Anatomy
Cranial Nerve Lesions
Regarding the olfactory nerve, which of the following statements is CORRECT:
Answer:
The olfactory nerve is the shortest cranial nerve and originates from the cerebrum; it is one of two nerves that do not originate from the brainstem, the other being the optic nerve. Olfactory nerve fibres enter the skull via the cribriform plate of the ethmoid bone to enter to olfactory bulb in the anterior cranial fossa before passing into the olfactory tract which lies inferior to the frontal lobe.Cranial Nerve I: Olfactory Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Nose and Paranasal Region
Last Updated: 11th April 2019
The olfactory nerve (CN I) is a sensory nerve responsible for the sense of smell.
Table: Overview of the Olfactory Nerve
| Cranial Nerve | Olfactory Nerve (CN I) |
|---|---|
| Key anatomy | Nerve fibres from nasal epithelium enter cranial cavity via cribriform plate of ethmoid bone, fibres then enter olfactory bulb in anterior cranial fossa, second-order neurons pass posteriorly into olfactory tract which runs inferiorly to frontal lobe |
| Function | Sensory: sense of smell |
| Assessment | Enquire about changes in smell/taste, formal assessment of smell |
| Clinical effects of injury | Anosmia (loss of smell) and taste |
| Causes of injury | Local nasal disease, tumour of frontal lobe, meningitis, trauma (e.g. fractured ethmoid bone, head injury) |
Anatomical Course
The olfactory nerve consists of a collection of unmyelinated axons of bipolar neurons located in the nasal mucosa which enter the skull via the foramina of the cribriform plate of the ethmoid bone. Once in the cranial cavity, the fibres enter the olfactory bulb which lies in the anterior cranial fossa and synapse, before second-order neurons pass posteriorly into the olfactory tract which runs inferiorly to the frontal lobe.
The olfactory nerve is the shortest cranial nerve and originates from the cerebrum; it is one of two nerves that do not originate from the brainstem, the other being the optic nerve.
Olfactory Nerve. (Image by Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC BY 2.5 , via Wikimedia Commons)
Assessment
The olfactory nerve is usually tested by asking the patient if they have noticed any change in their sense of smell or taste; it can be formally tested using recognisable smells e.g. peppermint, asking the patient to close their eyes and examining each nostril in turn.
Likely Causes of Disease or Injury
Examples of causes of damage to the olfactory nerve include:
- Local nasal disease
- External pressure from a tumour of the frontal lobe
- Meningitis
- Trauma (most common, e.g. contrecoup injury from occipital head trauma, direct coup injury from frontal head trauma, fracture of cribriform plate)
Trauma is the most common cause of olfactory nerve injury and the olfactory nerve is the most commonly damaged nerve in head injury. All patients presenting with olfactory nerve dysfunction, especially following trauma, should be considered for CT scan.
Common Clinical Effects
Injury to the olfactory nerve causes anosmia (loss of sense of smell) with altered sense of taste.
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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 |
