Anatomy
Cranial Nerve Lesions
A 55 year old woman presents to ED complaining of vertical diplopia with difficulty walking downstairs. She is tilting her head to one side as she says this helps ease the diplopia. Which of the following extraocular muscles is responsible for this picture:
Answer:
The superior oblique, innervated by the trochlear nerve, acts to cause intorsion, depression and abduction of the eyeball. In trochlear nerve palsy the patient complains of difficulty reading or walking downstairs and of vertical diplopia. The eye is extorted and may be elevated and the patient may head tilt to the opposite side of the palsy to compensate.Cranial Nerve IV: Trochlear Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Orbit and Eye
Last Updated: 11th April 2019
The trochlear nerve (CN IV) is a motor nerve supplying the superior oblique muscle of the eye.
Table: Overview of the Trochlear Nerve
| Cranial Nerve | Trochlear Nerve (CN IV) |
|---|---|
| Key anatomy | Arises from midbrain, travels through lateral aspect of cavernous sinus, exits skull through superior orbital fissure |
| Function | Motor: superior oblique muscle of eye (intorsion, depression and abduction of eye) |
| Assessment | Eye movements |
| Clinical effects of injury | Weakness of downward gaze (difficulty reading/walking downstairs), vertical diplopia, eye is extorted and may be elevated (patient head tilts to opposite side to compensate) |
| Causes of injury | Idiopathic, trauma, microvasculopathy, cavernous sinus disease, raised intracranial pressure |
Anatomical Course
It is the smallest cranial nerve but has the longest cranial course. It arises from the trochlear nucleus and decussates within the midbrain, emerging from the posterior aspect of the midbrain. It runs anteroinferiorly within the subarachnoid space before piercing the dura and travelling along the lateral wall of the cavernous sinus, before entering the orbit of the eye via the superior orbital fissure.
Trochlear Nerve. (Image by Henry Vandyke Carter [Public domain], via Wikimedia Commons)
Function
The superior oblique primarily rotates the top of the eye towards the nose (intorsion). Secondarily, it moves the eye downward (depression) and outward (abduction). It prevents the unopposed action of the superior rectus which would otherwise rotate the globe.
Assessment
The trochlear nerve should be assessed together with the oculomotor nerve (CN III) and the abducens nerve (CN VI) by testing ocular movements.
Likely Causes of Disease or Injury
Causes of damage include:
- Idiopathic (most commonly)
- Trauma
- Microvasculopathy (associated with diabetes and hypertension)
- Multiple sclerosis
- Lesions in the midbrain
- Cavernous sinus disease
- Raised intracranial pressure
Common Clinical Effects
Trochlear palsy causes weakness of downward gaze. The patient complains of difficulty reading or walking downstairs and of vertical diplopia. The eye is extorted and may be elevated and the patient may head tilt to the opposite side of the palsy to compensate.
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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 |
