Craniofacial Muscles

8 Masticatory Muscle Response to Neuromuscular Diseases and Speci fi c Pathologies
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SCA indicates that masseter re fl ex could be a useful diagnostic tool in distinguishing
between different subtypes of SCA.
Most cranial skeletal muscles are also spared or less affected in critical illness
myopathy—an acute acquired myopathy in critically ill patients (Larsson 2008 ) . The
limb and trunk muscles become extremely weak and undergo atrophy in critical illness
myopathy, and 80% of the patients who develop this condition can display persistent
quadriplegia and generalized weakness after a month-long ICU stay. Masseter,
along with other craniofacial muscles, is functionally and morphologically spared
(Aare et al. 2011 ) . While the mechanism is unknown, the difference between the
retention of normal morphology and function in masseter muscle compared to the
signi fi cant decrease in muscle size and function limb muscle is quite striking.
Sparing of masticatory muscles in DMD and ALS presents a more complex picture,
in contrast to the clearly demonstrable sparing in the extraocular and laryngeal
muscles. In ALS with primary lower motor neuron involvement, an electromyographic
(EMG) study showed that a relatively high percentage of patients showed
evidence of “motor unit potential (MUP) reinnervation” in their masseter muscles—
despite no clinical evidence of involvement (Preston et al. 1997 ) . However, it was
also noted that evidence for active denervation in the masseter was seen only in a
few patients, while this was relatively common in the tongue muscles examined.
A similar story is seen in the analysis of masticatory muscles in DMD patients (see
below). It appears that in the continuum of muscle phenotypes, the masticatory
muscles are closer to the extraocular and laryngeal muscles than to limb muscles in
their morphologic and functional sparing in these two diseases, but they are not
completely and unequivocally spared from signs of degenerative changes.
8.4 Masticatory Muscle Speci fi c Diseases/Conditions
8.4.1 Primary Pathology
The masticatory muscles as a group are susceptible to a greater number of skeletal
muscle diseases than extraocular or laryngeal muscles. The disease pro fi le of these
muscles in DMD is illustrative of these differences. During the disease course of
DMD, both masseter and temporalis muscles show evidence of pathological
changes, with increased numbers of myo fi bers with centralized nuclei and some
increased fi brosis, although not as signi fi cant as that seen in limb muscles (Spassov
et al. 2010 ) . Other studies have shown that compared to diaphragm or limb skeletal
muscle, the pathological changes in masseter are more limited (Muller et al. 2001 ) .
In human DMD patients, however, it is clear that as the disease progresses there is
a signi fi cant loss in masticatory muscle force with resultant changes in orofacial
anatomy and function (Kiliaridis and Katsaros 1998 ; Botteron et al. 2009 ) .
Other diseases of limb skeletal muscle directly involve the masticatory muscles
in the early stages of the disease. While the initial phase of myasthenia gravisinduced
muscle weakness occurs in the extraocular muscles, the masticatory muscles
show an early involvement in the course of the disease (Yarom et al. 2005 ) .