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Thymopharyngeal Duct Cyst
An Unusual Variant of Cervical Thymic Anomalies
Matthew R. Kaufman, MD;
Shane Smith, MD;
Michael A. Rothschild, MD;
Peter Som, MD
Arch Otolaryngol Head Neck Surg. 2001;127:1357-1360.
ABSTRACT
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Background The thymus develops from the third pharyngeal pouch and descends from
the neck into the anterior-superior mediastinum. Thus, it is possible to have
thymic remnants in the neck, which most often present as a cervical mass during
childhood. One type of cystic thymic remnant is the thymopharyngeal duct cyst,
a remnant of one of the paired tracts of embryological thymic descent. Thymopharyngeal
duct cysts are rare lesions that can have a similar presentation to more commonly
encountered childhood neck masses.
Objectives To review the embryological development of cervical thymic remnants
and to report our experience with the thymopharyngeal duct cyst.
Design Case series.
Setting Tertiary care center.
Patients Two children who presented with asymptomatic neck masses that were caused
by cystic remnants of the thymopharyngeal duct.
Results Both patients underwent preoperative computed tomography, which revealed
a multiloculated mass coursing adjacent to the carotid sheath. Surgical treatment
was the definitive therapy for both patients, although neither patient had
a definitive preoperative diagnosis. In both cases, the mass was approached
through an incision anterior to the sternocleidomastoid muscle, and dissection
proceeded along the length of the carotid sheath. A fibrous cord extending
into the mediastinum was found in both patients. There were no postoperative
complications. Histopathologic evaluation revealed the presence of mature
thymic elements within the wall of a multiloculated cyst.
Conclusions Thymopharyngeal duct cysts must be considered in the differential diagnosis
of pediatric neck masses. Computed tomography is helpful to delineate the
relationship to the carotid sheath. Complete surgical excision is the appropriate
therapy in a majority of cases, with minimal morbidity when careful attention
is paid to vital structures.
INTRODUCTION
THE WORD thymus derives from the Greek thymos, meaning "soul" or "spirit." Galen theorized that
the function of the thymus was that of a cushion, to protect the mediastinal
vessels from the overlying sternum.1 It was
not until the late 1700s that an association was suggested between the development
of the lymphatic system and the thymus. It is currently accepted that the
thymus serves as a central lymphoid organ that is responsible for maturation
of T lymphocytes and induction of self-tolerance.1
The thymus is derived primarily from the third pharyngeal pouch in association
with the inferior parathyroid glands, with a small contribution from the fourth
pharyngeal pouch. Beginning in the sixth gestational week, the right and left
portions of the thymic primordium separate from the pharynx and descend into
the anterior-superior mediastinum along paired thymopharyngeal stalks. The
path of descent involves migration deep to the thyroid gland and sternocleidomastoid
muscle along the carotid sheath. In normal development, the inferior portions
of the thymopharyngeal stalks enlarge, while the proximal portions form epithelial
cords that eventually atrophy.
Cervical remnants of thymic tissue have been found in up to 30% of infants
studied at autopsy.2 When these remnants present
clinically as a mass in the neck, it is important to consider the range of
variants that exist, based on the embryological path of development. Zarbo
et al3 categorized 7 cervical thymic lesions,
distinguishing them by location and by whether the tissue was solid or cystic.
Whereas accessory cervical thymus and cervical thymic cyst variants represent
more than half of all cervical thymic anomalies, persistent thymopharyngeal
duct cysts represent only 7% of these fetal remnants.4
We describe 2 patients with lateral neck masses composed of cystified
remnants of the thymopharyngeal duct.
REPORT OF CASES
CASE 1
A 9-year-old boy was referred for evaluation of a mass in the left side
of the neck that had been apparent for 4 months. He was asymptomatic and had
no comorbidities. On physical examination, a ballotable, nontender mass with
indistinct borders was noted in the left side of the neck, extending from
the level of the hyoid to the clavicle. The carotid pulse was palpable and
transmittable through the mass. A contrast-enhanced computed tomographic scan
revealed a somewhat poorly defined, multiloculated cyst in the middle and
lower areas of the left side of the neck, extending toward the thymus (Figure 1).
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Figure 1. A, Contrast-enhanced computed
tomographic scan demonstrating a poorly defined cystic mass (arrow) splaying
the internal jugular vein and common carotid artery. B, The mass (arrow) is
noted to be descending into the superior mediastinum toward the thymus gland.
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A decision was made to remove the mass surgically. An incision was made
along the anterior border of the sternocleidomastoid muscle, extending from
mastoid tip to the sternal notch. Superiorly, the mass was encountered adjacent
to the internal jugular vein and superficial to the carotid artery and vagus
nerve. Blunt dissection was used to elevate the mass from these structures,
proceeding in an inferior direction. A dense capsule was identified surrounding
the mass as it extended into the mediastinum. As the mass was bluntly retracted
into the neck, a fibrous cord was found to be trailing inferiorly toward the
thymus. This cord was transected. The patient had an uneventful postoperative
course and was discharged home on postoperative day 3.
Gross examination revealed a multiloculated cyst measuring 26 cm in
greatest length and 5 cm in greatest diameter. Microscopic examination demonstrated
the presence of thymic tissue within the cyst wall, characterized by a lymphoid
predominant matrix with numerous Hassall corpuscles (Figure 2).
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Figure 2. A, On low-power magnification,
mature thymic elements are depicted within the cyst wall. B, At higher magnification,
it is easy to appreciate the presence of Hassall corpuscles (arrow); also
note cystification within a Hassall corpuscle (arrowhead) (hematoxylin-eosin,
original magnification x40).
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The patient has been followed up for 18 months, without evidence of
recurrence.
CASE 2
A healthy 11-year-old boy presented with a 2-month history of an enlarging
left neck mass. On physical examination, the mass was palpable deep to the
sternocleidomastoid muscle and was noted to extend from the mandibular angle
to the level of the cricoid cartilage. Although the carotid pulse was palpable
through the mass, no bruits were auscultated.
A contrast-enhanced computed tomographic scan revealed a multiloculated
cystic mass adjacent to and ramifying within the carotid sheath structures
(Figure 3). At the level of the
subglottis, the lesion was noted to be splaying the internal jugular vein
and the common carotid artery. Based on the findings of imaging, which showed
that a multiloculated mass was present throughout multiple fascial layers,
the preoperative diagnosis was lymphangioma.
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Figure 3. A, Contrast-enhanced computed
tomographic scan demonstrating a cystic mass (arrow) at the level of the mandibular
angle. B, More inferiorly in the neck, the mass (arrow) is well defined and
ramifies within the carotid sheath structures.
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Intraoperatively, the mass was approached through an incision anterior
to the sternocleidomastoid muscle. Blunt dissection was used to free the mass
from the internal jugular vein between the mandibular angle and the omohyoid
muscle. Coursing inferiorly into the mediastinum was a fibrous stalk, which
was transected. The patient had an uncomplicated postoperative course and
was discharged home on postoperative day 2.
Gross examination of the specimen revealed a multiloculated cystic mass,
measuring 7.0 x 1.2 x 1.5 cm. Microscopic examination revealed
a cyst wall lined by squamous epithelium. Hassall corpuscles, cholesterol
clefts, and lymphoid aggregates were found within the cyst wall.
The patient has been followed up for 4 months, without evidence of recurrence.
COMMENT
The mechanisms that account for a retained cervical thymus include arrest
in migration, failure of involution, or sequestration of thymic tissue during
descent. Any of these processes results in cervical rests of solid thymic
tissue, which will generally be asymptomatic. Cervical thymic cysts are more
important clinically, as they will have a similar clinical appearance to other
congenital neck masses and may demonstrate progressive growth. The transition
from solid to cystic thymic tissue has been hypothesized to occur as a result
of cystic degeneration of Hassall corpuscles or cystic change in persistent
remnants of the thymopharyngeal duct.3 A classification
system for cystic thymic remnants has been proposed, differentiating these
lesions based on structures of origin.5 True
cysts are those lesions that originate in maldevelopments of the thymopharyngeal
duct and may span the entire length of the neck with or without a patent fistula
tract to the pyriform sinus. Mixed cysts contain elements of second branchial
cleft and third pharyngeal pouch origin and are considered thymic fistulas.
Those lesions derived from branchial cleft remnants, with incidental thymic
tissue present, are referred to as false thymic cysts.
Cervical thymic cysts are rare in comparison to other congenital neck
masses, such as thyroglossal duct cysts, lymphangiomas, and branchial cleft
cysts. Nicollas et al,6 in their retrospective
review of 191 congenital neck masses, found that only 2% were thymic cyst
malformations. All cervical thymic cysts are characterized as congenital lesions;
accordingly, the majority of patients present in the first decade of life.
Males are more commonly affected, and there is a higher incidence of left-sided
lesions.7 Lesions are most often asymptomatic,
although there have been reports of associated hoarseness, dysphagia, and
stridor, especially in neonates.8 On physical
examination, thymic cysts are usually palpable deep to the sternocleidomastoid
muscle. Because of the intimate relationship between these lesions and the
carotid sheath, the carotid pulse will often be detectable through palpation
of the mass. By definition, when these masses span the length of the neck
from mandibular angle to clavicle, they are termed thymopharyngeal
duct cysts. It may be difficult to completely palpate the inferior
aspect of these masses, because in the 50% of cases that maintain a connection
with the mediastinal thymus, there is an atrophic fibrous cord attachment.9 Occasionally, a thymopharyngeal duct cyst will have
a fistula tract to the pyriform sinus, in which case a barium esophagram may
be of diagnostic assistance.
Contrast-enhanced computed tomographic scans are important for distinguishing
thymic cysts from other congenital neck masses, such as second branchial cleft
cysts or lymphangiomas. Second branchial cleft cysts occur superficial and
lateral to the internal jugular vein and common carotid artery, while lymphangiomas
occur most often in the posterior triangle of the neck, often violating normal
fascial planes. It is only thymic cysts that maintain their relationship with
the carotid sheath, based on the embryological path of migration. Magnetic
resonance imaging has been used to confirm an association between the cervical
neck mass and the mediastinal thymus because of its ability for greater soft
tissue delineation. It also may assist in determining whether the cervical
mass represents the only source of thymic tissue. Complete excision of a cervical
thymic mass should be deferred in a young child with no evidence of mediastinal
thymic tissue. Immune dysfunction after total thymectomy in children has been
reported.10
Although not always clinically indicated, fine-needle aspiration may
assist in proper management. If the findings of computed tomography support
a diagnosis of thymic cyst, the results of fine-needle aspiration can confirm
the presence of thymic tissue and thus promote further investigation as to
the presence of a mediastinal thymus prior to surgical therapy. Complete surgical
excision of the mass is the recommended therapy in most cases. When undertaking
surgery for suspected thymopharyngeal duct cysts, one must be prepared for
dissection along or within the carotid sheath into the superior mediastinum.
The presence of an atrophic fibrous cord can be a clue as to the inferior
extent of dissection. As previously mentioned, if workup reveals the absence
of a mediastinal thymus, one may consider excisional biopsy or deferring surgery
until the patient has completed immune system development.
Thymic cysts are most often multiloculated lesions with a smooth, fibrous
capsule. Unlike other congenital neck masses, the cyst has an epithelial lining
of the columnar, cuboidal, or squamous variety. Within the cyst wall, it is
common to find lymphoid aggregates, granulomas, and cholesterol clefts. The
pathognomonic histological finding in thymic lesions is the presence of Hassall
corpuscles, which are formed from swirls of keratinizing epithelial cells
originating in the thymic medulla.
Prognosis is excellent after complete surgical excision, with minimal
morbidity. Of the cases of thymic cysts described in children, to our knowledge
there have been no recurrences reported in the literature.
AUTHOR INFORMATION
Accepted for publication July 17, 2001.
Corresponding author: Matthew R. Kaufman, MD, Department of Otolaryngology,
The Mount Sinai Medical Center, One Gustave L. Levy Place, Box 1189, New York,
NY 10029 (e-mail: kaufmanmatthew{at}hotmail.com).
From the Department of Otolaryngology, The Mount Sinai Medical Center,
New York, NY.
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