The nerves of the human body are required for a variety of tasks, ranging from carrying sensory input to controlling various muscles throughout the body. The twelve pairs of nerves coming from the brain and brainstem are known as the cranial nerves - in other words, the cranial nerves are those which come directly from the brain instead of branching off from the spinal cord.

The cranial nerves are among the most important in the body as they accept input from the eyes, nose, ears and tongue and are responsible for skin sensation and various muscles in the face. Detailed coverage of the nerves is available in the Cranial Nerves Entry. This Entry, on the other hand, looks at the medical examination of the cranial nerves.

Causes of Cranial Nerve Damage

There are a number of diseases which may affect each of the cranial nerves or a combination of them on either or both sides, and so this Entry will not pretend to be able to cover them all. However, there are a number of causes which can affect any of them. These include trauma to the head, diabetes mellitus, multiple sclerosis (MS), systemic lupus, stroke, brain tumours, damage to the nerves or brainstem, sarcoidosis¹, vasculitis², chronic meningitis³ and syphilis⁴. If you are really worried that you may have a problem with your cranial nerves, you're best off talking to a doctor.

I - Olfactory

The olfactory nerve is the sensory nerve that deals with smell. Thus, the first cranial nerve is tested by getting the patient to differentiate between different everyday smells such as mint and cinnamon, one nostril at a time. A lack of smell is known as anosmia, but may sometimes be due to something as mundane as an infection.

II - Optic

The optic nerve is the sensory nerve that deals with sight. It is therefore the most laborious to test as it is necessary to go through four steps:

• Acuity - the patient's visual acuity, ie the clearness of their vision, is tested using a Snellen chart⁵. The visual acuity is tested for each eye on its own without glasses and is then tested using glasses and/or a pinhole if the patient's vision is poor unaided. In the modern Snellen chart, metres are used instead of feet, and so perfect vision is now 6/6⁶ instead of 20/20. Visual acuity may additionally be tested by getting the patient to read a book (for near vision), and by using a set of Ishihara plates, which consist of dotted pictures with hidden numbers and are used to test for colour blindness.

• Fields - these can be tested quite easily by sitting in front of the patient and asking them to cover one eye. Cover your matching eye⁷ and then wiggle your fingers at the edge of your own vision. This should correspond with the visual fields of a normal person, and thus a patient with normal visual fields should always be able to see your hand. There are also more complicated ways of measuring a patient's visual fields, but these are more time consuming and require fancy bits of kit.

• Reflexes - both pupils should constrict in response to a light shone into one eye, and so there are four tests to do. Check each pupil's reaction to light shone into it, and check its 'consensual' reaction to light being shone into the other eye. The size, shape and symmetry of the pupils is also checked.

• Opthalmoscopy - a light is shone into each eye using an opthalmoscope, thus allowing the back of the eye (retina) along with its blood vessels, the fovea (the centre of the retina) and the optic disc (the point where the optic nerve enters) to be visualised.

III, IV and VI - Occulomotor, Trochlear and Abducens

The third, fourth and sixth nerves are motor nerves that control the movement of the eyes, and are thus bundled into one easy test. All in all, there are six muscles per eye, arranged to move the eye in six directions:

• Medial rectus - moves the eye medially.
• Lateral rectus - moves the eye laterally.
• Superior rectus - moves the eye upwards when in the lateral position.
• Inferior rectus - moves the eye downwards when in the lateral position.
• Superior oblique - moves the eye downwards when in the medial position.
• Inferior oblique - moves the eye upwards when in the medial position.

Note that lateral means 'away from the midline of the body', while medial means 'toward the midline of the body'. The above muscles are all controlled by the occulomotor nerve except for the lateral rectus and the superior oblique. The lateral rectus produces a movement known as 'abduction', or moving away from the midline, and is thus controlled by the abducens nerve. The superior oblique muscle passes over a pulley known as a 'trochlear' so that it pulls the back of the eye upwards (and therefore makes the eye look downwards), and is thus controlled by the trochlear nerve.

With this knowledge in mind, testing these three nerves is straightforward. The patient is presented with a target such as a pen lid or finger to look at, and is then asked to follow the target as it makes a large 'H' shape in front of them⁸. By watching the eyes, it is possible to detect signs of nerve damage. The accommodation reflex, which allows the eyes to focus on objects close-up, can be tested by moving the target closer to the nose and then asking the patient to focus on a wall and then on the target. Any double vision, especially when looking downwards past the nose (indicates damage to the trochlear nerve), should be noted, as should any nystagmus⁹.

V - Trigeminal

The trigeminal nerve has three aspects to it, each of which can be tested. The sensory part of the nerve has three branches covering the forehead (opthalmic region), the region above the upper lip (maxilla), and the jawbone (mandible). Sensation over each of these regions should be tested for both sides. The sensory part of the nerve also covers the corneal reflex, whereby the patient will blink if their cornea (the transparent covering at the front of the eye) is touched with a strand of cotton wool. While an absent corneal reflex is one of the earlier signs of trigeminal nerve problems, it is not a pleasant test and should only be conducted with reason. The third aspect of the trigeminal nerve is the motor branch. This controls the muscles of the jaw and can be tested either by getting the patient to open their mouth, or by tapping a finger placed in the dip below the lower lip in order to elicit the jaw jerk reflex.

VII - Facial

Out of all the nerves, the facial nerve is possibly the most fun to test, although a nervous patient may need to be led by example. The patient is asked to:

• Raise their eyebrows,
• Screw their eyes shut, and stop the examiner from gently opening them,
• Puff out their cheeks, and stop the examiner from gently deflating them,
• Show their teeth,
• Whistle.

Rarely, taste (via the facial nerve) is tested using sugary and salty water.

VIII - Vestibulocochlear

The vestibulocochlear nerve is responsible for both hearing and balance, but it is generally hearing that is tested here¹⁰. While problems with hearing may be due to nerve damage, other causes include ear infection and degeneration of hearing over time. A simple hearing test consists of the following:

• Whispering a number in one ear while blocking the other either by putting a finger in it or by rubbing two fingers together next to it. The number is repeated increasingly loudly if the patient fails to hear it the first time, and both ears are tested. Each ear should be tested independently.

• Rinne's test - a vibrating tuning fork is placed on the mastoid process (the bony mass just behind the ear), and is then quickly placed around 4cm from the ear hole. The sound should normally be louder next to the ear hole than it is when in contact with the mastoid process, as the bones of the middle ear that amplify sound are bypassed when the sound travels in through the mastoid process. Each ear should be tested independently.

• Weber's test - a vibrating tuning fork is placed against the middle of the forehead. The sound should be equally audible in both ears.

IX and X - Glossopharangeal and Vagus

The glossopharangeal nerve is responsible for sensation at the back of the mouth, and this can be tested by touching the back of the mouth with a swab. The gag reflex involves both the ninth and tenth nerves, but is unpleasant and is only present in some people. It is tested using a wooden tongue depressor at the back of the tongue. The vagus nerve can be tested by asking the patient to say 'ah'¹¹, upon which the roof of the mouth should rise, with the uvula¹² deviating towards the side of a vagus nerve lesion if present.

XI - Accessory

The accessory nerve controls the sternomastoid muscles and the trapezii. For the former, the patient is asked to turn their head to the left and to the right; for the latter, the patient shrugs their shoulders. Both tests are performed against the examiner's hands in order to provide resistance for the muscles to work against.

XII - Hypoglossal

The patient is asked to stick out their tongue - the tongue will deviate towards the side of a twelfth nerve lesion. Leading by example is usually unnecessary.