A medley of midbrain maladies: a brief review of midbrain anatomy and syndromology for radiologists

Abstract

The midbrain represents the uppermost portion of the brainstem, containing numerous important nuclei and white matter tracts, most of which are involved in motor control, as well as the auditory and visual pathways. Notable midbrain nuclei include the superior and inferior colliculus nuclei, red nucleus, substantia nigra, oculomotor nuclear complex, and trochlear nucleus. In addition, white matter tracts include the brachium conjunctivum, medial and lateral lemniscus, spinothalamic tracts, and the fiber tracts within the cerebral peduncles. Although neurologically vital, many of these small midbrain nuclei and white matter tracts are not easily individually identified on neuroimaging. However, given their diverse functions, midbrain pathology often leads to distinct clinical syndromes. A review and understanding of the location and relationships between the different midbrain nuclei and fiber tracts will allow more precise correlation of radiologic findings with patient pathology and symptomatology. Particular syndromes associated with midbrain pathology include the Weber, Claude, Benedikt, Nothnagel, and Parinaud syndromes. The oculomotor and trochlear cranial nerves also reside at this level. An understanding of their functions as well as their projected courses from the midbrain towards the eye allows identification of distinct locations which are particularly vulnerable to pathology.

Figure 1

Major afferent connections to the inferior colliculus nucleus. Afferent axons from the medial geniculate, contralateral inferior colliculus, lateral lemniscus, cerebellar cortex and primary auditory cortex connect to the inferior colliculus nuclei. Efferent connections include the medial geniculate body, contralateral inferior colliculus, superior colliculus, lateral lemniscus, and cerebellum (Figure 2).

Figure 2

Major efferent connections to the inferior colliculus nuclei. Inferior colliculus nuclei project to the medial geniculate, superior colliculus, cerebellar cortex, contralateral inferior colliculus and lateral lemniscus.

Figure 3

Major white matter tracts within the tegmentum at the level of the inferior colliculus. White matter tracts include ascending sensory fibers within the medial lemniscus, trigeminal lemniscus, and spinothalamic tracts. Decussating cerebellar fibers within the brachium conjunctivum are involved in motor coordination.

Figure 4

Major nuclear groups within the tegmentum at the level of the inferior colliculus.

Figure 5

Pathway of the Trochlear Nerve. Elevation and enlargement of the brainstem exposes fibers originating from the trochlear nucleus exiting the dorsal aspect of the midbrain as the Trochlear nerve. This nerve wraps anteriorly and passes between the posterior cerebral artery and superior cerebellar artery along with the oculomotor nerve and continues through the superior orbital fissure to innervate the superior oblique muscle. The dorsal view of the trochlear nerve demonstrates its close proximity to the pineal gland.

Figure 6

Midbrain anatomy at the level of the Superior Colliculus. The red nucleus and oculomotor nuclear complex are noted at this level in addition to other essential nuclear groups.

Figure 7

Pathway of the oculomotor nerve. Projections of the somatic and parasympathetic fibers of the oculomotor nerve extend from the midbrain through the cavernous sinus and into the orbit.

Figure 8

Ventral Midbrain. The ventral midbrain is comprised of the substantia nigra and cerebral peduncles (corticospinal, corticobulbar and corticopontine fiber tracts).

Figure 9

Substantia Nigra. The normal substantia nigra anatomy is shown on a myelin stained section. This pigmented band of tissue dorsal to the cerebral peduncle is divided into two zones. The zona compacta represents the dorsal layer and appears slightly brighter on MRI and sectioning (1). The zona reticulata is the more ventrally located zone which is slightly darker in hue (2). The red nucleus (3) and cerebral peduncle (4) are also identified on these images.

Figure 10

Vascular supply of the midbrain at the level of the superior colliculus is divided into 3 zones. The superior colliculus and tectum are supplied by the superior cerebellar artery. Vascularization of the medial zone is provided by paramedian branches of the basilar artery. The lateral zone is supplied by the posterior cerebral artery.

Figure 11

Carcinomatosis involving the left oculomotor nerve. Axial (a) and coronal (b) postcontrast and fat saturated images at the level of the midbrain demonstrate enhancement of the left cranial cranial nerve III. This is compatible with a diagnosis of carcinomatous lymphomatosis.

Figure 12

Parinaud's Syndrome. T1 weighted sagittal (a) and postcontrast sagittal (b) images in a 28 year old with a pineocytoma compressing the midbrain tectum. Patient presented with paralysis of upward gaze.

Figure 13

Schematic of Weber's syndrome showing a paramedian midbrain infarct involving the cerebral peduncle and 3rd nerve fascicles.

Figure 14

Weber's syndrome. FLAIR (a) and DWI (b) images showing a left paramedian midbrain infarct in a patient with Weber's syndrome. Images used courtesy of Dr. Frank Gaillard at Radiopaedia. org (http://radiopaedia.org/images/94/).

Figure 15

Infarct of the crus cerebri. Axial T2 (a), and DWI (b) MR images of the midbrain with subacute infarction involving the left cerebral peduncle. This patient presented with right-sided face, arm and leg weakness due to involvement of the corticobulbar and corticospinal tracts within the left cerebral peduncle.

Figure 16

Claude Syndrome. Injury to the dorsal tegmentum, including the oculomotor nerve fascicles, and dentato-rubro fibers.

Figure 17

Claude Syndrome. Severely motion degraded axial T2-weighted (a) and DWI (b) images of the midbrain demonstrate a subacute infarct within the left midbrain tegmentum, with extension from the Sylvian aqueduct to the red nucleus. This patient presented with downward abduction of the left eye and right leg incoordination with poor gait. The visual disturbance is related to the involvement of the oculomotor nerve fascicles. Injury to the dentato-rubro fibers and/or red nucleus resulted in ataxia.

Figure 18

Benedikt syndrome. Localization of oculomotor fascicle injury and symptoms of Benedkit syndrome.

Figure 19

Benedikt Syndome. Ovoid lesion within the midbrain demonstrates isointensity on axial T1-weighted image (a), and slight hypointensity on axial T2-weighted (b) image. These findings are consistent with a cavernous hemangioma within the right midbrain, presenting with symptoms of Benedikt Syndrome.

Figure 20

Nothnagel Syndrome. MRI images of the midbrain with enlargement of the midbrain tectum, including the quadrageminal plate, noted on sagittal T1-weighted (a) image. There is associated abnormal T2 hyperintensity seen on accompanying axial T2-weighted (b) image. These findings likely represent tectal glioma with involvement of the oculomotor nuclear complex and decussating fibers of the superior cerebellar peduncle.