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Anatomy

Central Nervous System

Question 157 of 180

Which of the following is most likely to cause a homonymous hemianopia:

Answer:

A posterior cerebral stroke will most likely result in a contralateral homonymous hemianopia with macular sparing.

Visual Pathway

RETINAL BIPOLAR AND GANGLION CELLS:

There are two types of photoreceptors in the human retina, rods and cones. Rods and cones are the first-order receptor cells that respond directly to light stimulation. Bipolar neurons are the second-order neurons that relay stimuli from the rods and cones to the retinal ganglion cells.

OPTIC NERVE:

The optic nerve is formed by the convergence of axons from the retinal ganglion cells. The optic nerve leaves the bony orbit via the optic canal, a passageway through the sphenoid bone. It enters the cranial cavity, running along the surface of the middle cranial fossa (in close proximity to the pituitary gland).

OPTIC CHIASM:

Within the middle cranial fossa, the optic nerves from each eye unite to form the optic chiasm.

OPTIC TRACTS:

At the optic chiasm, fibres from the medial (nasal) half of each retina crossover, forming the right and left optic tracts.

  • The left optic tract contains fibres from the left lateral (temporal) retina and the right medial retina.
  • The right optic tract contains fibres from the right lateral retina and the left medial retina.

Each optic tract travels to its corresponding cerebral hemisphere to reach its lateral geniculate nucleus (LGN) located in the thalamus where the fibres synapse.

OPTIC RADIATION:

Axons from the lateral geniculate nucleus (LGN) then carry visual information, via the upper and lower optic radiations, to the visual cortex in the occipital lobe:

  • The upper optic radiation carries fibres from the superior retinal quadrants (corresponding to the inferior visual field quadrants) and travels through the parietal lobe to reach the visual cortex.
  • The lower optic radiation carries fibres from the inferior retinal quadrants (corresponding to the superior visual field quadrants) and travels through the temporal lobe to reach the visual cortex of the occipital lobe.

VISUAL CORTEX:

At the visual cortex, the brain processes the sensory information and responds appropriately.

By Miquel Perello Nieto (Own work) [CC BY-SA 4.0 , via Wikimedia Commons

Visual Pathway. (Image by Miquel Perello Nieto (Own work) [CC BY-SA 4.0 , via Wikimedia Commons)

Blood Supply

Table: Blood Supply of the Visual Pathway

Structure Blood Supply
Optic nerve (retinal and orbital part) Central retinal artery (branch of the ophthalmic artery)
Optic nerve (intracranial part) and optic chiasm Ophthalmic artery, anterior cerebral and anterior communicating arteries
Optic tract Posterior communicating artery and anterior choroidal artery
Lateral geniculate nucleus Posterior cerebral artery and anterior choroidal artery
Optic radiations Middle and posterior cerebral arteries, anterior choroidal artery
Visual cortex Posterior cerebral artery (macular dually supplied by middle cerebral artery)

Likely Causes of Disease or Injury

Damage to the optic nerve may be caused by:

  • optic neuritis in multiple sclerosis or secondary to measles or mumps
  • optic nerve compression secondary to orbital cellulitis, glaucoma or ocular tumours
  • optic nerve toxicity secondary to ethambutol, methanol and ethylene glycol
  • optic nerve damage secondary to orbital fracture or penetrating injury to the eye
  • optic nerve ischaemia

Compression at the optic chiasm may occur due to:

  • pituitary tumour
  • craniopharyngioma
  • meningioma
  • optic glioma
  • aneurysm of the internal carotid artery

Damage to the visual tract central to the optic chiasm may be caused by:

  • cerebrovascular event
  • brain abscess
  • brain tumours

Common Clinical Effects

Table: Clinical Effects of Lesions of the Visual Pathway

Site of Lesion Visual Defect
Optic nerve Ipsilateral visual loss
Optic chiasm Bitemporal hemianopia
Optic tract Contralateral homonymous hemianopia
Parietal upper optic radiation Contralateral homonymous inferior quadrantanopia
Temporal lower optic radiation Contralateral homonymous superior quadrantanopia
Occipital visual cortex Contralateral homonymous hemianopia with macular sparing

By FRCEM Success

Clinical Effects of Lesions of the Visual Pathway. (Image modified by FRCEM Success. Original by KDS444 [Public domain], via Wikimedia Commons)

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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