DISEASES OF MASTICATION:
The trigeminal nerve give rise to the sensory nerves for the skin and structures of the eye,
nose, mouth and face and the motor nerves to the muscles of mastication.
It consists of 3 major
branches:
the ophthalmic nerve which is sensory to the structures of the orbit and skin of the
dorsum of the nose;
the maxillary nerve which is sensory to the skin of the cheek, side of the
nose, muzzle, mucous membrane of the nasopharynx, maxillary sinus, soft and hard palates and
the teeth and gingiva of the upper jaw;
and , the mandibular nerve which is sensory to the
remaining portions of the face and mouth and is motor to the muscles of mastication.
Diseases
of the trigeminal nerve or the muscles of mastication are not uncommon and must be
differentiated from one another.
Trigeminal neuritis (an immune disorder affecting the myelinated pathways in the
trigeminal nerve) is usually transient, but can present as a significant problem.
The cardinal
signs of trigeminal neuritis if a dropped jaw with the inability to close the mouth.
It affects adult
dogs and cats with no breed or sex predilection. It must be differentiated from fracture or
subluxation of the temporomandibular joint (evaluated by skull radiographs).
On pathologic
examination, there is bilateral nonsuppurative neuritis of the trigeminal nerve.
As an idiopathic
immune-related disorder, the condition will usually improve over 1-3 weeks.
On the other hand,
methylprednisolone therapy may help reduce the severity of an attack.
Additional measures
includes the use of antioxidant medications such as vitamin E and C, n-acetylcysteine and
ginkgo biloba. These latter measures may help prevent reoccurrence of episodes, which are
occasionally seen. Other measures including feeding liquified food and/or introduction of a PEG
tubes to support nutrition while the neuritis slowly responses. Some have supported the patients
nutrition by placing a wide rubber band around the mouth (which helps close the mouth) while
the patient is allowed to eat.
Masseter and temporalis myositis is a chronic progressive which presents with acute
exacerbations and remissions.
It is an auto-immune disease directed at the unique antigenic
markers of the muscles innervated by the trigeminal nerve.
The cardinal signs of this myositis is
the inability to open the jaw, which differentiates it from primary diseases of the trigeminal
nerve.
In the acute phase, there is elevation of serum muscle enzymes (CPK, AST, LDH and
aldolase).
On the CBC, there is often an elevation of eosinophils (giving the condition its names,
eosinophilic myositis).
On the other hand, in the chronic phase, the amount of remaining muscle
and, therefore, the amount of inflammation are reduced.
The diagnosis can be confirmed on
muscle biopsy and determination of serum antibody titers to the 2M antigen.
The treatment of
acute masseter-temporalis myositis is with immunosuppressive medication. We use oral
prednisolone at 1 mg/kg/day divided into 2 or 3 doses for 1-2 weeks, followed by 0.5 mg/kg/day for 1-3 weeks.
There is no evidence to suggest that keeping patients on alternate day steroid
therapy between attack will reduce the chances for progression. Here is another area where
using dietary supplements might be useful. In the chronic phase, the jaw may be locked shut.
In
these cases, it may be necessary to manually open the mouth under anesthesia. This may lead to
fracture of the jaw. The hope is that, once the fibrosis is broken, the remaining muscle mass will allow enough function for the patient to be able to feed itself. The jaw must open about 1-1.5 inches for this to happen.
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