17.2 Head II: Meninges & Dural Venous Sinuses, Brain Regions & Ventricles, Blood Supply, Cranial Nerves, Pituitary & Pineal, Brainstem Syndromes

Why this area matters

The TOS head list continues with:

• cranial meninges (dura mater, pia-arachnoid, meningeal spaces);
• the brain (parts, ventricular system, blood supply, venous drainage);
• cranial nerves;
• the pituitary and pineal glands.

Neuroanatomy is the basis for localizing strokes, head injuries, tumors and infections in Medicine, Surgery and Pediatrics.

Cranial meninges and meningeal spaces

Dural venous sinuses:

• The superior sagittal sinus runs in the upper edge of the falx and receives CSF through the arachnoid granulations.
• The inferior sagittal sinus joins the great cerebral vein (of Galen) to form the straight sinus.
• These meet at the confluence of sinuses and drain through the transverse and sigmoid sinuses to the internal jugular vein at the jugular foramen.
• The cavernous sinuses lie on each side of the sella turcica (contents below). They receive the ophthalmic veins and drain through the petrosal sinuses.

Parts of the brain and the ventricles

• Cerebrum:
  • frontal lobe: motor cortex, Broca area, personality;
  • parietal lobe: somatosensory cortex, spatial awareness;
  • temporal lobe: hearing, Wernicke area, memory (hippocampus);
  • occipital lobe: vision.
  • The internal capsule (genu and posterior limb) carries corticobulbar and corticospinal fibers. A small lacunar infarct there causes pure motor hemiplegia of the face, arm and leg.
• Diencephalon: the thalamus (sensory relay) and the hypothalamus (endocrine and autonomic control, temperature, thirst, hunger).
• Brainstem:
  • midbrain: CN III and IV nuclei, the cerebral peduncles;
  • pons: CN V to VIII;
  • medulla: CN IX to XII, and the respiratory and cardiovascular centers.
• Cerebellum: coordination. It connects to the brainstem through the superior, middle and inferior peduncles.
• Ventricles: the lateral ventricles, the third ventricle (between the thalami) and the fourth ventricle (between the pons, medulla and cerebellum). The CSF pathway is described below.

Pituitary and pineal glands

• Pituitary (hypophysis):
  • sits in the sella turcica under the diaphragma sellae, with the optic chiasm just above it (bitemporal hemianopia from tumors) and the cavernous sinuses on each side;
  • the anterior lobe (adenohypophysis) develops from Rathke's pouch, which is also the origin of craniopharyngioma;
  • the posterior lobe (neurohypophysis) is a downgrowth of the hypothalamus;
  • it is reached surgically through the sphenoid sinus (transsphenoidal approach).
• Pineal gland: in the roof of the third ventricle; secretes melatonin; calcifies in adults, which makes it a midline marker on X-ray. Pineal tumors compress the dorsal midbrain and cause Parinaud syndrome (paralysis of upward gaze) and hydrocephalus by blocking the aqueduct.

Arterial supply and venous drainage of the brain

• Arteries: the internal carotid arteries supply the anterior circulation (ophthalmic, posterior communicating, anterior choroidal, anterior cerebral and middle cerebral arteries). The vertebral arteries join to form the basilar artery (posterior circulation: PICA, AICA, superior cerebellar and posterior cerebral arteries).
• The circle of Willis links them through the anterior and posterior communicating arteries. Berry aneurysms form at its junctions, most commonly at the anterior communicating artery. A posterior communicating artery aneurysm compresses CN III, causing a dilated pupil, ptosis and a "down and out" eye.
• Venous drainage: superficial cerebral veins drain to the dural sinuses; deep veins drain to the great cerebral vein (of Galen) and the straight sinus. Cerebral venous sinus thrombosis (pregnancy, dehydration, oral contraceptives) causes headache, seizures and venous infarcts.

Cranial Nerves I through XII & Clinical Neurological Localization

Mastery of cranial nerve trajectories, functional components, and reflex arcs is critical for bedside clinical localization:

Summary of Cranial Nerve Functions & Clinical Deficits

• CN I (Olfactory): Pure sensory; olfaction. Lesion: anosmia (head trauma, Kallmann syndrome).
• CN II (Optic): Pure sensory; vision, afferent limb of pupillary light reflex. Lesion: monocular blindness, Marcus Gunn pupil (relative afferent pupillary defect).
• CN III (Oculomotor): Somatic motor to Superior Rectus, Inferior Rectus, Medial Rectus, Inferior Oblique, Levator Palpebrae Superioris; Parasympathetic to ciliary muscle (accommodation) and pupillary sphincter (miosis). Lesion: "down-and-out" eye, complete ptosis, dilated non-reactive pupil. Diabetic microangiopathy affects the central ischemic core of the nerve (sparing peripheral parasympathetic fibers: normal pupil, "pupil-sparing motor palsy"), whereas compressive aneurysms (PComA) compress external parasympathetic fibers first (fixed dilated pupil).
• CN IV (Trochlear): Somatic motor to Superior Oblique (intorsion and depression during adduction). Unique: only cranial nerve emerging from the dorsal surface of the brainstem and decussating completely. Lesion: vertical diplopia worsening with downward, inward gaze (reading, walking down stairs); patient compensates by tilting the head to the contralateral side.
• CN V (Trigeminal): Three divisions:
  • $V_1$ (Ophthalmic): Sensory from forehead, cornea, dorsum of nose.
  • $V_2$ (Maxillary): Sensory from midface, upper lip, palate, maxillary teeth.
  • $V_3$ (Mandibular): Sensory from lower face, mandibular teeth, anterior 2/3 of tongue (general touch/temp); Motor to muscles of mastication (masseter, temporalis, medial and lateral pterygoids), mylohyoid, anterior belly of digastric, tensor veli palatini, tensor tympani. Jaw deviates toward the side of the lesion on opening (due to unopposed action of contralateral lateral pterygoid).
• CN VI (Abducens): Somatic motor to Lateral Rectus (abduction). Longest intracranial subarachnoid course; highly vulnerable to elevated intracranial pressure (false localizing sign). Lesion: horizontal diplopia, failure of ipsilateral eye abduction.
• CN VII (Facial): Branchial motor to muscles of facial expression, stapedius, posterior belly of digastric, stylohyoid; Parasympathetic to lacrimal, submandibular, and sublingual glands; Special visceral sensory: taste to anterior 2/3 of tongue via chorda tympani; Somatic sensory from external ear.
• CN VIII (Vestibulocochlear): Pure sensory; cochlear division (hearing), vestibular division (acceleration, spatial orientation).
• CN IX (Glossopharyngeal): Branchial motor to stylopharyngeus; Parasympathetic to parotid gland (via otic ganglion); Sensory from posterior 1/3 of tongue (general sensation and taste), palatine tonsil, oropharynx, middle ear cavity, carotid sinus (baroreceptors via Hering nerve), and carotid body (chemoreceptors).
• CN X (Vagus): Branchial motor to pharyngeal constrictors, intrinsic laryngeal muscles, levator veli palatini; Parasympathetic to thoracic and abdominal viscera up to the splenic flexure; Sensory from aortic arch baroreceptors/chemoreceptors, epiglottis, larynx. Lesion: uvula deviates to the contralateral normal side, hoarseness, loss of ipsilateral gag reflex.
• CN XI (Accessory): Pure motor to Sternocleidomastoid (SCM) and Trapezius. Lesion: weakness in turning the head to the opposite side (SCM deficit) and ipsilateral shoulder droop / inability to shrug (trapezius deficit).
• CN XII (Hypoglossal): Pure motor to all intrinsic and extrinsic tongue muscles (Genioglossus, Hyoglossus, Styloglossus) except Palatoglossus (innervated by CN X). Lesion: tongue deviates toward the side of the lesion on protrusion (due to unopposed push of normal contralateral genioglossus).

Upper vs. Lower Motor Neuron Facial Paralysis

Understanding the supranuclear organization of the facial nucleus is high-yield for differentiating central stroke from peripheral Bell's palsy:

• The facial motor nucleus in the caudal pontine tegmentum is divided into dorsal and ventral tiers.
• The dorsal tier (innervating the upper facial muscles: frontalis and orbicularis oculi) receives bilateral corticobulbar innervation from both cerebral motor cortices.
• The ventral tier (innervating the lower facial muscles: perioral muscles, buccinator) receives strictly contralateral corticobulbar innervation.

Cavernous Sinus Anatomy & Syndrome

The cavernous sinuses are paired dural venous sinuses flanking the sella turcica and sphenoid body. Transiting neurovascular structures are strictly stratified:

• Structures in the Lateral Wall (superior to inferior): CN III, CN IV, $CN\ V_1$, and $CN\ V_2$.
• Structures Passing Freely Through the Central Venous Lumen: CN VI (abducens nerve) and the Internal Carotid Artery (ICA).

Because the abducens nerve runs directly through the turbulent venous bloodstream adjacent to the ICA (rather than being protected within a fibrous dural wall), CN VI is the earliest and most severely affected structure in Cavernous Sinus Thrombosis (CST) or expanding ICA aneurysms. Patients present initially with isolated lateral gaze palsy, rapidly progressing to complete external ophthalmoplegia, severe chemosis, proptosis, and periorbital sensory loss in the $V_1\text{--}V_2$ dermatomes.

Cranial Nerve Reflex Arcs

Cerebral Vasculature, CSF Hydrodynamics & Brainstem/Spinal Syndromes

The Circle of Willis & Stroke Vascular Syndromes

• Anterior Cerebral Artery (ACA): Supplies the medial surfaces of the frontal and parietal lobes (motor and sensory representations of the contralateral lower extremity and genitalia on the homunculus). ACA stroke manifests with contralateral sensorimotor paralysis of the lower limb > upper limb, urinary incontinence, and frontal release signs / abulia.
• Middle Cerebral Artery (MCA): Supplies the entire lateral convexity of the cerebral hemispheres (motor and sensory representations of the contralateral face and upper extremity). Stroke manifests with contralateral hemiparesis and hemisensory loss (face/arm > leg), contralateral homonymous hemianopia, aphasia (if dominant left hemisphere: Broca's expressive or Wernicke's receptive), or hemineglect / apraxia (if non-dominant right hemisphere).
• Posterior Cerebral Artery (PCA): Supplies the occipital lobe and inferior temporal lobe. PCA stroke manifests with contralateral homonymous hemianopia with macular sparing (macular cortical representation receives collateral dual blood supply from the MCA).

CSF Circulation & Hydrocephalus

• Path of Flow: Lateral ventricles $\rightarrow$ paired Foramina of Monro $\rightarrow$ Third ventricle $\rightarrow$ Cerebral Aqueduct of Sylvius (narrowest conduit; common site of congenital stenosis) $\rightarrow$ Fourth ventricle $\rightarrow$ exits via the paired lateral Foramina of Luschka and single median Foramen of Magendie into the subarachnoid space $\rightarrow$ reabsorbed into the dural venous sinuses via arachnoid granulations.
• Communicating Hydrocephalus: Non-obstructive; impaired CSF resorption by arachnoid granulations following post-meningitic fibrosis or subarachnoid hemorrhage.
• Non-Communicating Hydrocephalus: Physical structural obstruction within the ventricular tree (aqueductal stenosis, colloid cyst of third ventricle, Chiari malformation).
• Normal Pressure Hydrocephalus (NPH): Chronic form of communicating hydrocephalus affecting the elderly; ventriculomegaly with normal opening lumbar pressure; classic clinical triad: Wet, Wacky, and Wobbly (urinary incontinence, cognitive dementia, and magnetic/apraxic gait).

Brainstem Stroke Syndromes

1. Lateral Medullary (Wallenberg) Syndrome:

   • Arterial Occlusion: Posterior Inferior Cerebellar Artery (PICA) or vertebral artery.
   • Nucleus Ambiguus (CN IX, X): Dysphagia, dysarthria, hoarseness, loss of ipsilateral gag reflex, uvula deviation to contralateral side.
   • Vestibular Nuclei: Severe vertigo, nausea, vomiting, rotatory and horizontal nystagmus.
   • Spinal Trigeminal Nucleus & Tract: Ipsilateral facial loss of pain and temperature sensation.
   • Spinothalamic Tract: Contralateral trunk and extremity loss of pain and temperature sensation (crossed/alternating sensory loss).
   • Inferior Cerebellar Peduncle: Ipsilateral limb ataxia, dysmetria.
   • Descending Sympathetic Tract: Ipsilateral Horner syndrome (ptosis, miosis, anhidrosis).
   • Pathognomonic feature: Corticospinal motor tract and medial lemniscus are situated medially and completely spared; no limb motor weakness occurs.

2. Medial Medullary Syndrome (Dejerine Syndrome):

   • Arterial Occlusion: Anterior Spinal Artery (ASA) or paramedian branches of vertebral artery.
   • Hypoglossal Nucleus / CN XII: Ipsilateral flaccid paralysis and atrophy of the tongue; tongue deviates toward the side of the lesion.
   • Corticospinal Tract: Contralateral spastic hemiparesis of the trunk and extremities.
   • Medial Lemniscus: Contralateral loss of proprioception, vibration, and fine discriminatory touch.

3. Weber Syndrome (Paramedian Midbrain Syndrome):

   • Arterial Occlusion: Paramedian branches of the Posterior Cerebral Artery (PCA) or basilar bifurcation.
   • Oculomotor Fascicles (CN III): Ipsilateral CN III palsy (down-and-out pupil, complete ptosis, dilated fixed pupil).
   • Crus Cerebri (Corticospinal & Corticobulbar Tracts): Contralateral spastic hemiparesis and contralateral lower facial droop.