Basal ganglia and pituitary

Deep grey matter: anatomy

• Caudate head forms the lateral wall of the frontal horn. Lentiform nucleus = putamen (lateral) + globus pallidus (medial), lying lateral to the internal capsule. Striatum = caudate + putamen.
• From medial to lateral at the level of the thalamus: thalamus, posterior limb of the internal capsule, globus pallidus, putamen, external capsule, claustrum, extreme capsule, insula.
• Subthalamic nucleus lesion (small lacune) gives contralateral hemiballismus.
• Pathways: the nigrostriatal pathway carries dopamine from the substantia nigra pars compacta to the caudate and putamen; its loss causes Parkinson disease. Striatal output neurons use GABA: in Huntington disease the GABAergic neurons of the caudate die first, so the caudate heads atrophy and the frontal horns become box-shaped.
• Hypertensive haemorrhage arises from lenticulostriate arteries (Charcot–Bouchard microaneurysms) and occurs most often in the putamen, then thalamus, pons and cerebellum.

Signal changes in the basal ganglia

Thalamus

• Smell is the only sense with no thalamic relay, so a thalamic stroke spares it.
• Thalamic haemorrhage: sensory loss greater than weakness, eyes deviated down and in, small pupils; later thalamic pain.
• Artery of Percheron: one trunk from a PCA supplying both paramedian thalami and the rostral midbrain: drowsiness, vertical gaze palsy, memory loss. Bilateral thalamic lesions may also be deep venous thrombosis (internal cerebral veins).

Hypothalamus

• Wernicke encephalopathy (alcohol or prolonged vomiting in pregnancy): T2 bright mammillary bodies, medial thalami and periaqueductal grey. A normal MRI does not exclude it.
• Kallmann syndrome: GnRH neurons fail to migrate from the olfactory placode; hypoplastic olfactory bulbs and anosmia.
• Hypothalamic hamartoma: small non-enhancing mass, isointense to grey matter, hanging from the tuber cinereum; gelastic seizures and precocious puberty.

Pineal region

• Pineal calcification is common in adults; calcification in a child under about 10 years suggests a tumour.
• Germinoma (commonest pineal tumour, children and young adults, mainly males): hyperdense mass engulfing the calcification, often with a synchronous suprasellar lesion. Pineoblastoma tends to scatter calcification to the edge.
• A pineal mass compresses the tectum, causing upward gaze palsy, and the aqueduct, causing obstructive hydrocephalus.

Pituitary: development and relations

• The posterior lobe has pituicytes (glia) and Herring bodies (axon swellings holding hormone). Anterior lobe basophils make FSH, LH, TSH and ACTH; acidophils make GH and prolactin.
• ADH and oxytocin are made in the supraoptic and paraventricular nuclei of the hypothalamus and only stored and released by the posterior lobe, so posterior lobe injury causes diabetes insipidus that may recover once the stalk stump releases hormone. The anterior lobe receives hypothalamic releasing hormones through the hypophyseal portal system.
• Prolactin is the only anterior hormone under mainly inhibitory control (dopamine), so stalk compression or removal of hypothalamic input raises prolactin while the others fall. Prolactinoma is the commonest functioning adenoma (galactorrhoea, amenorrhoea); somatotroph adenoma is next. Bromocriptine and cabergoline are dopamine D2 agonists that suppress prolactin and shrink the tumour.
• Sheehan syndrome: the gland enlarged by pregnancy has a fragile low-pressure portal supply, so postpartum haemorrhage and shock cause ischaemic necrosis; failure of lactation is often the first sign.
• A remnant of Rathke pouch can leave a Rathke cleft cyst (ciliated cuboidal lining, non-enhancing, between the lobes) or an ectopic adenoma in the sphenoid sinus along the craniopharyngeal canal.
• Relations: roof, diaphragma sellae (pierced by the stalk); floor, sphenoid body over the sphenoid sinus; above, optic chiasm; laterally, cavernous sinus. Tumours erode downwards into the sphenoid, and surgeons reach the gland by the transsphenoidal route through the nose and sphenoid sinus.
• Cavernous sinus: ICA and VI run inside the sinus (VI is often first hit in sinus thrombosis); in the lateral wall, from above down, lie III, IV, V1 and V2 (lowest).
• Cavernous sinus thrombosis usually follows infection of the "danger area" of the face (a furuncle on the nose or upper lip) spreading through the valveless facial and ophthalmic veins, or sphenoid sinusitis.
• Blood supply: the superior hypophyseal artery feeds the capillary plexus in the median eminence; portal veins carry releasing hormones to the anterior lobe, which has almost no direct arterial supply. So on dynamic MRI the normal gland brightens early and a microadenoma stays dark then fills in late.

Sellar and suprasellar masses

• Pregnancy: lactotroph hyperplasia under oestrogen makes the gland up to about 10–12 mm tall with a convex top; normal for pregnancy only.
• Visual fields: an adenoma presses the chiasm from below, giving upper bitemporal quadrant loss first; a craniopharyngioma from above gives lower quadrant loss first; both end as bitemporal hemianopia.

Remember Posterior pituitary bright on T1 because it stores hormone; anterior pituitary late to enhance because its blood arrives through portal veins.

Exam trap Lentiform nucleus means putamen plus globus pallidus; the caudate is not part of it.

Exam trap Upward growth of a pituitary mass affects the chiasm; lateral growth affects the cavernous sinus nerves (in apoplexy most often III).

How FCPS asks about this

• Thalamic nucleus relaying taste: ventral posteromedial (VPM).
• Teenager with tremor, raised liver enzymes, T2 bright putamina: Wilson disease; ATP7B copper transporter.
• Pulvinar brighter than the putamen: variant CJD.
• Commonest pineal tumour: germinoma; Parinaud upward gaze palsy.
• Lowest nerve in the cavernous sinus lateral wall: maxillary (V2); VI lies inside the sinus.
• Mechanism of bromocriptine: dopamine D2 agonist suppressing prolactin.
• Posterior hypothalamic lesion: loss of heat conservation (poikilothermia); anterior lesion causes hyperthermia.