Patient protocol: A 26-year-old woman awoke one morning with her face "drawn to one side." Examination disclosed that on the left side she could not wrinkle her forehead, close her eye, pull back the corner of her mouth, or wrinkle the skin of her neck. She moved the right side of her face normally. Her complaint of "drawing" of her face was due to the unopposed pull of the intact right-side facial muscles, which pulled her lips to the right when she spoke or smiled (see the Pt in Fig. 6-7). The remainder of the examination was completely normal, including taste sensation and hearing.
Analysis of the clinical data will lead to a conclusion as to where and what the CrN VII lesion is.
In analyzing a motor deficit, consider first its distribution. Does it match a central or a pyramidal tract (upper motoneuron) distribution? Does it match a root or peripheral nerve or myopathic distribution?
Which distribution does the motor deficit of the present Pt match? □ upper motoneuron/□ peripheral nerve/□ myopathic.
☑ peripheral nerve
The paralysis involves the muscles of one nerve, CrN ___________.
VII
The distribution of the paralysis in the field of one nerve excludes a neuromyal junction disorder or myopathy; these are widespread disorders and not limited to a single nerve.
Because interruption of a single nerve explains the paralysis, the disorder consists of a □ mononeuropathy/□ polyneuropathy/□ myopathy.
☑ mononeuropathy
Having identified a mononeuropathy of CrN VII, we have to specify the location of the lesion along the course of the nerve. In analyzing a sensory disturbance or a reflex arc, we invoked the principle of starting at the ______________ to trace along the entire pathway of the nerve impulses.
receptor
Where should you start to trace along the course of the impulses in a motor nerve? __________________________________
At the nucleus or cell body of the lower motoneurons.
The CrN VII nucleus occupies the □ tectum/□ tegmentum/□ basis of the □ midbrain/□ pons/□ medulla.
☑ tegmentum; ☑ pons
Because of the close packing of tracts and nuclei, a brainstem lesion would rarely affect just one CrN nucleus. It most likely would involve the neighboring lemniscal, cerebellar, or CrN VIII pathways or neighboring CrNs. In addition to VII, the CrN motor nuclei in the pons are ______________.
V and VI. In Fig. 2-16, notice the relation of the intra-axial course of the axons of VII to the VI nucleus.
The Pt had no signs implicating structures in the vicinity of the CrN VII nucleus in the central nervous system (CNS); therefore, the lesion most likely interrupted the nerve □ inside/□ outside the brainstem.
☑ outside
After leaving the pontomedullary sulcus of the brainstem and before entering the internal auditory meatus, CrN VII must cross the ___________________ space.
subarachnoid
The subarachnoid space between the cerebellum and the brainstem is called the cerebellopontine angle. A lesion here, such as a neoplasm, would interrupt not only CrN VII, but also CrN ______.
VIII
As the neoplasm enlarged, in addition to CrNs VII and VIII, it would affect CrNs _______________________.
V and VI (and possibly IX, X, and XII).
Go to Fig. 2-20 to appreciate how a relatively common tumor, an acoustic neuroma, in the cerebellopontine angle can affect additional nerves. Start with your pencil on CrN VIII as the center and shade in a circle about 1 to 1.5 cm in diameter to see how the lesion would encroach on adjacent nerves and the brainstem as it grows.
If the lesion occupied the internal auditory meatus or canal, other than CrN VII, which CrN would it affect? ______________________.
VIII
If the lesion interrupted the trunk of CrN VII somewhere between its point of exit from the brainstem and the geniculate ganglion, the Pt should have lost __________________ sensation on the anterior two-thirds of the tongue; however, the Pt did not have ageusia.
taste (Figs. 6-5 and 6-6)
CrN VII innervates one muscle in the middle ear, the _________________ muscle. Contraction of this muscle dampens the vibration of the ossicles, protecting the inner ear from excessively loud sounds. After stapedius muscle paralysis, the Pt experiences ordinary sounds as uncomfortably loud, a symptom called hyperacusis.
stapedius
Because the Pt retained taste and had no hyperacusis, the CrN VII lesion must be /□ outside distal to/□ outside within the middle ear.
☑ distal to
If distal to the middle ear, the lesion might be in the facial canal, but a lesion deep within the canal in the temporal bone bars direct clinical examination. If a lesion interrupted CrN VII after its exit from the stylomastoid foramen, the Ex should find pain or swelling in the parotid region, as from an inflammatory or neoplastic mass, but the Pt had no mass or pain in the parotid region.
The next link in a motor nerve comes at the terminal tips of its axons, where the axons synapse on the muscles, a region called the ____________________ junction.
neuromyal (neuromuscular)
Explain why the Pt's lesion is not at the neuromyal junction or in the muscle itself.
_____________________________________________________
___________________________________________________________
Neuromyal junction lesions, such as myasthenia gravis, or myopathies are diffuse disorders and generally are not confined to a single nerve distribution.
Make a line across Table 6-5 at the most likely site of the Pt's lesion.
Your line should cross the facial canal distal to the chorda tympani nerve but proximal to the stylomastoid foramen.
The Pt had idiopathic facial paralysis (Bell's palsy), a common mononeuropathy of CrN VII. The lesion is usually an inflammation, frequently caused by viruses (several herpes viruses have been implicated, particularly HSV-1) or Borrelia burgdorferi (Roberg et al., 1991). The swelling compresses and sometimes completely transects the axons and may occur at various sites along the facial nerve. The Pt recovered good facial function by 6 weeks after onset.
This Pt shows how testing taste helps to localize a lesion along the course of CrN VII. Unless the Pt's symptoms and signs implicate taste, smell, or CrN VII, you may omit taste testing; but make the omission by discretion, not carelessness.
Read Birnberg's (1988) personal account to appreciate how loss of smell and taste impaired the quality of the Pt's life.