Craniofacial Muscles

132 S.L. Hebert et al.
type 3 and critical illness myopathy. However, these muscles do not seem as resistant
to pathology as other craniofacial muscles, such as Duchenne muscular dystrophy
(DMD) or amyotrophic lateral sclerosis (ALS). They are also vulnerable to functionspeci
fi c pathology. The high forces and impact produced during chewing make the
masticatory muscles prone to pathology related to these high stresses and impact.
Further, masticatory muscles are adaptable, and so their phenotype and pathology is
in fl uenced by external factors such as stress, dentition, and diet, as well as to changes
in temporomandibular joint and respiratory function.
8.2 Developmental Anomalies
The masticatory muscles develop from cranial mesoderm of the fi rst pharyngeal
arch. The core of each pharyngeal arch is comprised of mesodermal and neural crest
cells. Proper development of the masticatory muscles is dependent on the speci fi c
migration and interaction between these two cell types. Syndromes of the fi rst pharyngeal
arch are typically caused by either improper migration or development of
mesodermal or cranial neural crest cells (Kapur et al. 2008 ; Passos-Bueno et al.
2009 ; Heude et al. 2011 ; Johnson et al. 2011 ) .
Hemifacial microsomia, a syndrome of the fi rst pharyngeal arch, encompasses a
wide variety of phenotypes including defects of the masticatory muscles, jaw, external
ear, as well as microphthalmia. As a result, this syndrome is also referred to by
a multitude of names—Goldenhar–Gorlin syndrome, fi rst arch syndrome, lateral
facial dysplasia, unilateral craniofacial microsomia, otomandibular dysostosis,
oculoauriculovertebral dysplasia, auriculo-branchiogenic dysplasia, and oculoauriculovertebral
spectrum. Patients with hemifacial microsomia may display defects
on one or both sides of the face.
The defects of the masticatory muscles differ widely across patients with hemifacial
microsomia. The affected side(s) of the face in these patients can exhibit reduced
size or complete absence of the masticatory muscles. Typically, the masseter, temporalis,
and medial and lateral pterygoids are hypoplastic and show reduced activity on
the affected side(s) though in some cases the masseter and temporalis are completely
absent (Moss and James 1984 ; Kapur et al. 2008 ; Heude et al. 2011 ) . While the exact
cause of hemifacial microsomia is not currently known, it has been hypothesized to
be due to a defect in the cranial neural crest cells (Heude et al. 2011 ) .
8.3 Sparing in Skeletal Muscle Disease
The masticatory muscles are completely spared in very few skeletal muscle diseases.
Masseter function is spared in some forms of SCA, but not in others. Patients with
SCA type 3 display a bilaterally normal masseter re fl ex response, while patients
with SCA type 2 have an abnormal masseter re fl ex (Garcia et al. 2009 ; Alvarez-
Paradelo et al. 2011 ) . The preferential sparing of masseter in certain subtypes of