Thalamus
Relevant Nuclei
The thalamus is complicated. It contains between 38 and 60 unique nuclei, depending on the source. Some of the major ones are depicted in Figure 1. There are nuclei in the thalamus that send and receive signals via tracts that go to specific, localized cortical areas, while others are more diffuse. The function of a nucleus is determined by the strength and location of these connections.
Thalamic nuclei are not well visualized on routine brain MRs, but they can be localized based on a few landmarks. The thalamus is bordered medially by the third ventricle and laterally by the posterior limb of the internal capsule (Figure 2.). Scroll to the level of the habenulae in the axial plane and draw a horizontal line through them (Figure 3.).
The area of the thalamus posterior to the line is called the pulvinar, which means couch or chair lined with pillows in Latin; think of it of the couch that the rest of the thalamus sits on. There are three nuclei that abut the line anteriorly. From lateral to medial, they are the ventral posterolateral (VPL) nucleus, centromedian (CM) nucleus, and dorsomedian (DM) nucleus. On top of the VPL sits the ventral lateral (VL) nucleus, and on top of that the ventral anterior (VA) nucleus (Figure 4.). The anterior nucleus (AN) is located one slice superior, with the mammillothalamic tract located inferior (see Figure 10. in the next section).
Another clinically relevant nucleus to recognize is the lateral geniculate nucleus (LGN). Scroll down a few slices to the top of the midbrain at the level of the red nucleus and look lateral (Figure 5.). Alternatively, in the coronal plane, find the red nucleus and scroll 1 slice posterior (assuming 5 mm slice thickness); the LGN is located above and medial to the hippocampus. A third slightly more involved way is to follow the optic chiasm posteriorly until it ends, which provides a hint to the LGN's function.
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Question 1
a. Where is the lesion?
b. What deficits are expected, sensory or motor?
Question 2
a. What nucleus does this lesion involve?
b. What type of deficits are expected?
Function
Sensory
The VPL takes sensory information from the medial lemniscus (fine touch, vibration, proprioception) and the spinothalamic tract (crude touch, pain, temperature, and pressure) and connects to the primary somatosensory cortex located in the post-central gyrus (Figure 6.).
The LGN takes sensory information from the retina and connects to the primary visual cortex. The LGN has 6 layers. From dorsal to ventral, the first four are the parvocellular layers (parvo = small) which have inputs from retinal midget cells (form and color). The bottom two are magnocellular layers which have inputs from retinal parasol cells (motion).
The medial geniculate nucleus (MGN), located slightly posterior and medial to the LGN, transmits auditory information to the auditory cortices.
Motor
The VL transmits motor information from the basal ganglia and cerebellum to the motor, pre-motor, and supplemental motor cortices (Figure 7.).
The VA transmits motor information from the basal ganglia and deep cerebellar nuclei diffusely to the cortex of the frontal lobe, including motor cortices like the VL but also to the prefrontal cortex (Figure 8.). As a result, the VA is involved in higher order motor function and motor decision making.
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Higher order/associative
The AN transmits information from the hippocampus and mamillary bodies via the mammillothalamic tract to the cingulate gyrus (Figure 9.), which is an important pathway for memory and a component of the Papez circuit (Figure 10.).
The DM transmits information between the amygdala, limbic basal ganglia, and olfactory cortex to the prefrontal association cortex. It is important in memory, attention, and higher order functions (Figure 11.).
The pulvinar transmits information between the tectum and the parietotemporal-occipital association cortex. It is important in the processing of visual information and attention.
The CM transmits information diffusely between the basal ganglia, cortex, and brainstem. It is important in arousal, motor coordination, and pain processing.
Question 3
A 83 year-old-woman presents with status epilepticus.
a. What thalamic nucleus is involved?
Question 4
A 35 year-old right-handed male presents with aphasia.
a. Where is the lesion?
b. How does the location of this lesion relate to aphasia?
Question 5
A 51-year-old woman presents with somnolence, double vision, and impaired memory.
a. Where is the lesion?
functional neurosurgery
The thalamus is an excellent target for stimulation (deep brain stimulation (DBS)) and ablation (stereotactic radiosurgery (SRS), radiofrequency (RF), MR guided focused ultrasound (MRgFUS), or laser interstitial thermal therapy (LITT)) for the treatment of neurological and psychiatric disorders gives its wide array of functions and connectivity throughout the brain.
Essential tremor and Parkinson disease
Thalamic target(s): Ventral intermediate nucleus (VIM).
See Figure 12. and Figure 13. for an example of an appropriately positioned ablation zone in the left VIM, achieved with MRgFUS.
Seizures
Thalamic target(s): Centromedian (CM) and anterior nuclei (AN).
See Figure 14. for an example of CM nucleus DBS lead placement for the treatment of drug-resistant epilepsy. Reflecting its function, CM stimulation also increases arousal, a welcome effect given the prevalence of epileptic encephalopathy in these patients.
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Vascular Anatomy
Arterial
The thalamus is often described as having four vascular territories: the tuberothalamic, inferolateral, paramedian, and posterior territories (Figure 15.). Arterial supply to thalamus is highly variable. Often the arteries supplying the thalamus are clustered in groups of small perforators, each originating from the posterior communicating (PCOM) arteries or posterior cerebral arteries (PCA). In other people, these smaller vessels may arise from a single arterial trunk; in this case, a proximal occlusion may lead to infarction of an entire vascular territory.
The lateral thalamus also receives partial blood supply from the anterior choroidal artery, which usually arises from the posterior wall of the distal internal carotid artery; recall an anterior choroidal artery occlusion can infarct the LGN (Question 2). However, the majority of thalamic blood supply comes from the posterior circulation.
Venous
Venous drainage of the thalamus occurs through small perforating veins that converge on the paired internal cerebral veins and basal vein of Rosenthal (Figure 16.).
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Question 6
Returning to the image from Question 1, which demonstrates a lesion in the right ventral posterolateral (VPL) nucleus.
a. An occlusion of what artery could cause this lesion?
Question 7
20-day-old girl presenting with a seizure after a few days of fuzziness and vomiting.
a. What is the diagnosis?
Variants
The paramedian arteries have variable origins with four types (Figures 16-19.). Type I is considered normal, where each paramedian artery arises from the ipsilateral PCA (Figure 17.). Type IIa results in the paramedian territories being supplied by one PCA, but through separate trunks (Figure 18.). Type IIb, also known as the artery of Percheron, results in the paramedian territories being supplied by one PCA through a single trunk (Figure 19.). Type III results in both paramedian arteries arising from a bridging vessel between the PCAs (Figure 20.). There is also variability in the extent of each vascular territory. Like the balance between the medial and lateral lenticulostriate vessels in the anterior circulation, or the arteries that supply the cerebellum in the posterior circulation, there is a balance between the arteries that supply the thalamus. For example, the tuberothalamic (anterior) territory may receive the majority or all of its blood supply from the paramedian arteries, especially if the tuberothalamic artery is small or absent. Since the paramedian arteries arise from the PCOM, a hypoplastic or absent PCOM, which is a frequent finding on vascular imaging, likely contributes to this territorial variance.
Artery of Percheron Infarct
With the Percheron variant, an occlusion of the single trunk or the PCA proximal to its origin may result in an infarction of the bilateral DM nuclei, or paramedian territories, as shown on the T2 weighted image to the right (Figure 21.). Importantly, the infarct will be centered around and relatively confined to the DM nuclei. If there is diffuse involvement of the thalami bilaterally, the differential is broad but should not include an artery of Percheron infarct. The infarct may extend to the anterior nuclei (see the previous paragraph), but the entire thalamus should not be involved. There may be involvement of the superior midbrain (Figure 22.), termed the "V-sign," which can help narrow the differential when present. More on the differential for bilateral thalamic lesions in the next section.
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Question 8
a. What anatomic variant is present?
