PERSISTENCE OF THE BRANCHIAL ARCHES AND THEIR REMNANTS

During embryological development, the six branchial arches go through various stages of development and regression. They each originate from embryological mesenchyme and contain a core of cartilage, muscle, an artery and nerve supply from a cranial nerve.

The cartilage of the first arch develops into the malleus and incus (middle ear bones) and an associated ligament, the second into the stapes (third middle ear bone) and styloid and part of the hyoid bone, the third into the rest of the hyoid, and part of the fourth and sixth arch into the larynx, and most of the rest of the cartilage disappears. The muscles of the arches develop into facial muscles, each keeping their original nerve supply. The arteries become paired aortic arches, but the only ones which remain are the third (carotids), fourth (right subclavian on right and aortic arch on left) and sixth (right pulmonary artey on right and left pulmonary artery plus ductus arteriosus on left – which with its nerve supply from the vagus, the tenth cranial nerve, explains how the recurrent laryngeal nerve loops under the ductus arteriosus on the left and under the subclavian on the right).

Persistence or failure of complete regression of the branchial arches gives rise to many congenital abnormalities which are present at birth.

The first arch anomalies include cleft lip and palate (see p. 496). Upper preauricular sinuses and skin tags (which invariably contain cartilage) are usually superficial and can easily be excised, but a low preauricular sinus may have a deep internal connection to the first arch – a surprising finding for the unsuspecting surgeon who follows a track and ends up within the middle ear – and close to the facial nerve, with its consequent problems if damaged.

A branchial fistula arises from the second arch, from the anterior border of the bottom third of the sternomastoid muscle, and travelling up inside the neck to open in the tonsillar fossa in the pharynx. This fistula may present as a discharging dimple on the neck (as the fistula is lined by mucus-secreting glands), and it needs to be excised (usually requiring two separate neck incisions) in its entirety from the lower neck up to the pharynx to prevent continuous discharge, infection, or the rare possibility of subsequent malignant transformation. This second arch remnant may also present as a skin or cartilaginous tag at the site of the dimple.

A branchial cyst also originates from a remnant of the second arch without external connection, and contains the glairy fluid containing cholesterol crystals which typifies the mucus-secreting glands within the cyst. It should be excised before it gets infected.

Fistulae from the third or fourth arches are rare, but will also connect deeply internally, and can get infected and should be excised, again with the full awareness of the anatomy which may be involved.

LINGUAL THYROID AND THYROGLOSSAL CYSTS

The thyroid gland begins as an outgrowth from the midline of the tongue in the primitive pharynx, which moves caudally into the neck, looping around or through the hyoid bone and moving caudally further to its final site in the lower neck.

The thyroid may fail to descend, and remain as a small gland in the tongue at the site where it started its outgrowth (the foramen caecum, in the midline at the junction of the anterior two-thirds and posterior third of the tongue), or it may partially descend and be mistaken for a thyroglossal cyst – see below. This ‘ectopic’ or incompletely descended thyroid tissue is invariably hypoplastic, and should be removed, as it may suffer from all the pathological problems of a normally sited thyroid. The patient should be investigated to see if they have any normally sited thyroid, which most of them do not. The patient will usually become hypothyroid, as this gland is hypoplastic, and if it is their only thyroid tissue, and it is removed, the patient will certainly become hypothyroid, but without potential problems from the gland.

The thyroid may keep its connection to the tongue as the thyroglossal duct, and a cyst can develop anywhere along the line of this duct, the commonest site being at the level of the body of the hyoid bone. This thyroglossal cyst can present at any age, as a midline swelling in the upper neck, which moves upward when the tongue is protruded. Excision is advised, along with the whole thyroglossal tract (and consequently the midportion of the hyoid bone), in order to stop infection, following which excision is so much more difficult.

DERMOID CYSTS

Dermoid cysts develop at an area where fusion of sections of the embryo has occurred, and are most common in the midline of the neck, at the external angle of the eye, and behind the pinna. They should be removed to prevent secondary infection.

CLEFT LIP AND PALATE

This is one of the more common congenital anomalies, occurring in 1 in 600 live births.

The face forms during the fifth to the eighth week, from the maxillary and mandibular prominences of the first branchial arch. They grow and fuse together, and if this fusion is incomplete, unilateral or bilateral cleft lip may arise (Fig. 16.2).

Fig. 16.2 Cleft lip. unilateral. bilateral.

The palate develops after the eighth week, and fusion occurs between the primary palate (the anterior section of the premaxilla and attached four front teeth) and the secondary palate (the hard and soft palate). The palate may be cleft posteriorly only, a cleft soft palate, or it may extend anteriorly to include the hard palate, cleft palate only, and more commonly it extends further anteriorly to join up with either a unilateral or bilateral cleft lip (Fig. 16.3).

Fig. 16.3 Types of cleft palate. cleft of soft palate. partial cleft palate. unilateral complete cleft palate. bilateral complete cleft palate.

Surgical correction of cleft lip and palate needs to take the embryological origins and in particular the blood supply into consideration, in order to allow an optimum repair and subsequent growth.

CYSTIC HYGROMA (LYMPHANGIOMA)

The primitive lymph sacs develop in the mesenchyme in the sixth week, and the largest is in the neck, and should resolve, but persistence and sequestration produces a multicystic swelling within the neck which is a lymphangioma, a benign hamartoma (overgrowth of normal tissue), which is also called a cystic hygroma when it occurs in the neck. Occasionally this is very large and causes respiratory distress in the neonatal age group, but more usually is just a soft swelling in the neck which may extend into the axilla, or even the chest. A degree of spontaneous resolution can be hoped for, but often it comes to surgical debulking – a difficult prospect because of the multicystic nature, which makes it difficult to be sure that every bit of the abnormal tissue is removed. If there is a haemangiomatous element as well as the lymphangiomatous part, spontaneous resolution is unlikely. An MRI scan is recommended to delineate the full extent and nature of the lesion, and the normal structures which are involved, to help plan surgical excision.

CONGENITAL DIAPHRAGMATIC HERNIA

The diaphragm develops between the thoracic and abdominal cavity, and this is a complex process which is finished before the end of the eighth week (Fig. 16.4). As the embryo folds and carries the primitive heart and septum transversum caudally and ventrally, it carries part of the yolk sac dorsally to develop as the foregut. The lateral mesenchyme develops into the pericardioperitoneal canals, from which the pericardial cavity and the lungs develop, and is separated from the peritoneal cavity by the closure of the diaphragm. The motor nerve supply travels with the diaphragm as it descends, and so comes from a more cranial region than may be expected: C3–5. This explains the clinical observation that diaphragmatic inflammation/irritation, e.g. due to intraperitoneal blood, can demonstrate referred pain and can cause shoulder tip pain (which area is also supplied by C4).

Fig. 16.4 Development of the diaphragm.

The diaphragm develops from the fusion of four parts:

There are different types of congenital diaphragmatic hernia, depending on which section has failed to close.

The most common defect is the posterolateral Bochdalek hernia (through the pleuroperitoneal canal) which is more common on the left, as that side closes last. Absence of the diaphragm can also occur, or absence of the central tendon. These three hernias tend to present early with respiratory distress soon after birth. The presence of the intestines within the pleural cavity antenatally prevents the normal development of the lung on the ipsilateral side, and mediastinal shift also prevents normal development of the contralateral lung. If the lung hypoplasia is severe, it is not compatible with life. Urgent supportive ventilation is required, and nasogastric aspiration of the gut, to decrease direct pressure on the lungs.

These diaphragmatic hernias must be dealt with surgically, after resuscitation of the patient (this is sometimes not possible in a neonate because of the severity of the lung hypoplasia) – up to 50% of babies born with congenital diaphragmatic hernias will die even today with all the modern management possibilities of oscillatory and jet ventilation, or extracorporeal membrane oxygenation (bypass).

Morgagni hernias are small defects in the anterior diaphragm close to the sternum, and are rarely associated with lung hypoplasia. They may be a coincidental finding on a chest x-ray taken for another reason. These hernias also require surgical repair.

OESOPHAGEAL ATRESIA

The foregut starts to divide into the oesophagus and the laryngotracheal tube during the fourth week. If it fails to do so correctly, there can be pure oesophageal atresia (in 8% of cases), or atresia associated with tracheo-oesophageal fistula – the commonest (in 80% of cases), being a fistula between the lower trachea and the distal oesophagus (Fig. 16.5).

Fig. 16.5 Types of oesophageal atresia. oesophageal atresia with distal tracheo-oesophageal fistula – the usual type, with an incidence of 80%. isolated oesophageal atresia – the second commonest form, with an incidence of 8%.

The baby presents soon after birth, unable to swallow saliva, and an attempt to pass a tube into the stomach fails. An x-ray taken then will show the tube in the proximal oesophagus, and either no gas below the diaphragm (in pure oesophageal atresia) or gas below the diaphragm (in patients with oesophageal atresia and tracheo-oesophageal fistula). There is a high incidence (50% of babies) of associated anomalies described by the acronym VACTERL:

Management involves protection of the lungs from aspiration of saliva prior to surgical repair. This is usually by a right thoracotomy to ligate the fistula, freeing the distal oesophagus which is then anastomosed primarily to the upper oesophageal pouch. If primary repair is not possible, a feeding gastrostomy is performed to feed the baby until it has grown enough to perform a delayed primary anastomosis or oesophageal substitution, e.g. with stomach, colon, or small bowel.

INFANTILE HYPERTROPHIC PYLORIC STENOSIS

The stomach develops from a simple tubular part of the foregut by localised dilatation. The stomach rotates clockwise so that the vagus which followed the left side of the oesophagus supplies the anterior stomach. The mesentery which suspends the stomach from the posterior abdominal wall enlarges and becomes the greater omentum. The exit of the stomach into the duodenum is the pyloric canal.

All of the gastrointestinal tract has two layers of muscle: circular and longitudinal. The circular muscle only of the pylorus can become hypertrophied in some babies. This is often called ‘congenital’ hypertrophic pyloric stenosis, but does not actually exist at birth. The baby usually presents after 10–50 days (most commonly 3–5 weeks), as the pyloric canal is narrowed by the hypertrophied muscle, and milk is prevented from leaving the stomach. The stomach becomes full and peristalses vigorously to try to empty. This peristalsis may be visible on the baby’s abdomen. The baby will then vomit forcefully, which is described as projectile.

As the baby vomits fluid and gastric hydrochloric acid, the baby becomes dehydrated, hypochloraemic and alkalotic. This is reflected in the baby’s electrolytes and blood gases at presentation. The diagnosis is made by feeding the baby, to relax the baby. The visible peristalsis may be seen, and the abdomen is palpated to feel for the pylorus, which can be felt as a lump in the right upper quadrant, about the size and shape of an olive. After rehydration and correction of the acid-base balance, the baby is taken to theatre for a laparotomy, and the hypertrophied muscle is split, without opening the mucosa – a pyloromyotomy. This is a curative operation, and the baby will be fully fed within 24–36 h postoperatively and discharged home.