Anatomy
Cranial Nerve Lesions
A patient complains of headaches and dizziness. Imaging reveals a tumour at the cerebellopontine angle. Which of the following nerves will most likely be affected by this tumour:
Answer:
The vestibulocochlear nerve and facial nerve both emerge from the brainstem at the cerebellopontine angle so are the nerves most likely to be affected by a tumour in this location.Cranial Nerve VIII: Vestibulocochlear Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Ear
Last Updated: 20th August 2019
The vestibulocochlear nerve (CN VIII) is a sensory nerve which transmits sensory information regarding head position and movement via the vestibular nerve, and regarding the reception of sound via the cochlear nerve.
Table: Overview of the Vestibulocochlear Nerve
| Cranial Nerve | Vestibulocochlear Nerve (CN VIII) |
|---|---|
| Key anatomy | Comprised of vestibular and cochlear components which combine in the pons, emerges from the brainstem at the cerebellopontine angle, enters internal acoustic meatus of temporal bone |
| Function | Sensory: hearing and balance |
| Assessment | Hearing, Weber and Rinne tests |
| Clinical effects of injury | Sensorineural deafness, tinnitus, vertigo, loss of equilibrium, nystagmus |
| Causes of injury | Infection, cerebellopontine angle tumours, basal skull fracture, drugs |
Anatomical Course
The vestibulocochlear nerve is comprised of two parts. The vestibular and cochlear component combine in the pons to form the vestibulocochlear nerve which emerges from the brainstem at the cerebellopontine angle to enter the internal acoustic meatus of the temporal bone. Within the distal aspect of the internal acoustic meatus, the vestibulocochlear nerve splits, forming the vestibular nerve innervating the vestibular system and the cochlear nerve innervating the cochlear.
Vestibulocochlear Nerve. (Image by Zina Deretsky, National Science Foundation [Public domain], via Wikimedia Commons)
Assessment
The vestibulocochlear nerve can be assessed by:
- Testing hearing by whispering numbers into each ear separately and asking the patient to repeat it
- Performing Rinne and Weber tuning fork tests to differentiate between conductive and sensorineural hearing loss
Table: Rinne and Weber Tuning Fork Tests
| Tuning Fork Test | Weber’s test | Rinne’s test |
|---|---|---|
| Screening | Test of lateralisation to assess for asymmetric conductive or sensorineural hearing loss. | Test of comparison of perceived air conduction to bone conduction to assess for conductive hearing loss. |
| Method | Strike tuning fork, place on midline of forehead and hold for up to 4 s. Ask patient to report where the tone is heard: centrally (in the head or in both ears) or towards the left or right. | Start with ear that Weber test has lateralised to. Strike tuning fork and hold about 25 mm from ear canal entrance for about 2 s. Immediately then place against the mastoid and hold for a further 2 s. Ask patient to report whether tone is louder next to the ear (Air conduction: AC) or behind the ear (Bone conduction: BC). |
| Normal hearing | Sound is heard centrally. | AC > BC in both ears (Rinne positive). |
| Unilateral conductive hearing loss | Sound lateralised to affected ear. | In affected ear BC > AC (Rinne negative).
In unaffected ear AC > BC (Rinne positive). |
| Unilateral sensorineural hearing loss | Sound lateralised to unaffected ear. | In affected ear AC > BC (Rinne positive).
In unaffected ear AC > BC (Rinne positive). |
| Bilateral hearing loss | Sound is heard centrally. | Sensorineural: AC > BC in both ears (Rinne positive).
Conductive: BC > AC in both ears (Rinne negative). |
Likely Causes of Disease or Injury
Possible causes of damage to the vestibulocochlear nerve include:
- Infections e.g. vestibular neuritis, mastoiditis and herpes zoster
- Cerebellopontine angle tumours (85% are acoustic neuromas, others include meningiomas, cholesteatomas and primary malignancies of the posterior fossa)
- Tumours invading the temporal bone e.g. brainstem glioma
- Vascular malformations at the cerebellopontine angle
- Drugs e.g. aspirin, furosemide, phenytoin, cytotoxics, alcohol
- Paget's disease
- Fracture of the petrous temporal bone
- Basal skull fracture
Common Clinical Effects
Damage to the vestibulocochlear nerve results in (ipsilateral):
- Sensorineural deafness
- Tinnitus
- Loss of equilibrium
- Nystagmus
- Vertigo
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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 |
