Anteriorly in the medulla at this level are the medullary pyra- mids containing the corticospinal and corticobulbar tracts. Just dorsolateral to the pyramids, are the large inferior olivary nuclei (seeFig. 2.4).
The connections of the inferior olivary nuclei will be more fully discussed when we review the cerebellum. Briefly, the inferior olives receive inputs from the cerebral cortex, basal ganglia and spinal cord, each of which projects to both olivary complexes. Also, there is a large input to each inferior olivary nucleus from the ipsilateral red nucleus. The inferior olives provide the climbing fiber input to the Purkinjie cells of the contralateral cerebellar cortex. In a feedback circuit, they also receive significant input from the deep cerebellar nuclei.
Between the pyramids and the olives, the fibers of the 12th (hypoglossal) cranial nerves exit the medulla. Posterior to the
pyramids in the midline lie the wedge-shaped medial lemnisci which extend posteriorly to the medullary tegmentum. Posterior to the medial lemnisci in the midline are the medial longitudinal fasciculi (MLFs).
In the mid-medulla, between the pyramids and the floor of the fourth ventricle, are the multiple nuclei of the meduallry reticular formation, including the parvicellular nucleus and the magnocellular–gigantocellular nucleus. Among other functions, the reticular formation, possibly via connections with the thalamus, is important for consciousness, wakeful- ness and attention.
In the posterior medulla are found two of the four vestibular nuclei for the 8th cranial nerve (the medial and inferior ves- tibular nuclei), the spinal trigeminal nucleus and tract for cranial nerve 5, the spinothalamic tracts, the inferior cerebel- lar peduncles, and the descending preganglionic sympathetic fibers, called Horner’s tract.
The nuclei which subserve cranial nerve 9 (glossopharyn- geal), cranial nerve 10 (vagus), and cranial nerve 12 (hypoglos- sal) are also found in the posterior medulla. These include the hypoglossal nuclei for cranial nerve 12, and the nucleus ambig- uus, the nucleus solitarius and tract, the inferior salivatory nucelus, as well as the dorsal motor nucelus of the vagus for cranial nerves 9 and 10. The anatomy of the lower cranial nerves will be discussed in more detail in the cases that follow. Fig. 2.4. Cross-section of the medulla at the level of the inferior olives and inferior cerebellar peduncles (again oriented like an axial MRI).
Case 2.1
51-year-old male patient presents with slowly progressive dyspha- gia and hoarseness. The patient complains that he aspirates often while eating. Clinical testing reveals loss of the gag reflex on the right, and leftward deviation of the uvula. Careful swabbing of the back of the pharynx with a cotton swab suggests decreased sensa- tion on the right. ENT testing reveals right vocal cord paralysis. Further neurologic testing detects some weakness in turning the
head to the left against resistance, and some weakness in elevat- ing the right shoulder against resistance. The tongue was normal, without atrophy or fasciculations, and protruded in the midline.
What cranial nerves do you think are involved? Loss of the gag reflex on the right suggests involvement of either the right 9th (glosso- pharyngeal) or 10th (vagus) nerves, or both. The paralysis of the right vocal cord suggests a right 10th nerve lesion. The decreased
sensation along the pharyngeal mucosa suggests a 9th nerve lesion. Shrugging the shoulder and turning the head are functions of the trapezius and sternocleidomastoid muscles respectively. These are innervated by cranial nerve 11. Therefore, the history and physical findings strongly suggest involvement of the 9th, 10th and 11th cranial nerves.
Where do think the lesion may be located? As will be discussed below, the nuclei of the 9th and 10th nerves are in close proximity in the dorsolateral medulla, and so are often involved together. However, the nuclei of cranial nerve 11 subtending the trapezius and sternocleidomastoid muscles are actually in the cervical cord. Therefore, an intra-axial lesion would be unlikely. More likely, the lesion involves the nerves after they have exited the medulla and cervical cord and while they are traveling together. Possible loca- tions would be in the perimedullary cistern at the level of the foramen magnum, at the level of the jugular foramen, or in the neck along the course of the carotid sheath.
Look at the images (Figure2.5 (a)and (b)) What images are provided and what are the findings?
Two axial post-contrast CT images are provided. The left-hand image (Fig. 2.5(a)) is at the level of the skull base, and shows an enhancing lesion just below the right jugular foramen and right carotid canal. The right-hand image (Fig. 2.5(b)) is in the high cervical region, and shows an intensely enhancing mass in the right carotid space, displacing the right ICA anteriorly. The right parapharyngeal fat is displaced anteriorly as well. These findings are characteristic of a carotid space mass. It is noted that the carotid space is often also referred to as the retrostyloid paraphar- yngeal space, especially in neurology books. The three leading differential diagnostic possibilities for a carotid space mass are glomus tumors, schwannomas, or adenopathy. The extension superiorly to the level of the skull base and the intense enhance- ment make adenopathy less likely (deep jugular lymph nodes do not usually extend beyond the level of the jugulodigastric lymph node, which is the apex of the chain). Therefore, we are left with
two possibilities: glomus tumor and vascular schwannoma. Very briefly, glomus tumors are paragangliomas, which arise from neural crest paraganglion cells adjacent to nerves. They are multi- ple about 5 percent of the time (except if familial, where the multiplicity may be as high as 20–30%). Glomus tumors are named for the level at which they occur: glomus tympanicum at the level of the middle ear cavity, glomus jugulare at the level of the jugular foramen, glomus vagale at the level of the nasopharyn- geal or oropharyngeal carotid space, and carotid body tumor at the level of the carotid bifurcation.
Glomus tumors and highly vascular schwannomas are often not separable on imaging, although schawannomas are not typi- cally so highly vascular. In any case, both tumors are often pre- operatively embolized, and resected through the same surgical approach, so making a precise diagnosis is not critical.
This was a case of glomus vagale, producing cranial nerve 9, 10, and 11 findings.
For comparison, look at the case below, in a patient with a similar clinical presentation (Fig. 2.6 (a)and (b)).
Contrast CT, and post-contrast T1-weighted MRI images are provided. Once again, there is a large mass in the right carotid space, displacing the right internal carotid artery anteriorly. As opposed to the case of glomus vagale, however, this large mass shows minimal or no enhancement on post-contrast CT. The diagnosis in this case was a 10th nerve schwannoma. Typically, schwannomas are less vascular than glomus tumors. Once again, however, it is stressed that highly vascular schwannomas do exist, and that radiographic differentiation from glomus tumors in those cases may be impossible.
Another interesting facet of this case is that, although the mass does not show significant enhancement on CT, it avidly enhances on MRI. This discordance is often a source of confusion. However, it should be recalled that there are different contrast enhance- ment mechanisms in CT and MRI. CT functions basically as an electron density map (really, a linear X-ray attenuation coefficient
b
2 3
1 1
a
Fig. 2.5 (a),(b). Axial post-contrast CT images show an intensely enhancing lesion (label #1) extending from the skull base inferiorly along the right carotid space, displacing the right ICA anteriorly (label #2). Note that the fatty parapharyngeal space is also displaced anteriorly (label #3).
map, but this is closely related to electron density), correlating brightness to the number of electrons per voxel of the CT image. What is visualized with CT contrast enhancement is the iodine contrast molecule, with its relatively high atomic number (53), and hence large number of electrons increasing the electron den- sity per voxel. With MRI, however, only hydrogen protons are visualized. Gadolinium atoms provides ‘‘contrast enhancement’’ by shortening the T1 time of hydrogen protons in their vicinity, therefore causing them to appear bright on T1-weighted MRI images, in just the same fashion that protons within intrinsically short T1 time constants, such as hydrogen protons in fat, are bright on T1-weighted MRI. A smaller concentration of gadoli- nium is required to achieve this T1 shortening in MRI than the concentration of iodine needed to achieve appreciable increases in density on contrast CT examination. Therefore, a mass may appear as non-enhancing on contrast CT, yet as avidly enhancing on contrast MRI, as in this case.
Diagnosis Vernet’s syndrome.
Of course, you are probably thinking, ‘‘What the heck is Vernet’s syndrome?’’ It is an eponym found in various textbooks for a syndrome which involves cranial nerves 9, 10 and 11. Vernet’s syndrome is typically defined as ipsilateral loss of taste and sensa- tion of the posterior third of the tongue, ipsilateral loss of gag reflex, ipsilateral vocal cord paralysis, and ipsilateral weakness of the sternocleidomastoid and trapezius muscles. The most common causes are tumors and skull base fractures.
Discuss the functional anatomy of the9th and 10th nerves.
The 9th and 10th nerves are somewhat complicated, because they are both mixed sensory and motor nerves which subserve multiple modalities. Both nerves have general sensory afferent (GSA), general visceral afferent (GVA), special visceral afferent (SVA), special visceral efferent (SVE) and general visceral efferent (GVE) components, which will be described below.
To be able to perform these multiple functions, both nerves receive contributions from multiple brainstem nuclei, including the spinal trigeminal nucleus, the nucleus solitarius, and the nucleus ambiguus, which they have in common, as well as the inferior salivatory nucleus for cranial nerve 9 and the dorsal motor nucleus of the vagus for cranial nerve 10.
Parenthetically, it may seem confusing that the spinal trigem- inal nucleus and tract, which are associated with cranial nerve 5 (the trigeminal nerve), also contribute to the function of other cranial nerves (7, 9 and 10). Also, it is probably confusing that one nucleus, such as the nucleus ambiguus, would contribute to both the 9th and 10th nerves. However, the medulla can be thought of as a transition zone between the spinal cord, where the deep gray matter functions as essentially one large nucleus contributing to numerous spinal nerves, and the pons and midbrain, where the deep nuclei are more discrete in their correlation with cranial nerves.