Neurosurgery notes/Anatomy/Cortex/Occipital lobe

Occipital lobe

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Cortex

General

  • The occipital lobe is one of the five lobes of the cerebral hemispheres.
  • It expands to all three cerebral surfaces: lateral, medial, and basal.

Borders

  • On the medial surface, it is separated from the parietal lobe by the parieto-occipital sulcus.
  • On the lateral surface (convexity), it lies behind the parietotemporal line (which runs from the upper end of the parieto-occipital sulcus impression to the preoccipital notch). The occipital convexity is generally not separated from the temporal and parietal lobes by clearly defined sulci.
  • On the basal surface, it is situated behind lines extending from the junction of the calcarine and parieto-occipital sulci medially to the preoccipital notch laterally.

Surfaces

  • Lateral Surface (Convexity):
    • This surface is composed of irregular convolutions with considerable variability.
    • The lateral occipital sulcus, which is typically short and horizontal, divides this surface into the superior and inferior occipital gyri.
    • The transverse occipital sulcus may descend behind the posterior part of the parieto-occipital arcus.
  • Medial Surface:
    • The medial surface is defined by two major sulci: the parieto-occipital sulcus and the calcarine sulcus.
    • The calcarine fissure/sulcus extends forward from the occipital pole toward the splenium. It divides the medial surface into two main convolutions:
      • The Cuneus (upper part), a wedge-shaped lobule bounded anteriorly by the parieto-occipital sulcus and below by the calcarine sulcus.
      • The Lingula or Lingual Gyrus (lower part), a narrow, tongue-like convolution located below the calcarine sulcus.
    • The parieto-occipital sulcus descends to join the anterior part of the calcarine sulcus, creating a Y-shaped configuration.
    • The primary visual receiving area (striate cortex) is located on the upper and lower banks and in the depths of the posterior part of the calcarine sulcus.
  • Basal Surface:
    • The basal surface is continuous with the temporal lobe's basal surface.
    • It is formed by the lower part of the lingula, the inferior occipital gyrus, and the posterior part of the occipitotemporal gyrus.
    • The markings on this surface are the posterior extensions of the collateral and occipitotemporal sulci.

Internal Structures (Ventricles and White Matter)

  • The occipital horn of the lateral ventricle extends posteriorly into the occipital lobe from the atrium. Its size is highly variable.
  • The medial wall of the occipital horn is formed by two prominences:
    • The bulb of the corpus callosum, which overlies and is formed by the forceps major (fibers connecting the occipital lobes).
    • The calcar avis, a prominence that overlies the deepest part of the calcarine sulcus.
  • The roof and lateral wall of the occipital horn are formed by the tapetum (fibers from the corpus callosum).
  • The optic radiations (geniculocalcarine fibers) pass backward to reach the calcarine sulcus on the medial aspect of the occipital lobe. These fibers course along the lateral wall of the atrium and occipital horn.
    • Fibers responsible for the lower visual field course directly backward to end in the upper lip of the calcarine fissure.
    • Fibers destined for the inferior bank of the calcarine fissure (Meyer’s loop) initially loop forward and downward in the temporal lobe before turning back.

Arterial Supply

  • Posterior Cerebral Artery (PCA)
    • Principal blood supply
    • The two main terminal branches of the PCA supplying the cortical surface are:
      • The parieto-occipital artery, which runs in the parieto-occipital fissure and supplies the cuneus and adjacent lateral occipital gyrus.
      • The calcarine artery, which courses within the calcarine fissure and supplies the visual cortex, lingual gyrus, and inferior cuneus.
    • Branches of the PCA also extend around the occipital pole and lower border to reach the adjacent lateral surface.
  • Middle Cerebral Artery (MCA)
    • supplies parts of the lateral occipital gyri, specifically through the angular artery (supplying superior parts) and the temporo-occipital artery (supplying inferior parts).

Venous Drainage

  • Veins draining the lateral surface of the occipital lobe are predominantly ascending veins (occipital veins) that join the superior sagittal sinus, or a few may descend to join the transverse or tentorial sinus.
  • The medial surface is drained by the anterior and posterior calcarine veins. The anterior calcarine vein (also referred to as the internal occipital vein) drains into the deep venous system.
  • The inferior surface is drained by the occipitobasal vein.

Examination

  • Visual acuity
  • Evaluate colour vision
  • Observe for visual phenomena such as visual hallucinations or illusions.

Clinical disease

  • Anton’s syndrome
    • A form of anosognosia restricted to vision
    • Preservation of the pupillary reaction to light
    • Patient denies there is any visual disturbance despite being functionally blind.
      • Patient will confabulate
    • Damage:
      • Primary visual cortex.