Anatomy
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Cranial Nerve Lesions
A 49 year old man presents to ED complaining of visual disturbance in his right eye. On examination you note when you shine a light in his right eye, neither his right or left pupil constricts. When you shine a light in his left eye, both his left and right pupil constrict. Which of the following nerves is most likely affected:
Answer:
Loss of the afferent pupillary light reflex is seen in complete palsy of the optic nerve. The ipsilateral direct and contralateral consensual reflexes are lost as the afferent optic nerve does not sense the light shone in the affected eye but the contralateral direct and ipsilateral consensual reflexes are intact as the efferent oculomotor nerve is normal.Cranial Nerve II: Optic Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Orbit and Eye
Last Updated: 11th April 2019
The optic nerve (CN II) is a purely sensory nerve, which carries visual information from the retina to the visual cortex.
Table: Optic Nerve
| Cranial Nerve | Optic Nerve (CN II) |
|---|---|
| Key anatomy | Formed from convergence of axons of neurons in ganglion layer of retina, surrounded by cranial meninges, enters skull via optic canal of sphenoid bone, receives blood supply from combination of anterior cerebral, ophthalmic and central retinal arteries |
| Function | Sensory: vision, afferent pathway of pupillary light reflex |
| Assessment | Visual acuity (Snellen chart), colour vision (Ishihara plates), pupillary light response, optic disc (fundoscopy), visual fields (tests visual pathway) |
| Clinical effects of injury | Ipsilateral monocular visual loss, loss of colour vision, abnormal pupillary light reflex, visual field defects if damage to visual pathway |
| Causes of injury | Optic neuritis in multiple sclerosis, optic nerve compression in orbital cellulitis or glaucoma, optic nerve toxicity, trauma (e.g. orbital fracture, penetrating injury to eye), ischaemia secondary to vascular disease |
Anatomical Course
The optic nerve is not a true cranial nerve but rather an extension of the brain carrying afferent fibres from the retina of the eyeball to the visual centres of the brain. It is one of two cranial nerves that do not arise from the brainstem, the other being the olfactory nerve.
The optic nerve is surrounded by the cranial meninges, including the subarachnoid space, which extend as far forwards as the eyeball. Any increase in intracranial pressure will therefore result in increased pressure in the subarachnoid space surrounding the optic nerve. This impedes venous return along the retinal veins, causing oedema of the optic disc (papilloedema).
The optic nerve leaves the orbit through the sphenoidal optic canal.
The optic nerve receives its blood supply from the anterior cerebral, ophthalmic and central retinal arteries.
Optic Nerve. (Image by OpenStax [CC BY 4.0 , via Wikimedia Commons)
Assessment
To assess the optic nerve:
- The patient should be asked if they have noticed any changes in their vision
- Visual acuity should be assessed with a Snellen chart
- Visual fields should be assessed, testing each visual quadrant in turn
- Colour vision should be assessed with Ishihara plates
- Pupillary response should be tested using a swinging light to assess direct and consensual reflexes (this tests both the afferent optic nerve and the efferent oculomotor nerve)
- The optic disc should be assessed using fundoscopy
Likely Causes of Disease or Injury
Causes of damage to the optic nerve include:
- 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 trauma secondary to orbital fracture or penetrating injury to the eye
- Optic nerve ischaemia secondary to arterial disease
Common Clinical Effects
Lesions of the optic nerve result in:
- Visual loss in the ipsilateral eye
- Loss of colour vision in the ipsilateral eye
- Abnormal pupillary light reflex
- Loss of pupillary light reflex seen in complete transection of the optic nerve:
- Ipsilateral direct reflex lost
- Contralateral consensual reflex lost
- Contralateral direct reflex intact
- Ipsilateral consensual reflex intact
- Relative afferent pupillary defect (RAPD) seen in other optic nerve disease:
- Paradoxical direct and consensual dilatation when light is shone in the affected eye directly after being shone in the unaffected eye (the affected eye still senses light and constricts, but to a lesser extent than when light is shone in the unaffected eye, therefore the pupils appear to dilate)
- Loss of pupillary light reflex seen in complete transection of the optic nerve:
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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 |
