external capsule

**Anatomy (from medial to lateral at the level of the basal ganglia):** Lateral ventricle → caudate / thalamus → internal capsule → globus pallidus → putamen → **external capsule** → claustrum → **extreme capsule** → insular cortex.

- The external capsule is a thin sheet of myelinated fibers that separates the putamen from the claustrum. - The extreme capsule is a separate, more lateral lamina between the claustrum and the insular cortex. - The putamen and globus pallidus together form the lentiform (lenticular) nucleus. The internal capsule separates them from the caudate and thalamus.

**Fiber content:** - Cholinergic projections from the basal forebrain (nucleus basalis of Meynert) to the neocortex. These are relevant to Alzheimer disease, in which cholinergic neurons are lost and ACh is decreased. - Corticocortical association fibers, including parts of the uncinate fasciculus and the inferior fronto-occipital fasciculus (some descriptions place these in the extreme capsule). - Corticostriatal and claustrocortical connections. - Some descriptions also include fibers to the thalamus and brainstem.

**Clinical significance:** - **Internal vs. external capsule.** The internal capsule is a compact projection-fiber bundle (corticospinal, corticobulbar, thalamocortical). Small-vessel lacunar infarcts there cause pure motor hemiparesis. The external capsule is usually not the focus of board questions, but you may be asked to tell it apart from the internal capsule on axial MRI or CT. - **Vascular supply.** The lenticulostriate branches of the MCA supply the basal ganglia and internal capsule. Hypertensive hemorrhage (Charcot-Bouchard microaneurysms) most commonly affects the putamen, and it can extend to adjacent capsular white matter, including the external capsule. - **Imaging.** Putaminal hemorrhage can track along the external capsule. The "slit" or "rim" hyperintensity lateral to the putamen on T2 or FLAIR corresponds to the external capsule and extreme capsule, and the claustrum lies between them. The external capsule is also involved in some metabolic and toxic processes, such as extrapontine myelinolysis, carbon monoxide poisoning, and Wilson disease, which affect the basal ganglia. - **Demyelination and metabolic disease.** Because it is white matter, it can be involved in MS and leukodystrophies, though not characteristically.

**High-yield pearls:** 1. Order of structures, lateral to medial from the insula: insula → extreme capsule → claustrum → external capsule → putamen → globus pallidus → internal capsule. 2. The external capsule carries cholinergic input to the cortex (nucleus basalis of Meynert), which is relevant to Alzheimer disease. 3. Do not confuse it with the internal capsule, whose posterior limb carries the corticospinal tract. 4. This is an anatomy and neuroimaging concept more than a clinical syndrome, so it is tested mainly through anatomy identification questions.