Anatomy
Cranial Nerve Lesions
Regarding the abducens nerve, which of the following statements is CORRECT:
Answer:
The abducens nerve originates in the pons and enters the orbit through the superior orbital fissure to innervate the lateral rectus muscle. Abducens nerve palsy results in a convergent squint and horizontal diplopia.Cranial Nerve VI: Abducens Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Orbit and Eye
Last Updated: 24th October 2020
The abducens nerve (CN VI) is a motor nerve supplying the lateral rectus muscle which acts to abduct the eye.
Table: Overview of the Abducens Nerve
| Cranial Nerve | Abducens nerve (CN VI) |
|---|---|
| Key anatomy | Arises from pons, travels through cavernous sinus, exits skull through superior orbital fissure |
| Function | Motor: lateral rectus muscle of eye (abducts eye) |
| Assessment | Eye movements |
| Clinical effects of injury | Convergent squint with inability to abduct eye, horizontal diplopia |
| Causes of injury | Idiopathic, brain tumours, extradural haematoma, cavernous sinus disease, diabetes mellitus, Wernicke-Korsakoff syndrome |
Anatomical Course
The nerve originates in the pons and exits the brainstem from the inferior pontine sulcus to travel in the subarachnoid space. It traverses the cavernous sinus where it runs alongside the internal carotid artery, and enters the orbit through the superior orbital fissure.
Origins of Right Ocular Muscles and Nerves Entering by the Superior Orbital Fissure. (Image by Henry Vandyke Carter [Public domain], via Wikimedia Commons)
Assessment
The abducens nerve should be assessed together with the oculomotor nerve (CN III) and the trochlear nerve (CN IV) by testing ocular movements.
Likely Causes of Disease or Injury
Causes of CN VI palsy include:
- Idiopathic
- Trauma (up to one-third of cases)
- Diabetes, hypertension
- Pontine stroke
- Extradural haematoma
- Demyelination
- Wernicke's encephalopathy
- Giant cell arteritis
- Tumours (e.g. cerebellopontine angle tumours)
- Basilar artery aneurysm
- Cavernous sinus disease
- Infections e.g. subacute meningitis, tuberculosis
Common Clinical Effects
CN VI palsies result in a convergent squint at rest (eye turned inwards) with inability to abduct the eye because of unopposed action of the rectus medialis. The patient complains of horizontal diplopia when looking towards the affected side. With complete paralysis, the eye cannot abduct past the midline.
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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 |
