Anatomy
Cranial Nerve Lesions
Question 127 of 180
Which of the following nerves is the afferent pathway of the corneal blink reflex:
Answer:
The ophthalmic division of the trigeminal nerve is the afferent pathway of the corneal blink reflex. The facial nerve is the efferent pathway of the corneal blink reflex.Cranial Nerve V: Trigeminal Nerve
Anatomy / Cranial Nerve Lesions / Head and Neck / Face
Last Updated: 23rd April 2020
The trigeminal nerve (CN V) is the largest cranial nerve, originating from three sensory nuclei and one motor nucleus extending from the midbrain to the medulla and exiting the brainstem from the pons.
Table: Overview of the Trigeminal Nerve
| Cranial Nerve | Trigeminal Nerve (CN V) |
|---|---|
| Key anatomy | Arises from several nuclei in the brainstem, exits brainstem from pons |
| Sensory function | Face, oral and nasal cavities, frontal sinus, external ear, afferent pathway of corneal reflex |
| Motor function | Muscles of mastication, tensor tympani, tensor veli palatini, mylohyoid, anterior belly of digastric, parasympathetic fibres to lacrimal and nasal glands |
| Assessment | Sensation of face, jaw jerk, corneal blink reflex, power/bulk of muscles of mastication |
| Clinical effects of injury | Flaccid paralysis of muscles of mastication, jaw deviation towards affected side, loss of sensation to face, loss of afferent corneal reflex, loss of jaw jerk |
| Causes of injury | Trauma, anaesthetic block, tumours, cavernous sinus disease |
Anatomical Course
The trigeminal nerve is a mixed motor and sensory nerve. It has three main divisions:
- V1 ophthalmic
- V2 maxillary
- V3 mandibular
Trigeminal Nerve. (Image by Henry Vandyke Carter [Public domain], via Wikimedia Commons)
Function
The trigeminal nerve supplies:
- Sensation to the face, mucous membranes of the nasal and oral cavities and frontal sinus, teeth, hard palate, soft palate and deep structures of the head (proprioception from muscles and the TMJ), the dura of the anterior and middle cranial fossa and the external ear
- The afferent pathway for the corneal reflex
- The muscles of mastication (temporalis, masseter, lateral and medial pterygoids)
- The tensor tympani muscle of the middle ear
- The tensor veli palatini muscle of the soft palate
- The mylohyoid and the anterior belly of the digastric muscles
- Parasympathetic fibres to lacrimal and nasal glands
Assessment
The trigeminal nerve can be assessed by:
- Testing sensation of the face (testing both light touch and pin prick)
- Testing muscles of mastication by asking the patient to clench their teeth and palpating for contraction in the temporalis and masseter muscle (or by asking the patient to open their mouth and move their jaw from side to side)
- Testing the corneal reflex
- Testing the jaw reflex
Likely Causes of Disease or Injury
The trigeminal nerve may be damaged by:
- Fractures of the middle third of the face (V2)
- Trauma to the mandible (V3)
- Anaesthetic block of the inferior alveolar nerve (V3)
- Basal skull fractures
- Tumours e.g. of the maxillary antrum and nasopharynx (V3)
- Cavernous sinus pathology (V1)
- Trigeminal neuralgia (sensory disorder characterised by severe shooting pains usually in the distribution of V2 or V3)
All three branches have bilateral cortical representation so a unilateral central lesion, for example a stroke, does not usually produce a deficit.
Common Clinical Effects
CN V palsy results in:
- Flaccid paralysis of the muscles of mastication
- Jaw deviation to the paralysed side (due to unopposed action of the opposite lateral pterygoid)
- Loss of sensation over the areas innervated by the three divisions of the trigeminal nerve
- Loss of the corneal reflex (afferent pathway)
- Loss of jaw jerk
- Paralysis of tensor tympani muscle leading to hypoacusis
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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 |
