Week 27

• What is the optic nerve?

  The optic nerve (Cranial Nerve II) is a purely sensory (afferent) nerve that extends from the retina to the optic chiasm and is part of the central nervous system (CNS).
• How many sections is the optic nerve divided into?

  The optic nerve has four sections:

  1. Intra-ocular (1mm)
  2. Intra-orbital (25-30mm)
  3. Intra-canalicular (4-10mm)
  4. Intra-cranial (3-16mm)
• What is the total length of the optic nerve?

  Approximately 50mm.
• How many retinal ganglion cell axons does the optic nerve contain?

  About 1 million axons.
• What is the physiological blind spot?

  A region with no photoreceptors in the optic nerve head, creating a blind spot (~15° temporal to fixation) that is normally unnoticeable due to binocular vision.
• What is the function of the lamina cribrosa?

  A mesh-like collagen structure in the sclera that allows retinal nerve fibre axons to exit the eye and undergoes structural changes in glaucoma.
• How is the optic nerve head (ONH) supplied with blood?

  By the short posterior ciliary arteries, giving the ONH its pink/orange colour. Reduced supply leads to ONH pallor.
• What are the three protective meninges surrounding the optic nerve?

  1. Dura mater (outer, tough)
  2. Arachnoid mater (vascular)
  3. Pia mater (inner, thin)
• What is the purpose of myelination in the optic nerve?

  Myelination protects axons and usually begins post-optic nerve head; in ~1:200 cases, it extends to the retina, appearing as a white patch obscuring retinal features.
• What are the key types of glial cells in the optic nerve?

  Astrocytes, Oligodendrocytes, and NG2 cells, which provide structural support and homeostasis.
• Where does the optic nerve terminate?

  At the optic chiasm, where fibres partially decussate for visual processing.
• What is the normal size and shape of the optic disc?

  About 1.7mm (vertical) x 1.5mm (horizontal), typically a vertical oval.
• What is the ISN’T rule regarding the neuroretinal rim (NRR)?

  The NRR thickness follows this order: Inferior > Superior > Nasal > Temporal.
• What are common glaucoma indicators in the optic nerve head?

  • NRR thinning, especially vertically
  • Increased cup size and pallor
  • Loss of circumlinear vessels following the cup edge
• What is the cup-to-disc (CD) ratio, and what does it indicate?

  Ranges from 0.0 to 1.0; a high ratio may indicate glaucoma or optic atrophy.
• How can the optic disc and cup size be estimated?

  Using direct ophthalmoscopy, indirect ophthalmoscopy with slit beam length, and OCT for precise measurements.
• What is spontaneous venous pulsation (SVP), and why is it important?

  A visible pulsation of the central retinal vein due to intraocular pressure variation; its absence may indicate raised intracranial pressure.
• What is peripapillary atrophy (PPA), and where is it more common?

  Atrophy around the optic disc, seen as scleral or choroidal crescents, more common in high myopia and glaucoma.
• What congenital abnormalities can affect the optic disc?

  • Optic Disc Pit: Small grey oval defect
  • Optic Disc Coloboma: Developmental fissure defect
  • Optic Disc Drusen: Calcium/protein deposits appearing raised
• How can you find your own blind spot?

  1. Close your left eye.
  2. Hold your left thumb and a pen next to each other at arm's length.
  3. Fixate on your left thumb.
  4. Move the pen slowly to the right.
  5. When it reaches the blind spot, part of the pen will disappear.