Thalamus
Relevant Nuclei
The thalamus is complicated. It contains between 38 and 60 unique nuclei, depending on the source. 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 1.). Scroll to the level of the habenulae in the axial plane and draw a horizontal line through them (Figure 2.).
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 3.).
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 4.). 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
Where is the lesion?
What deficits are expected, motor or sensory?
Answer:
VPL nucleus of the thalamus.
Sensory deficits. A lesion here may cause Dejerine-Roussy syndrome or central post-stroke pain (CPSP), characterized by hypersensitivity to pain.
Explanation:
Note the ovoid T2 hyperintensity in the right thalamus. Draw a horizontal line through the habenulae. The bright spot is centered within the lateral most nucleus that is sitting on top of the habenula line; the VPL. This area has reciprocal connections with the somatosensory cortex, so sensory deficits are expected. More about this in the next section.
question 2
What nucleus does this lesion involve?
What deficits are expected?
Answer:
Left LGN.
Visual deficits.
Explanation:
The left coronal T2 shows a focal hyperintensity superior and medial to the left hippocampus. In the same patient, the right axial FLAIR shows a focal hyperintensity lateral to the red nucleus in the expected position of the LGN. Visual deficits are expected, classically a contralateral homonymous hemianopia.
More on the function of the LGN in the next section.
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 5.).
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 6.).
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 7.). 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 8.), which is an important pathway for memory and a component of the Papez circuit (Figure 9.).
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 10.).
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 35 year-old right-handed male presents with aphasia.
Where is the lesion?
Can you make sense of why he has aphasia?
Answer:
Left dorsomedian (DM) nucleus.
The DM connects to the prefrontal cortex, including to Broca's area. A left-sided DM lesion in a right-handed person may cause an expressive aphasia, because the relay station between what you want to say and the muscles required to say it is damaged.
The pulvinar connects to the parieto-temporal association cortex, including to Wernicke's area. A left pulvinar lesion may cause a fluent aphasia, because the relay station between what you hear and how you interpret it is damaged.
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Note this is a simplified model for conceptualizing language, and modern theories support distributed cortical networks over language processing being confined to focal Broca's and Wernicke's areas.
question 4
A 51-year-old woman presents with somnolence, double vision, and impaired memory.
Where is the lesion?
Answer:
Right DM.
These are some of the classically described symptoms of a right DM 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 Figures 11 and 12 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).
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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 11). 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 or posterior cerebral arteries. 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.
Venous
Venous drainage of the thalamus occurs through small perforating veins arising from the paired internal cerebral veins and basal vein of Rosenthal (Figure 12.).
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question 5
Returning to the image from Question 1, which demonstrates a lesion in the right VPL. An occlusion of which artery could cause this lesion?
Answer:
Right thalamogeniculate artery.
Explanation:
The VPL is located in the inferolateral territory, which is typically supplied by the thalamogeniculate artery arising from the PCA (Figure 13.). Recall an infarct here is classically associated with Dejerine-Roussy syndrome, more recently referred to as central post-stroke pain (CPSP).
question 6
20-day-old girl presenting with a seizure after a few days of fuzziness and vomiting.
What is the etiology of this finding?
Answer:
Venous infarct with hemorrhagic conversion.
Left: GRE sequence with susceptibility artifact in the left thalamus and occipital horn of the left ventricle, representing hemorrhage.
Right: Saggital T1 with hyperintensity in the superior saggital sinus, straight sinus, vein of galen, and internal cerebral veins, representing thrombosis.
Variants
The paramedian arteries have variable origins with four types (Figures 13-16.). Type I is considered normal, where each paramedian artery arises from the ipsilateral PCA (Figure 14). Type IIa results in the paramedian territories being supplied by one PCA, but through separate trunks (Figure 15). Type IIb, also known as the artery of Percheron, results in the paramedian territories being supplied by one PCA through a single trunk (Figure 16.). Type III results in both paramedian arteries arising from a bridging vessel between the PCAs (Figure 17).
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 18). 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. More on this in the next section.
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