CN I (olfactory nerve)

Smell, if tested, requires soft musks, floral and ketone smells rather than astringents, such as ammonia or cloves. The reason for this is that astringents are also noxious. They may stimulate trigeminal (CN V) nerve endings in the nose, causing perception of the stimulus even with completely severed CN 1. Each nostril is tested individually and not necessarily with a different scent, so that the patient is asked if the test scent is the same or different in each nostril. Often the patient will say they are different when they are the same, making interpretation difficult.

A CN 1 palsy should alert the GP to the possibility of a meningioma of the olfactory groove. This is a slow growing tumour that may be ignored because its effects come on slowly. The patient may not be aware that they have lost the sense of smell. Loss of smell, associated with the flu, may be permanent. Sensation of smell and taste are intertwined so a patient complaining of altered taste may be identifying problems with smell.

CN II (optic nerve)

This requires four separate tests: field, fundi, acuity and pupils.

Fields are tested by confrontation, by standing in front of the patient and randomly wiggling fingers in each of the four quadrants of the visual fields (Fig 3.1).

FIGURE 3.1 Testing visual fields.

The patient is asked to point at the wiggling fingers, and at times it is worth wiggling fingers in more than one quadrant to encourage the patient to pay extra attention. If this test suggests abnormality, then each eye should be tested individually by covering the other eye. The crude test has the doctor wiggle the finger in each of the four quadrants of the visual field and the patient must identify the wiggling finger. More sophisticated testing has the doctor move the fingers in on the diagonal in each quadrant (see Fig 3.2).

FIGURE 3.2 Testing visual fields.

The patient is asked to indicate as soon as the finger is seen, and the doctor compares this to their own perception of the finger to see if there is parity between doctor and patient. Use of a red object, such as a red pin, produces a more precise definition of the field of vision. The patient is asked to nominate when the pin is clearly perceived as red, thereby relying on colour vision, rather than a relatively large moving object. Loss of vision must respect the horizontal and vertical meridians to be anatomically sound (see Fig 3.2).

When the patient is uncooperative and will not point to the moving finger or object, an alternative method for testing visual fields is to use ‘menace’. Menace employs a motion as if the examiner is going to poke the patient in the eye, either with a fist or flattened hand, stopping just short of the point of contact. If the patient has preserved vision within the quadrant being tested, it is almost impossible for the patient to avoid blinking. This is an innate, self-protective reflex evoked by the patient seeing the menacing object, such as a fist, approach. Failure to blink in the face of such menace suggests blindness in the fields being tested. It is not absolute but it is very suggestive.

Fundi are tested using an ophthalmoscope. It is important to use the right eye to examine the right eye, to approach the patient from directly in front to avoid the forehead covering the line of vision of the other eye (the one not being examined) and to place the disc (that is, optic nerve) where it is most convenient to view it, rather than looking for it. The way to do this is to ask the patient to look at a convenient object that allows the examiner to approach using that line which follows an imaginary line drawn between the object and the eye (Fig 3.3).

FIGURE 3.3 Ophthalmoscopic examination.

Once the optic nerve has been identified the doctor needs to examine it for colour, contour and for signs of raised intracranial pressure. Asking someone who has little experience of ophthalmoscopy to note colour changes is often difficult but if it looks very white, rather than golden yellow, it suggests optic atrophy. If it is very pink it may reflect increased pressure. These are value judgments that may be too hard to call.

The optic disc contains within it an internal ‘cup’. If this is very obvious it may simply be a variant of normal but it may also herald the first sign of glaucoma and, hence, if noted, should alert the examiner to seek measurement of intraocular pressure from either the optometrist or ophthalmologist.

If the disc margins are ‘blurred’, this may reflect any number of normal conditions, such as medullated nerve fibres (fibres with myelin still in place) but it may also reflect papilloedema. Papilloedema is usually accompanied by haemorrhages and an angry looking disc (see Fig 3.6). Perhaps the first sign of papilloedema is a loss of venous pulsations, which can be seen in 90% of normal people. To seek out venous pulsations, the operator must identify the disc and focus upon it. The relative ratio of arteries to veins is 2 : 3, with the veins being the wider and darker vessels. The doctor selects the end of the vein before it disappears into the nerve, and focuses on that end. If venous pulsations are present then the vein will fill and empty while being observed. Alternatively, the vein may change from fatter to thinner in rhythmic fashion while pulsating. The presence of venous pulsations automatically excludes the presence of raised intracranial pressure but, as already stated, up to 10% of the normal population do not demonstrate such pulsations. Thus, absence of venous pulsations need not reflect raised intracranial pressure, while their presence excludes it (see Figs 3.4 and 3.5).

FIGURE 3.4 The basis of venous pulsations.

FIGURE 3.5 Ophthalmoscopic disc examination.

FIGURE 3.6 Papilloedema.

(Kliegman. Nelson Textbook of Pediatrics, 18th edn. 2007 Saunders; Figure 591-1 (a) Mild papilloedema. Blurred disc margins and venous congestion. (b) Moderate papilloedema. Disc oedematous and raised. Vessels buried within substance of nerve tissue. (c) Severe papilloedema. Haemorrhages are evident within disc (arrow), and there are microinfarcts (soft exudates) in the nerve fibre layer. (d) Macular star (arrow) with oedema residues distributed within the Henle layer of the macula.)

If, after numerous attempts, it is impossible to gain access to the disc through the lens, the doctor must question if there is opacity of the lens and, hence, presence of cataracts. This too should result in referral to the ophthalmologist. Often doctors were taught to look for the ‘red reflex’ but most will not know exactly what this is nor how to identify it. It follows that it is not a helpful tool.

While not directly part of CN II testing, when examining the eyes it makes sense also to look for ptosis (droopy eyelid). Ptosis may obscure the pupil from fundoscopy. If it is necessary to hold the eyelid up while performing fundoscopy, then ptosis should be considered. Equally, pupillary examination is part of CN II examination and pupil size helps to differentiate causes of ptosis. It follows that this is the time to remember to look for ptosis, even though the only cranial nerve palsy to actually cause ptosis is CN III, discussed below.

There are basically only four causes of ptosis:

Acuity can be tested using either or both Schnellan charts and/or near chart tests, and pupils are tested for direct and consensual response using a shielded light source directed at one and then the other pupil.

Shining the light into the right pupil and shielding that light from the left eye should evoke constriction of both pupils. The right eye constricts because of direct response of the light onto the right optic nerve. The left pupil constricts because that message from the right CN II is transmitted consensually to the left pupil via CN III and parasympathetic fibres. Thus blindness in the right eye will prevent the process happening, and damage to the transmission via CN III, assuming functioning right eye, will result in no pupillary response on the left.

Having evoked the response from the right pupil, the shielded light is focused onto the left eye. This should result in a repetition of the above response. If the left eye is blind and does not constrict, then both pupils may dilate as a relaxation of the previous constriction. This does not imply the dilatation is evoked by the left eye, but rather that the left eye cannot perceive the light and does not prevent the dilatation consequent to the earlier constriction. This is known as the ‘swinging lantern sign’.

CN III, IV, VI (oculo-motor, trochlear and abduscens nerves)

Eye movement is tested formally using an ‘H’ manoeuvre (see Fig 3.7).

FIGURE 3.7 Muscles and cranial nerves.

The distant object, namely the image that is furthest away, is generated from the affected eye because the image of the object falls furthest away from central vision on the retina. The distant object is lost by covering the affected eye, and the muscle involved is determined by the ‘H’ manoeuvre.

Where there is loss of distant object, irrespective of which eye is covered, it is unlikely to be due to individual muscle or cranial nerve damage. While testing eye movement, the examiner should also look for nystagmus. As a simple rule of thumb, horizontal nystagmus suggests peripheral deficit and vertical nystagmus suggests central damage. This is not an absolute as cerebellar damage may cause horizontal nystagmus and drug toxicity may cause nystagmus in all directions.

In the unconscious patient, these three cranial nerves can be tested by holding the eyelids up and moving the head in the ‘H’ pattern. The eyes will move by actually remaining in the same position, hence moving within the socket. For example, when the head is moved quickly to the left, the horizontal of the ‘H’, the eyes will move to the right within the socket. This tests the right abduscens (CN VI) and the left oculmotor (CN III) or, alternatively, right lateral rectus and left medial rectus. This tests the equivalent of passive movement of these muscles and hence the competence of the brainstem connections. Where the brainstem is damaged, the eyes will move with the head rather than in the opposite direction—namely, just continue to look ‘straight ahead’ in whatever direction the face is pointing.

A CN III palsy allows the competing muscles (those muscles not innervated by CN III) to dominate eye movement. This causes the eye to be turned down and out. The levator palpebrae (eyelid elevator) is weak, causing ptosis. The parasympathetic innervation of the pupil is weak, allowing the sympathetic autonomic function to dominate, causing an enlarged pupil. The parasympathetic fibres travel on the outside of the nerve so pressure from the outside will affect those fibres first, causing midriosis. Partial ptosis with midriosis should raise suspicion of a partial CN III palsy. This may be caused when the CN III nerve passes between the posterior cerebral artery and the posterior communicated artery of the circle of Willis. Thus, partial CN III palsy may result from an aneurysm of the posterior communicating artery. This should alert the doctor to order a computer tomography (CT angiogram), which is not invasive and available to most GPs. It is important to advise the radiologist what to look for.

A CN IV lesion is often overlooked. It innervates the superior oblique muscle of the eye, causing the eye to turn down and in (see Fig 3.7). It is not the only muscle doing this as CN III turns the eye in (medial rectus) and pulls the eye down (inferior rectus). Thus competing muscles may obscure a CN IV palsy presentation. What should alert the doctor is the patient who complains of diplopia and presents with the head held in a tilted position. The tilt is to overcome the diplopia. Remember that full adduction of the eye switches off most of the inferior rectus function (see Fig 3.7). Downward movement in this position is reliant on the superior oblique (innervated by CN IV). This two-step testing helps to identify the CN IV palsy on clinical testing.

CN VI abducts the eye on the horizontal plane. A CN VI palsy prevents the eye from full lateral movement. Such palsy allows there to be sclera between the iris and the margins of the palpebral fissure. If there is still ‘eye white’ lateral to the iris in the fully abducted eye, then a CN VI palsy should be considered. The CN VI has the longest, and hence most precarious, passage through the cranial vault. It follows that a partial CN VI palsy (incomplete abduction) may be a false localising sign of raised intracranial pressure. It should alert the GP to the possible need for repeat examination of the fundus, looking for papilloedema (see Figs 3.5 and 3.6), the first sign of which is loss of venous pulsations. It should also alert to the need for cerebral CT scanning looking for a space occupying lesion causing raised intracranial pressure. Unlike CN IV, where competing muscles produce some of the same eye movement functions, the competitors to the lateral rectus, the only muscle innervated by CN VI, is the medial rectus which adducts the eye. Hence a complete CN VI palsy will cause the eye to be turned in on the horizontal plane.

CN V (trigeminal nerve)

CN V is tested with respect to its three divisions, namely, first, ophthalmic branch, second, maxillary branch, and third, mandibular branch (Figs 3.8 and 3.9). It is important to note that sensory changes, which do not respect these anatomical demarcations, provide positive evidence of psychological, non-organic disease.

FIGURE 3.8 Trigeminal nerve (I).

FIGURE 3.9 Trigeminal nerve (CN V).

While motor testing of CN V is said to be important, motor function is rarely affected without sensory loss. Testing sensation with a motor response is performed via the corneal reflex, which is a useful way to test trigeminal sensation in the unconscious patient. It is not my chosen route, because if tested using cotton wool on the cornea, it is theoretically possible to leave a wisp of cotton wool on the cornea and thereby provoke a corneal ulcer. My chosen route, in unconscious patients, is to approach from the side with a rolled up tip of tissue and stimulate inside the nostril. This will usually evoke twitching of the nose due to an unpleasant sensory stimulation inside the nose. If the patient is conscious but uncooperative, it is best to stimulate surreptitiously so that the patient does not see the tissue approach, thereby evoking an unconscious response that truly reflects sensation of an unpleasant nature, rather than anticipation of that stimulus.

Testing eye closure can be achieved without the need to try to prise the eyes open. The eyelashes provide a natural ‘manometer’ of eye closure. Strong eye closure will result in burying of the eyelashes. If the lashes are evident on one side but hidden on the other with strong eye closure, it is evidence of weak eye closure on the side where the lashes are seen. The crow’s-feet wrinkles at the side of the eye with tight closure offer an alternative manometer of strength.

CN VII (facial nerve)

The maxim ‘lower is upper and upper is lower’ is important when considering the CN VII. Involvement of the lower face, below the eye, suggests upper motor lesion. Involvement of the forehead is indicative of lower motor neurone, namely CN VII involvement. Failure to close the affected eye, with the eye rolling upwards to place the pupil behind the partially closed upper eyelid, is termed Bell’s phenomenon. The easiest way to test spontaneous facial movement is to get the patient to smile or chuckle. This may be achieved by asking if the patient is ticklish or, alternatively, asking if the patient knows any jokes, particularly ‘dirty jokes’. It is important not to ask about ‘dirty jokes’ if it is felt this might offend the patient. Asking about jokes, especially ‘dirty jokes’ where appropriate, almost universally evokes at least a hesitant or self-conscious smile, even in the depressed patient. This is usually more useful in discerning subtle upper motor neurone weakness with the affected cheek moving less quickly than the unaffected side. The naso-labial groove may also be less pronounced on the affected side.

Taste, which travels via the chorda tympani, is affected in CN VII palsy, especially in Bell’s palsy. It joins the CN VII late in its passage, thus more distal involvement of CN VII in Bell’s palsy will affect taste. Testing taste is both difficult and usually unreliable. It will suffice in most cases to question sense of taste when taking a history and where testing is required to leave it to others. Taste involvement has localising value rather than therapeutic implication. It is important, as stated earlier, to remember that taste and smell sensation are interrelated.

The CN VII also sends a small branch to innervate the muscle to the stapedius in the middle ear. If this is affected the patient with Bell’s palsy may complain of hyperacusis. Hence if a patient with Bell’s palsy says that things sound louder, this does not suggest non-organic disease. The symptom has no therapeutic value but is also helpful in localising the lesion affecting the CN VII.

CN VIII (vestibular cochlear nerve)

CN VIII is usually tested by covering the patient’s vision (to exclude lip reading), rubbing the hair on the side opposite to the ear being tested (to confound hearing in that ear) and asking the patient to repeat numbers that are whispered. This tests lower frequency hearing. More formal testing may be achieved by using the same approach for low tones and a ticking wrist watch for high tones. A tuning fork adds additional sophistication with a 256 Hz frequency (middle C) tuning fork being preferred.

Weber’s test involves placing the vibrating fork in the middle of the forehead. It should be perceived in the midline, at the place it is positioned. Should it be perceived lateralised to one ear, that implies either middle ear problems in that ear (enhanced conduction to that side) or neurosensory deafness in the opposite ear. If the patient has difficulty with the tuning fork placed in the midline of the forehead, the same test can be repeated by placing the same vibrating tuning fork at the vertex on the top of the head. Some patients find this easier to perceive.

This testing can be further refined by placing the vibrating tuning fork next to the ear in the air (position 1) and then on the mastoid behind the ear (position 2). Normal hearing conducted via the cochlear nerve (VIII cranial nerve) will perceive air conduction (position 1) louder than bone conduction (position 2). The reverse implies middle ear pathology. This is known as Rinne’s test.

A simple confirmation of hearing deficit is to lightly tap the tuning fork, place it in the air next to the patient’s ear, and ask the patient to identify when it stops emitting a sound. If the doctor can still hear it when the patient cannot, the simple implication is that the patient has less preserved hearing than does the doctor. This is known as Schwabach’s test.

The final test of the ears is direct otoscopy. An ear full of wax provides less clear hearing than an ear without wax. Likewise, it is important to recognise an inflamed ear, or the presence of herpes zoster in the ear of someone with Bell’s palsy (lower motor neurone facial weakness) as it requires antiviral treatment in addition to steroids to treat the Bell’s palsy.

CN IX, X, XI (hypoglossal, vagus and accessory nerves)

The pharyngeal plexus is tested by shining a bright light into the pharynx and asking the patient to say ‘AHHHH!’. This should result in the midline elevation of the uvular and soft palate. Weakness will deviate these structures to the contralateral side. I rarely, if ever, perform the gag reflex as it is most unpleasant and one should be able to predict its response by observing the palatal movement and listening to the patient’s speech.

A lower motor neurone, palatal speech will sound as if air is escaping while the patient is trying to enunciate. Such deficit should cause paucity of gag response. Conversely, a spastic, tight, higher pitch speech should be accompanied by exaggerated gag reflex. A pointer to upper motor neurone ‘pseudobulbar palsy’ is emotional incontinence. This is different to emotional lability in which mood fluctuates apparently from highs and lows without obvious cause. With emotional incontinence, the patient reflects an over-expression of appropriate emotions. Examples of this may be the reflection of sadness with sobbing and crying rather than dull affect. Alternatively the patient may laugh uncontrollably, when the appropriate response may be a polite smile.

Further evidence of ‘pseudobulbar palsy’ will be bilateral representation of frontal lobe signs, such as grasp reflexes as discussed below.

The accessory component of the CN XI is not really a cranial nerve but rather high cervical roots travelling up into the cranium and then out to supply the sternocleidomastoid muscles, which turn the head and elevators of the shoulders, the trapezius muscles. Pushing the chin against a hand will activate the sternocleidomastoid muscle opposite to the direction of the force; that is, pushing to the right with the chin will activate the left sternocleidomastoid muscle (and vice versa).

CN XII (glossopharyngeal nerve)

CN XII innervates the tongue and is best tested by first looking at the tongue and then asking the patient to protrude the tongue in the midline. The tongue will deviate to the affected side, being pushed over. Suspicion of tongue deviation must be balanced by observing facial movement. If the patient has unilateral facial weakness, as occurs in Bell’s palsy, there may be the perception of the tongue deviating to the side of weakness, but this is almost universally wrong, namely the tongue protrudes in the centre of where the lips should have been were it not for the facial weakness.

The tongue is the only place in the body where the observer can see fibrillations with the naked eye. By definition a fibrillation is the spontaneous firing of a single muscle fibre (rather than a motor unit which produces fasciculations and reflects numerous muscle fibres spontaneously firing). The tongue is the only place in the body where single muscle fibres can be seen in the natural setting, and the presence of fibrillations is a most worrying sign suggestive of amyotrophic lateral sclerosis, a form of motor neurone disease.

TABLE 3.1 Summary of cranial nerves