Eur. Radiol. 10, 978-980 (2000) �9 Springer-Verlag 2000

Pediatric radiology European Radiology

Case report

Aberrant cervical thymus in an infant: an unusual cause of stridor

D. A. Hammoud, N.J. Khoury, M. C. Haddad

Department of Diagnostic Radiology, American University of Beirut Medical Center, E O.Box: 113-6044, Beirut, Lebanon

Received: 1 July 1999; Revised: 3 September 1999; Accepted: 6 September 1999

Abstract. We report an 8-month-old infant presenting with stridor caused by a nonpalpable neck mass dis- covered at imaging and surgery. The diagnosis of ab- errant thymic tissue was confirmed at histopathology. The authors reviewed the literature and discuss the embryology, imaging findings and differential diag- nosis of this rare disorder.

Key words: Neck masses - Thymus - Respiratory dis- tress in children - Stridor - Ultrasound - CT - Imag- ing

Introduction

Ectopic thymus is an unusual cause of asymptomatic neck mass in infancy and childhood. This aberrant mi- gration of the thymic tissue occurs in the base of skull, cervical reNon and the mediastinum specifically the tra- cheal bifurcation. Few cases have been reported previ- ously in the international literature. In a collective re- view of literature between 1901 and 1987 Nowak et al. reported 76 cases of aberrant solid and cystic thymic le- sions that presented as cervical masses. They also re- ported an additional 15 cases that presented as enlarged anterior mediastinal masses [1].We present a case of ab- errant cervical thymic tissue in an infant with respirato- ry distress.

Case report

An 8-month-old previously heathy male infant present- ed with stridor and barking cough of 1-week duration. On physical examination, he had suprasternal retrac- tions, inspiratory stridor and mild decrease in air entry bilaterally. No palpable neck mass was identified. He

Correspondence to: M. C. Haddad

was afebrile. Plain radiographs showed (Fig. 1) a soft tis- sue density compressing the trachea from the left side. A C T scan of the neck (Fig. 2) showed a relatively well- defined soft tissue mass anterior and left lateral to the trachea causing displacement and narrowing of the air- way. It was located at the thoracic inlet inferior to left thyroid lobe and superior to the left brachiocephalic vein.Ultrasound examination (Fig. 3) revealed a 17 • 7- mm hypoechoic lesion in the anterior compartment of the lower aspect of the neck, between the left major ves- sels laterally and the trachea medially. It showed inter- nal echoes with an echogenicity similar to the sterno- eleidomastoid muscle. At surgery, there was a longitudi- nal tan-white soft mass extending from the inferior as- pect of the thyroid gland down to the anterior mediasti- num. The mass was resected through a transcervical ap- proach. Permanent sections of the mass revealed lobu- tated normal thymic tissue with prominent Hassell's cor- puscles and minimal cystic dilatation. The thymic tissue was lymphocyte depleted. Postoperatively all symp- toms, including stridor, resolved.

Discussion

Aberrant cervical thymus is a rarely encountered clini- cal entity in children and is usually described in sporadic cases. The infrequency of reported clinical cases is ex- plained by the fact that most thymic vestiges in the neck remain dormant and thus may only be incidentally discovered during neck exploration for other reasons or at autopsy [2]. In fact, in a study done at the Royal Alex- andra Hospital for Children in Australia, abnormal po- sition of the thymus was recorded in 34 cases among 3236 paediatric necropsies which is significantly greater than the reported frequency of occurrence [3].

Embryologically, most of these masses arise as a con- sequence of migrational defects during thymic embryo- genesis. The thymus originates as paired ventral saccu- lations of the third pharyngeal pouches during the sixth week of embryologic development. The initially hollow

D. A. Hammoud et al.: Aberrant cervical thymus in an infant 979

Fig.1. a Anteroposterior and b lateral ra- diographs of the neck show lateral displace- ment of the trachea to the right side in the lower aspect of the neck

Fig.2 a, b. Consecutive contrast-enhanced CT scan sections of the neck showing a soft tissue mass (arrows) anterior and to the left of the trachea and medial to the major ves- sels at the thoracic inlet, causing compres- sion and displacement of the trachea to the right. Note suprasternal retractions compat- ible with respiratory distress

Fig.3. Sonography image of the neck show- ing a hypoechoic lesion (arrows) between the trachea (T) medially and the left carotid artery (.4) and left internal jugular vein (V) laterally

thymic primordia rapidly become solid epithelial bars. The lower ends enlarge and unite superficially during the eighth week. They become attached to the pericar- dium and gradually sink with it into a permanent posi- tion in the thorax. During its descent into the thorax

the upper ends of the thymus become drawn out and fi- nally vanish. It is during the stage of descent that pri- mordial thymic tissue may become arrested in the neck and present later as a malformation [4]. The cervical thymus is thus mostly localized along a line extending from the angle of the mandible to the manubr ium sterni.

Most of the repor ted cases of aberrant thymus in in- fants were discovered incidentally or as neck masses. Very few of these cases have presented with symptoms of respiratory distress [4. 5]. Although it has been claimed that the thymic gland, no mat ter how large, does not compress or displace normal structures, cases of aberrant thymus causing compression, displacement or deviation of neighbouring structures have been de- scribed. Locat ion of the gland in the thoracic inlet has the potential to compress the trachea and esophagus and cause symptoms, namely dyspnoea, stridor and dys- phagia [5]. The abnormally positioned thymus is re-

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ferred to as aberrant whenever it is located along the normal pathway of descent such as in the lateral neck or in the suprasternal area, or ectopic whenever located in any other place such as pharynx, trachea, posterior neck or oesophagus.

Aberrant thymic tissue can be cystic, solid or mixed. Most lesions are cystic, are more frequently lateral than in the midline, and occur with equal frequency on the left and right sides [6]. Sonography represents the most direct and practical initial imaging modality for the aberrant thymus presenting as a neck mass. It is a well-accepted modality for localization of the neck mass and determining its extent and internal character- istics [7]. The sono~aphic diagnosis is based on the ho- mogeneity and similarity to the normally positioned thymus [7]. The appearance of aberrant cervical thymus on CT is variable and non-specific. It usually shows as a more or less well-defined hypodense neck mass which could be cystic or solid. Magnetic resonance imaging is the best non-invasive method for identification of aber- rant thymus; however, it is not often included in the ini- tial diagnostic approach, and CT is usually performed when the sonographic findings are not diagnostic. In children, the normal thymic tissue is homogeneous and slightly more intense than muscle on Tl-weighted im- ages and slightly less intense or isointense relative to fat on T2-weighted images. The M R appearance of cer- vical thymic tissue parallels that of mediastinal thymus. Cystic components of the aberrant thymus, however, might look bright on Tl-weighted images, unlike in the mediastinal thymus where they appear as low signal in- tensity on Tl-weighted images and bright on T2-weight- ed images probably due to high protein content of the cyst [6].

The differential diagnosis of aberrant cervical thy- mus when it is cystic includes simple or complicated (haemorrhagic or infected) third branchial cleft cysts, lymphangiomas, venous malformations, cystic hygroma, necrotic or suppurative lymphadenopathy, thyrogtossat duct cysts and epidermoid and dermoid cysts [6].

D. A. Hammoud et al.: Aberrant cervical thymus in an infant

Finally, the diagnosis of cervical thymus is rarely made preoperatively but rather intra- or postoperative- ly. Similarly in our case, the diagnosis was established postoperatively at histopathological analysis. This is due to the rarity of this entity which makes it uncom- monly considered in the differential diagnosis of neck cysts and masses especially in infants and children. Awareness of this entity together with knowledge of its variable presentation is essential for unnecessary sur- gery to be avoided. Sonography is probably the best ini- tial diagnostic approach and occasionally leads to the correct diagnosis. Magnetic resonance imaging is anoth- er option if the findings on ultrasound are not diagnos- tic.

Acknowledgement, The authors thank R. Haddad for permission to publish this case report.

References

I. Nowak PA, Zarbo RJ, Jacobs JR (1988) Aberrant solid cervical thymus. Ear Nose Throat J 67:670-677

2. Spigland N, Bensoussan A, Blanchard H, Russo P (1990) Aber- rant cervical thymus in children: three case reports and review of the literature. J Pediatr Surg 25:1196-1199

3. Bale P, Sotelo-Avila C (1993) Maldescent of the thymus: 34 necropsy and 10 surgical cases, including 7 thymuses medial to the mandible. Pediatric Pathology 13:181-190

4. Sehackelford GD, McAlister WNM (1974) The aberrantly posi- tioned thymus. A cause of mediastinal or neck masses in chil- dren. Am J Roentgenol Rad Ther Nucl Med 120:291-296

5. Tovi K Mares AJ (1978) The aberrant cervical thymus. Embryol- ogy, pathology, and clinical implications. Am J Surg 136:631-637

6. Cure JK, Tagge EE Richardson MS, Mulvihill DM (1995) MR of cystic aberrant cervical thymus. AJNR 16:1124-1127

7. Koumanidou C, Vakaki M, Theophanopoulou M, Koutrouvehs H, Savvidou D, Pitsoulakis G, Kakavakis K (1998) Aberrant thymus in infants: sonographic evaluation. Pediatr Radiol 28: 987-989