Craniofacial microsomia

36 Craniofacial microsomia





The clinical association of ipsilateral congenital mandible and ear anomalies was first described in 1861. Since that time, various names have been used to designate this condition: first and second branchial arch syndrome, otomandibular dysostosis, oculoauriculovertebral sequence, Goldenhar syndrome, hemifacial microsomia, and CFM. Because the dysmorphology can be bilateral in 5–30% of cases, the terms unilateral and bilateral CFM are preferred. Goldenhar syndrome or oculoauriculovertebral sequence is a variant of CFM, constituting approximately 10% of patients. It is differentiated by the presence of ocular abnormalities (lipoma, lipodermoid, epibulbar dermoid, or coloboma) and cervical spine anomalies.



Basic science/disease process




Etiopathogenesis


The underlying cause or etiopathogenesis of CFM remains a subject of debate. The prevailing theory was that CFM was a sporadic event, possibly precipitated by exposure to teratogens. Other studies have instead suggested a fundamental role for genetic transmission in some patients. The etiology of CFM is probably heterogeneous among individuals, with variable contributions from extrinsic and intrinsic factors. Support for a teratogenic etiology of CFM is based largely on animal studies. Poswillo exposed mouse embryos to triazine by maternal administration of the drug, causing CFM-like phenotypes (Fig. 36.1). Focal hematomas arising from disruption of the stapedial artery were also observed. Although a “stapedial artery hemorrhage etiology” is attractive because the vessel is a second branchial arch derivative, a causative association between the bleeding and the deformities has not been made. The hemorrhages occurred 14 days after administration of the teratogen, and there was no clear temporal relationship between the hemorrhage appearance and the associated phenotypic deformity. When mice are exposed to triazine later in development (10 days of gestation), all animals developed deformities; however, only a third showed evidence of a hematoma. The authors concluded that triazine has a direct teratogenic effect and the stapedial artery findings were simply a side-effect. In contrast to those described by Poswillo, these animals demonstrated more evidence of bilateral deformities and inner-ear anomalies. Intermittent occlusion of the internal carotid system of fetal sheep late in gestation has been shown to result in deformities similar in appearance to CFM. The vascular disruption hypothesis, therefore, cannot be excluded.



Rats exposed to etretinate, a retinoic acid derivative, show deformities comparable to the first and second branchial arch syndromes. This finding is consistent with the finding that neural crest cells express large amounts of retinoic acid-binding proteins. Furthermore, when retinoic acid is administered early in development, it interferes with cell migration. When administered later in gestation, however, retinoic acid kills ganglionic placodal cells, resulting in a deformity similar to mandibulofacial dysostosis (Treacher–Collins).


Human case studies also support a teratogen-based etiology for CFM. Thalidomide and ethanol administration during pregnancy and gestational diabetes are exogenous other factors that have been implicated.


Support of a genetic etiology of CFM has come from both animal and human studies. A transgenic mouse model for CFM with an insertional deletion on mouse chromosome 10 has been described with an autosomal-dominant mode of transmission and 25% penetration. The affected animals display low-set ears, unilateral microtia, and jaw asymmetry, without evidence of middle-ear abnormality. Second branchial arch hematomas were also observed in the embryos.


Human genetics studies have documented a positive family history in 9.4% of 32 probands, 21% of 57 probands, 26% of 88 probands, and 44% of 82 probands. Kaye et al. performed segregation analysis on 74 families of probands with CFM and rejected the hypothesis that genetic transmission is not a causative factor. The evidence favored autosomal-dominant inheritance; however, recessive and polygenic models were not distinguishable. Despite the suggestion of autosomal-dominant transmission, they found only a 2–3% overall recurrence rate in first-degree relatives. This figure compares to the 10% recurrence risk in first- and second-degree relatives reported by the same group in an earlier study of 294 individuals with CFM. Graham et al. described a family with a strong autosomal-dominant transmission of Goldenhar-like phenotype with linkage to a mutation at locus 8q13.


The high variability and low penetrance of CFM may be explained in terms of genetic transmission by a number of theories. Compensation of defective genes by adjacent normal genes has been described in a cleft palate mouse model, and it may explain the variability of CFM. Another mechanism may be “maternal rescue,” a mechanism by which transplacental transfer of a normal maternal gene product may compensate for an abnormal fetal gene. The low penetrance of CFM may be explained by differential expression of maternal and paternal DNA sequences (genomic imprinting) or by only a limited number of cells possessing the abnormal gene (mosaicism).


Studies on the incidence and expression of craniofacial anomalies in twins have provided insight into the etiology of CFM. Mulliken’s group described 10 twin pairs with CFM. Only one of the pairs, who were monozygotic, was concordant for the anomaly. Other twin studies have noted a high level of discordance of CFM among monozygotic twins. Even among concordant monozygotic twins, the anomaly can be mirror image. One theory to explain the discordant findings is that vascular insufficiency of the first and second branchial arch occurs in monozygotic twins with a shared placenta (monochorionic) and unequal circulation. Arguing against this is the observation that discordance is not limited to monochorionic twins but is also observed in dichorionic pairs. Monozygotic discordance would appear to refute the teratogen theory of CFM; however, monozygotic twins have been reported to respond to in utero teratogens in a discordant manner.


In summary, the exact etiology of CFM is not known. It is likely to involve a number of factors ranging from abnormal genes with various intrinsic modifiers to extrinsic insults such as teratogens or vascular events. It is also probable that, as in patients with cleft palate, the population of patients with CFM is a heterogeneous group. Some individuals with the phenotype may be part of a family with a dominant abnormal gene expression, in which case the recurrence risk would approach 50%. In other individuals with a purely environmental etiology, recurrence would be negligible. The empirically stated recurrence rate of 2–3% for CFM should be recognized as a group summary and placed in context when an individual or family with this phenotype is counseled.



Embryology


The ear serves as a frame of reference in this syndrome because of its developmental relationship with the jaw. A brief review of the phylogeny and ontogeny of the auricle and hearing apparatus is helpful in understanding the embryogenesis of the malformation in the patient with CFM.


The two principal divisions of the organ of hearing are derived from different embryonic anlagen. The sensory organ in the inner ear is derived from the ectodermal otocyst; the sound-conducting apparatus in the external and middle ear comes from the gill structures.


The membranous labyrinth has its beginning in the image-week-old human embryo as a thickening of the ectoderm in the side of the head – the otic placode. This area is enfolded to become the otic pit and is subsequently pinched off to become the otocyst. By means of a series of folds, the otocyst differentiates in the 3-month-old fetus into the endolymphatic duct and sac, the semicircular endolymphatic ducts, the utricle, the saccule, and the organ of Corti. By the fifth month of fetal life, the sensory end organ of the ear attains adult form and size as the cartilaginous otic capsule ossifies.


It is speculated that the aquatic ancestors of humans swam in seas not yet as salty as today’s oceans and that endolymph, entrapped by the enfolding otocyst, closely resembles in chemical composition the dilute salt water of the primeval sea. Our ancient aquatic forebears did not require any special mechanism to transmit sound to the inner ear. As in today’s fish, sound was readily transmitted from the sea through the skin to the fluid of the inner ear.


When these ancestors struggled out of the seas on to dry land, a new problem appeared. A mechanical device was needed to convert air vibrations of large amplitude and small force into fluid vibrations of small amplitude and large force. The gill structures, no longer needed for breathing, became converted into such a mechanism. The first branchial groove became the external auditory meatus and canal; the first pharyngeal pouch became the eustachian tube and middle ear. Instead of the branchial groove and pharyngeal pouch connecting to become a gill cleft, a thin intervening layer of tissue remained to form the tympanic membrane.


The mandible, incus, and malleus develop from the cartilage of the first branchial arch (Meckel cartilage) (Fig. 36.2). The stapes (with the exception of the footplate, which originates from the otic capsule), styloid process, and hyoid bone develop from the cartilage of the second branchial arch (Reichert cartilage). The large area of the tympanic membrane, connected by the lever system of the ossicular chain to the small area of the oval window, provides the ear with an effective mechanism to overcome the sound barrier between air and water.



By the third fetal month, the external auricle has been formed from the first and second branchial arches on either side of the first branchial groove, which is the primary shallow, funnel-shaped external auditory meatus (Fig. 36.3). From the inner end of the primary meatus, a solid cord of ectodermal cells extends farther inward, with a bulb-like enlargement adjacent to the middle ear. It is not until the seventh fetal month that the cord canalizes, beginning medially to form the tympanic membrane and extending laterally to join with the primary meatus to form the completed external auditory meatus. The external and middle ears, although capable of transmitting sound to the inner ear, are not yet of adult form and size.



In the seventh fetal month, pneumatization of the temporal bone begins. At birth, the eustachian tube inflates; the fetal mesoderm tissue in the middle ear and antrum continues to resorb until the epithelium lies close to the periosteum, and pneumatization of the temporal bone proceeds.


The external auditory meatus, entirely cartilaginous at birth (except for the narrow incomplete ring of the tympanic bone), deepens by growth of the tympanic bone to form the adult osseous meatus. Except for pneumatization of the petrous apex, which may continue into adult life, the external and middle ears finally attain adult form and size in late childhood (in contrast to the inner ear, which becomes adult in fetal life). It is generally accepted that the first branchial arch furnishes the anterior part of the auricle; the second arch provides the structures of the remaining external ear.


The maxilla, palatine bone, and zygoma develop from the maxillary process of the first branchial arch, whereas the mandible forms from the mandibular process. Meckel cartilage, the primary jaw of lower vertebrates, represents the temporary skeleton of the first pharyngeal arch; the two symmetric cartilaginous bars describe a parabolic arch that serves as a model and guide in the early morphogenesis of the mandible.


Three main regions of Meckel cartilage should be considered: (1) the distal portion, which becomes incorporated into the anterior part of the body of the mandible; (2) a middle portion, which gives rise to the sphenomandibular ligament and contributes to the mylohyoid groove of the mandible; and (3) the proximal or intratympanic portion, which differentiates into the malleus, the incus, and the anterior malleolar ligament.



Pathology


A fundamental characteristic of the syndrome is the variable manifestation of the pathologic findings.


The deformity in CFM usually has the three major features of auricular, mandibular, and maxillary hypoplasia. The hypoplasia, however, can also involve adjacent anatomic structures: the zygoma, the pterygoid processes of the sphenoid bone, the temporal bone (the middle ear; the mastoid process is small and acellular), the frontal bone, the facial nerve, the muscles of mastication, the parotid, the cutaneous and subcutaneous tissues, the tongue, the soft palate, the pharynx and the floor of the nose.


Whereas the jaw and ear deformities are the most conspicuous in the majority of patients, the first and second branchial arches and the structures derived from them are intimately interlinked with the chondrocranium and membranous bones of the skull; associated deformities of the temporal bone and other cranial bones are inevitable. In extreme forms of the dysplasia, widespread craniofacial involvement is evident (Fig. 36.4). As Pruzansky stated, maldevelopment in one area may trigger a “domino effect,” with involvement of the entire craniofacial skeleton including microphthalmos, orbital dystopia, and orbitofacial clefts.




Skeletal tissue


The most conspicuous deformity of unilateral CFM is the hypoplasia of the mandible on the affected side. The ramus is hypoplastic or even absent, and the body of the mandible curves upward to join the vertically reduced ramus. The chin is deviated to the affected side. On the “normal” or “less affected” side, the body of the mandible is also characterized by abnormalities in the skeletal and soft-tissue anatomy. The body of the “normal” mandible shows an increased horizontal dimension and an increase in the gonial angle. The increase in length of soft- and hard-tissue structures on the less affected side may represent compensatory growth, secondary to the growth deficiency on the affected side.


Ramus and condyle malformations vary from minimal hypoplasia or blunting of the condyle to its complete absence in association with hypoplasia or agenesis of the ramus (Fig. 36.5). In all patients, condylar anomalies can be demonstrated, and this finding may be pathognomonic of the syndrome. As a consequence, the spatial relationships of the malformed or deficient skeletal parts, as well as the associated neuromuscular components, become of paramount importance in the diagnosis and planning of treatment.



The posterior wall of the glenoid fossa is partially formed by the tympanic portion of the temporal bone, which provides the bony portion of the external auditory canal in the normally developed ear. When there is hypoplasia of the temporal bone, the posterior wall of the glenoid fossa cannot be identified. The infratemporal surface is flat, and the hypoplastic ramus is often hinged on this flat surface at a point anterior to the contralateral “unaffected” temporomandibular joint.


Mandibular growth deficiency usually is closely related to the degree of hypoplasia of the condyle. In more severe conditions there is considerable disparity in condylar growth between the affected and contralateral sides. The cant of the occlusal plane (higher on the affected side) is caused by the short, hypoplastic ramus and by hypoplasia of the ipsilateral maxillary dentoalveolar process (Fig. 36.6). The floor of the maxillary sinus and of the nose on the affected side is canted at a higher level. In some patients, the base of the skull is elevated on an inclined plane similar to the inclined occlusal plane. Anteroposterior and superoinferior dentoalveolar and skeletal dimensions are reduced on the affected side. Crowded dentition, with a characteristic tilt of the anterior maxillary and mandibular occlusal planes upward on the affected side, is often noted.



Craniofacial bones other than the mandible or maxilla can be involved, especially the tympanic and mastoid portions of the temporal bone; the petrous portion usually is remarkably spared. The styloid process is frequently smaller on the affected side. The mastoid process can have a flattened appearance, and there can be partial or complete lack of pneumatization of the mastoid air cells (Fig. 36.7).



The zygoma can be underdeveloped in all its dimensions, with flattening of the malar eminence. A decrease in the span of the zygomatic arch results in a decrease in the length of the lateral canthal–tragal line on the affected side.


Disparities in the vertical axis of the orbit can be seen, with or without evidence of microphthalmos (Fig. 36.8). Often in this situation, there is flattening of the ipsilateral frontal bone – an appearance of plagiocephaly without radiographic evidence of coronal synostosis.




Nonskeletal (soft) tissue


The relationship between the soft tissue and skeletal dysmorphology of CFM is not fully understood. One theory is that they are independent manifestations of the same genetic or environmental event. Another possibility is that one is primarily involved, whereas the other is only a secondary event.


The “functional matrix” theory of Moss attributes the overall growth and development of the head to the development of the soft-tissue matrix and functional spaces. The matrix is composed of cells, tissues, organs, and air volumes that serve a functional role. The associated “hard” tissues, such as bone and cartilage, serve to protect and support the functional matrix. Their morphology is solely determined by the functional matrix. As summarized by Moss, “bones do not grow, they are grown.” This theory has been supported by animal research examining the effect of transposition of muscles of mastication on bone morphology. Investigation of the effect of soft tissue on bone shape in humans with unilateral CFM has been limited to computed tomographic (CT) analysis. Results of these studies do suggest that changes in the muscles of mastication can elicit a postnatal change in bone morphology but that the opposite – bone changes affecting muscle – does not take place (Fig. 36.9). It can be speculated that, if the functional matrix theory is validated, future treatment of CFM may be limited to early manipulation of the soft-tissue matrix to elicit a secondary effect on the associated bone.




Muscles of mastication


Muscle function, especially that of the lateral pterygoid muscle, is impaired in many patients with CFM. The right muscle is responsible for the lateral movement of the mandible to the left side, whereas the left muscle controls movement to the right. Both sides act synergistically in executing protrusive opening movements. In patients with CFM, a severe limitation of protrusive and lateral movements secondary to hypoplasia of the lateral pterygoid muscle is observed.


The impact of this factor is apparent both on the developing musculature and on the morphology of the attached bone. An alteration in mandibular movements (opening, lateral, and protrusive) comparable with the degree of mandibular deficiency is often noted.


When the patient opens the mouth, the deviation toward the affected side is produced not only by the skeletal asymmetry but also by the minimal or absent contribution of the ipsilateral medial and lateral pterygoid muscles in countering the opposing actions of the muscles on the unaffected side. The condyle on the less affected side is displaced abnormally downward and laterally when the mandible is depressed, almost to the point of dislocating the condyle from the glenoid fossa. No discernible condylar movement can be elicited on the affected side during opening and protrusive movements of the mandible. Thus, in testing for lateral pterygoid muscle weakness, one finds an inability to shift the jaw laterally toward the unaffected side and to deviate the midline of the chin toward the affected side during opening and during forceful protrusion.


In many cases, the coronoid process is absent, and there is reduction in the size of the temporalis muscle. The associated masseter and medial pterygoid muscles are also grossly deficient.



Ear


Auricular malformations are a usual manifestation of the syndrome. Meurman proposed a classification of the auricular anomalies based on the studies of Marx (1926): grade I, distinctly smaller malformed auricles with most of the characteristic components; grade II, vertical remnant of cartilage and skin with a small anterior hook and complete atresia of the canal; and grade III, an almost entirely absent auricle except for a small remnant, such as a deformed lobule (Fig. 36.10).



In a comprehensive study, Caldarelli et al., using air and bone conduction audiometry and temporal bone tomography, evaluated 57 patients with CFM. It was observed that the degree of Meurman auricular deformity does not correlate exactly with hearing function. The type of hearing loss, although usually assumed to be conductive in origin, can be determined only by audiometry. Tomography, not auricular morphology, is the only indicator of middle-ear structure. The unaffected ear may also harbor abnormalities in structure and function and should be evaluated.


Congenital hearing loss may be due to a malformed inner ear, hypoplasia of the cochlear nerve and brainstem auditory nuclei, or hypoplasia and impaired function of cranial nerves IX through XII.






Natural history


There are two schools of thought concerning the natural history or behavior of CFM (without any therapeutic intervention). One asserts that the severity of skeletal deformity is not progressive, with growth of the affected side paralleling that of the unaffected or less affected side. The other school of thought is that CFM is a progressive anomaly, with inhibited growth on the affected side resulting in increasing facial asymmetry with age. The natural history of the soft-tissue changes in CFM is not known since they are more difficult to document and quantitate.


Rune et al. examined the facial growth of 11 patients with unoperated unilateral CFM by use of metallic implants and roentgen stereophotogrammetry. They reported a mild increase in occlusal cant in 5 patients and stable or improving occlusal cant in the remaining 6. The authors concluded that asymmetry of the jaw does not increase with time; however, only 1 patient in their study group had reached skeletal maturity at the time of the study. Polley et al. retrospectively examined longitudinal posteroanterior cephalograms of 26 patients with unoperated unilateral CFM. The patients were divided into three groups on the basis of Pruzansky mandibular grading. Both vertical and horizontal asymmetries were analyzed by a combination of angular and linear measurements. They reported that growth on the affected side paralleled that on the unaffected side, regardless of the grade of severity or side that was affected.


Kearns et al. interpreted the significant changes in gonial height difference and in intergonial angle, as reported in the Polley paper, to indicate a progressive vertical asymmetry that is correlated with the severity of mandibular deformity. This reinterpretation was consistent with their findings when they retrospectively examined 67 patients with unoperated unilateral CFM by horizontal angular analysis of posteroanterior cephalograms. They divided the patients into two groups based on the Pruzansky classification; they found no significant changes in group I but significant changes in all measurements in group II.


If the true natural history of CFM is progression, early surgical intervention may be indicated in an attempt to minimize the deformity. If, instead, CFM remains relatively stable, one could argue that surgery can be deferred to minimize the need for revisionary procedures, provided that there is neither a significant functional (respiratory, masticatory) or dysmorphic problem. The topic is again discussed later in the section on growth studies.


Feb 21, 2016 | Posted by in General Surgery | Comments Off on Craniofacial microsomia

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