal ligament. Dissection of the tract through
and beyond the nuchal ligament required en bloc resection of the
mid-portion of the nuchal ligament (Figure 4) and immediate care
in the dissection beyond the nuchal ligament because the tracts
immediately changed direction from caudoventral to cranioventral.
In 2 of 8 tracts the most ventral aspect was dorsal to the nuchal liga-
ment, 5/8 tracts were attached to the dorsal spinous process of C2
and 1/8 was attached to the dorsal spinous process of C3.
The 2 tracts with a cutaneous opening caudal to the right pinna
extended caudomedially along the periphery of the right temporal
muscle to the right dorsal nuchal crest in dog 7 and to the right lat-
eral aspect of C1 in dog 6, after which each tract tapered into a
fibrous strand at the level of the right nuchal crest.
The most ventral aspect of 8 of the dermoid sinuses was located on
bone. The ventral aspect required removal of a portion of bone with
the attachment intact to ensure elimination of the entire dermoid
sinus. Bone was removed with an osteotome for 3 tracts and with
ronguers for the other 5 tracts.
All the wounds were closed with apposition of the deep fascia (sim-
ple continuous, 2/0 polydioxanone), subcutaneous tissue (simple
continuous, 3/0 or 4/0 poliglecaprone 25) and skin (3/0 nylon).
Closed suction drains were placed in all seven dogs with dorsal cervi-
cal tracts and all dogs were hospitalised for the duration of drainage.
Drains were left in place for 3-7 days (median, 4 days); five dogs
developed a seroma after drain removal and three dogs developed
Figure 3. Sagittal post-contrast computed tomography image of a 7-
month-old neutered male Rhodesian Ridgeback with two distinct tracts
noted on computed tomography (dog 2). The contrast-enhancing cra-
nial tract extends to the caudodorsal aspect of the spinous process of
the second cervical vertebrae. The caudal tract extended ventrally with
its deepest aspect ventral to the nuchal ligament.
Figure 2. Sagittal post-contrast computed tomography image of a 3-
year-old neutered male Rhodesian Ridgeback with four cutaneous
openings over the dorsal cervical region (dog 1). The four openings cor-
responded to four subcutaneous tracts dorsal to the nuchal ligament.
At surgery, the four tracts joined to form a single sinus that coursed
ventral to the nuchal ligament and terminated on the caudal aspect of
the spinous process of the second cervical vertebrae.
Figure 4. Excised dermoid sinuses from a 7-month-old neutered male
Rhodesian Ridgeback (dog 2). The caudal sinus (top) shows the en bloc
excision of the nuchal ligament (arrow), the thin fibrous strand (X) and
its attachment to excised bone from the spinous process of the third
cervical vertebrae (block arrow). The cranial sinus (bottom) shows the
terminal dilated sinus with en bloc resection of its attachment to the
caudodorsal aspect of the spinous process of the second cervical ver-
tebrae (arrow).
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minor wound dehiscence that did not require intervention. Re-
drainage was performed in three dogs with no further complications.
The two dogs with cutaneous openings caudal to the pinna did not
have drains placed; one of these developed a small seroma that
resolved without intervention.
All dogs received non-steroidal anti-inflammatory medication, either
carprofen (Norbrook, VIC, Aust) 4 mg/kg once daily PO (6/9 dogs)
or meloxicam (Boehringer-Ingelheim, NSW, Aust) 0.1 mg/kg once
daily PO (3/9 dogs), and tramadol (CSL, VIC, Aust) 2 mg/kg twice
daily PO for 3-5 days postoperatively.
All the excised tissue was examined under light microscopy and was
confirmed as dermoid sinus. Microscopically, all tracts were lined by
well differentiated, keratinising squamous epithelium with hair folli-
cles, sebaceous glands and apocrine glands.
13
Relationship between CT and surgical findings
Based on the surgical findings, there were 10 discrete dermoid
sinuses in nine dogs: 1 type Ia, 6 type IIa, 2 type IIIa and 1 type
Ib. In dog 1, the tracts converged to form a single dermoid sinus at a
more ventral aspect, whereas in dog 2, the two tracts were two dis-
crete dermoid sinuses. Thus there were 10 dermoid sinuses with
13 tracts in nine dogs.
The CT images were used to confirm the diagnosis and outline the
direction of the tract. The images identified 10 dermoid sinuses in
nine dogs; in 7 of the sinuses, the CT delineations of the tract were
inconsistent with the surgical findings.
The CT images consistently delineated the dilated segment of the
dermoid sinus but were unable to depict the thin fibrous strand. The
involvement of the nuchal ligament was evident in many but not all
the CT studies, but if the sinus transitioned into a thin fibrous strand
at this level, it was not evident if the sinus continued ventral to the
nuchal ligament.
Discussion
For all nine dogs in this series, CT with intravenous contrast showed
a superficial, peripherally contrast-enhancing, fluid-filled structure
with a contrast-enhancing, soft tissue-attenuating tract that extended
from the skin through the dorsal midline soft tissues often to the level
of the nuchal ligament. This had similar features to a subcutaneous
fluid-filled structure and tract, consistent with a diagnosis of dermoid
sinus. Knowing the consistent imaging features of a dermoid sinus is
important, particularly for diagnosing atypical presentation of a
draining fistula lateral to the ear,
8
which may initially not be recog-
nised as a possible dermoid sinus, or for differentiating other subcu-
taneous masses on the midline. CT assisted delineation of the tract
but was unable to consistently indicate the ventral aspect of the tract.
The involvement of the nuchal ligament was clear in 6 of the
10 sinuses, but results indicated that it should not be mistaken for
the termination of the tract, even if the tract dilated dorsal to the liga-
ment, as noted in 5 of the sinuses. An additional confounding feature
was that the tract usually changed direction ventral to the nuchal lig-
ament and tapered into a thin fibrous strand.
Imaging and surgical findings were consistent for sinuses that did
not taper into a fibrous strand at surgery. All the types I and III
sinuses could be evaluated on CT images to their most ventral
aspect. The slice thickness of the CT may prohibit the identification
of the thin fibrous strand; therefore thin slices should be evaluated in
such cases.
14
In addition, the fibrous strand will not enhance after IV
contrast administration. It is not possible to classify the sinus prior
to surgery, but the assumption should be that the tract may have its
ventral aspect connecting to bone. In dog 5 the imaging assessment
suggested that the tract extended to its ventral aspect on C2, but was
unable to confirm this definitively. The tract did in fact course to the
caudodorsal aspect of C2 and the slice captured the tapering edge of
the fibrous strand.
In light of our findings, the purpose of the classification scheme pro-
posed by Mann in 1966
1
is unclear as it cannot be accurately
assigned prior to surgery and hence does not facilitate any decision-
making prior to surgery. It should be noted that the current descrip-
tions in the classification system are imprecise for sinuses over the
cervical spine.
8
In the cervical region, the nuchal ligament, which is a
continuation of the supraspinous ligament, is distant from the verte-
bral bodies. Six sinuses in our study had a ventral aspect extending
towards the vertebral bodies and thus traverse the nuchal ligament
rather than ending at the supraspinous ligament/bone interface as
the classification assumes. Several sinuses in this series had their
most ventral aspect connected to bone (Figure 4), which is not a fea-
ture in the current classification system. For typing to be relevant for
sinuses in the cervical region, inclusion of a type with the ventral
aspect on bone (either spinous process or vertebral lamina) would be
required (Figure 5). The value of the classification for surgical
planning, however, remains unclear.
MRI and surgical findings have been compared in previous case
reports.
15,16
MRI was valuable in confirming the presumptive
Figure 5. Schematic illustration of the authors’interpretation of the six
types (I–VI) of dermoid sinuses as they present in the cervical spine. The
separation between the nuchal ligament and the vertebrae complicates
the classification in the cervical spine. Type I extends down as a dilated
sinus to the periosteum, type II extends down to the periosteum via a
thin fibrous strand, type III terminates dorsal to the nuchal ligament.
Compare with Figure 1. (Original illustration by Darren K. Fung.)
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diagnosis and was able to delineate the sinus, but was unable to iden-
tify the most ventral extent of the tract. In one dog (1-year-old
female Rhodesian Ridgeback
15
), the tract could be visualised on
MRI, but its ventral extent could not be clearly defined. Surgery
tracked the sinus to the nuchal ligament where it was considered to
terminate. No follow-up was reported. In dog 2 in this previous case
report (5-month-old female Rhodesian Ridgeback), the tract could
be visualised on MRI, but the ventral extent could not be clearly dis-
cerned. Surgery tracked the sinus to the nuchal ligament where it
was considered to terminate.
5
In another case report (13-month-old female Rhodesian Ridgeback),
MRI identified the dermoid sinus, but again, the most ventral aspect
could not be clearly defined.
16
The T2-weighted images delineated
the tract to the level of the nuchal ligament. On the short tau inver-
sion recovery images, a hyper-intense tract appeared to extend cau-
dally to the level of the first thoracic vertebrae. During surgery the
tract could be followed to the nuchal ligament where it appeared to
terminate. Based on the MRI findings, surgical exploration was con-
tinued ventral to the nuchal ligament, but no tract was identified
ventral to the nuchal ligament. Excised tissue was not sent for histo-
logical examination. Complete healing was reported at 3 months
after surgery.
16
Dissection of the tract beyond the nuchal ligament is
quite difficult and necessitates a small en bloc excision of the nuchal
ligament at the connection point to facilitate further dis-
section without disturbing the sinus or thin fibrous strand, while
keeping in mind that the tract will change direction, as shown by the
findings of our case series. Attention to the dissection is imperative.
The most ventral aspect in most cases in our series is on bone and
for completeness; the point of contact should be excised en bloc with
ronguers or an osteotome (Figure 5). We did not identify any cases
in which the tract was continuous with the dura mater, although this
has been described.
11
We recommend that surgeons should be pre-
pared to perform a dorsal laminectomy if continuity with the dura
mater is obvious at surgery, despite absence of findings on preopera-
tive imaging.
In our series, seroma formation was a frequent complication despite
postoperative closed suction drainage and confinement. The high
mobility in the area of the dorsal cervical neck and deep dis-
section required to remove the entire tract are likely predisposing
factors and we made extensive attempts to avoid this complication.
The duration of drain placement varied and was not consistent with
seroma formation after drain removal. In the light of these findings,
a case by case judgement for duration of drainage is likely required.
Conclusion
In the nine dogs assessed in this study, CT assisted the diagnosis and
delineation of the tract, but was unable to consistently identify the
most ventral aspect of the dermoid sinus, particularly ventral to the
nuchal ligament. Many sinuses transition into a thin fibrous strand
at their most ventral aspect and terminate on bone. The sinuses fol-
low a typical tangential path in a caudoventral direction from the
cutaneous fistula in the region dorsal to the nuchal ligament, then
travel through the nuchal ligament, changing to a cranioventral
direction ventral to the nuchal ligament. Dissection through the
nuchal ligament requires en bloc resection to obviate disruption of
the sinus and facilitate the complicated dissection at this point where
it dilates, transitions into a thin fibrous and changes direction.
Acknowledgments
We thank Perth Veterinary Specialists for sharing the CT image
of dog 4.
Conflict of interest
None to report
Source of funding
None to report
References
1. Mann GE, Stratton J. Dermoid sinus in the Rhodesian Ridgeback. J Small
Anim Pract 1966;7:631–642.
2. Fatone G, Brunetti F, Lamagna A et al. Derrnoid sinus and spinal malforma-
tions in a Yorkshire terrier: Diagnosis and follow-up. J Small Anim Pract
1995;36:178–180.
3. Perazzi A, Berlanda M, Bucci M et al. Multiple dermoid sinuses of type Vb
and IIIb on the head of a Saint Bernard dog. Acta Vet Scand 2013;55: 62.
4. Hillbertz NHCS. Inheritance of dermoid sinus in the Rhodesian ridgeback.
J Small Anim Pract 2005;46:71–74.
5. Hillbertz NHCS, Andersson G. Autosomal dominant mutation causing the
dorsal ridge predisposes for dermoid sinus in Rhodesian ridgeback dogs.
J Small Anim Pract 2006;47:184–188.
6. Hillbertz NHCS, Isaksson M, Karlsson EK et al. Duplication of FGF3, FGF4,
FGF19 and ORAOV1 causes hair ridge and predisposition to dermoid sinus in
Ridgeback dogs. Nat Genet 2007;39:1318–1320.
7. Barrios N, Gómez M, Mieres M et al. Spinal dermoid sinus in a Dachshund
with vertebral and thoracic limb malformations. BMC Vet Res 2014;10:54.
8. Bornard N, Pin D, Carozzo C. Bilateral parieto-occipital dermoid sinuses in a
Rottweiler. J Small Anim Pract 2007;48:107–110.
9. Anderson DM, White RA. Nasal dermoid sinus cysts in the dog. Vet Surg
2002;31:303–308.
10. Volstad N, Tyrrell D, Beck J. Sacrocaudal dermoid sinus in three Rhodesian
Ridgeback dogs. Aust Vet Pract 2016;46:46–49.
11. Studdert V, Gay C, Blood D. Saunders comprehensive medical dictionary. 4th
edn. Saunders Elsevier, 2012.
12. Kiviranta AM, Lappalainen AK, Hagner K et al. Dermoid sinus and spina
bifida in three dogs and a cat. J Small Anim Pract 2011;52:319–324.
13. Gross TL, Ihrke PJ, Walder EJ et al. Skin diseases of the dog and cat: clinical
and histopathologic diagnosis. 2nd edn. Wiley, 2005.
14. Goldman LW. Principles of CT: multislice CT. J Nucl Med Technol 2008; 36:
57–68; quiz 75–76.
15. Rahal S, Mortari AC, Yamashita S et al. Magnetic resonance imaging in the
diagnosis of type 1 dermoid sinus in two Rhodesian ridgeback dogs. Can Vet J
2008;49:871–876.
16. Davies ESS, Fransson BA, Gavin PR. A confusing magnetic resonance
imaging observation complicating surgery for a dermoid cyst in a rhodesian
ridgeback. Vet Radiol Ultrasound 2004;45:307–309.
(Accepted for publication 17 January 2016)
Australian Veterinary Journal Volume 94, No 12, December 2016 © 2016 Australian Veterinary Association
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