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Anatomy

Cranial Nerve Lesions

Question 19 of 180

Which of the following is NOT a typical feature of an abducens nerve palsy:

Answer:

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.

Cranial Nerve VI: Abducens Nerve

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

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