Internal capsule

● Lesion manifestations

▪ Anterior limb of internal capsule:

◦ Contains major pathways relevant to neuropsychiatric symptoms:

Frontostriatal fibres

Anterior thalamic radiation

Frontopontine fibres

◦ Disruption may produce:

Apathy and reduced initiation

Reduced motivation and goal-directed behaviour

Executive dysfunction

Cognitive slowing

Reduced emotional responsiveness

Depressive symptoms

Behavioural disinhibition (particularly with involvement of orbitofrontal circuitry)

▪ Genu of internal capsule:

◦ Contains predominantly corticobulbar fibres

◦ Lesions may produce:

Dysarthria

Facial weakness

Impaired voluntary control of bulbar musculature

◦ Neuropsychiatric manifestations are less typical unless adjacent anterior limb structures are involved

▪ Posterior limb of internal capsule:

◦ Contains major corticospinal and somatosensory projection fibres

◦ Lesions may produce:

Contralateral weakness (face, arm, and leg)

Contralateral sensory loss

▪ Bilateral internal capsule involvement:

◦ Greater disruption of frontal-subcortical and frontopontocerebellar circuits:

Severe apathy/abulia

Executive dysfunction

Reduced emotional responsiveness

Pseudobulbar affect (particularly with bilateral disruption of frontopontocerebellar pathways)

● Clinical pearls

▪ The internal capsule is a classic example of a structure where anatomical compactness creates disproportionate clinical effects. A small infarct can disrupt thousands of fibres because multiple pathways are tightly bundled together within a small anatomical space.

▪ Internal capsule lesions should be interpreted according to location:

◦ Posterior limb lesions commonly produce motor and sensory deficits.

◦ Anterior limb lesions are more relevant when assessing behavioural, affective, and executive symptoms.

▪ The anterior limb contains frontostriatal fibres and the anterior thalamic radiation, both of which contribute to broader frontostriatopallidothalamocortical (FSPTC) loops involved in motivation, executive function, behavioural control, and affective regulation.

▪ The anterior thalamic radiation connects anterior and medial thalamic nuclei with several cortical regions, including the medial prefrontal cortex, orbitofrontal cortex, and limbic-associated cortical regions, forming part of frontal-thalamic circuits involved in cognition and emotion.