Anatomy

The receptors for smell are in the nasal passages. Afferent fibres pass into the skull through a multitude of small holes, called the cribriform plate, in the base of the skull situated beneath the frontal lobes in the anterior cranial fossa. The olfactory nerves thus formed pass backwards under the frontal lobes to the temporal lobes and olfactory cortex on the same side.

Method of testing

It is best to use items that are familiar to most people, such as perfume or coffee (coffee may be more suitable because some patients may have an allergy to perfume). With the eyes closed the patient is asked to recognise the odour in each nostril separately while the other is occluded.

This is not a particularly useful test because the most frequent cause of anosmia (loss of the sense of smell) is the common cold. Anosmia can also occur following severe head injuries when the olfactory nerves are torn at the floor of the anterior cranial fossa. The nerve can be compressed by a subfrontal meningioma and the loss may be unilateral. Rarely, permanent anosmia can develop without any obvious cause (idiopathic) or following an influenza-like illness.

Anatomy

The visual pathways together, with the visual field abnormalities produced by lesions at certain sites along the pathway, are illustrated in Figure 4.1. Note that the lateral retina radiates back to the occiput on the same side via the optic nerve, chiasm and optic radiation, while the fibres from the medial retina cross at the optic chiasm and radiate to the opposite occipital lobe. The left occipital lobe receives fibres from the left lateral retina and the right medial retina (i.e. the right visual field), while the opposite is the case for the right occipital lobe.

An optic nerve lesion will produce a visual field loss in one eye. The optic chiasm lies above the pituitary gland and beneath the hypothalamus and is subject to compression from pituitary and hypothalamic tumours. Lesions of the optic chiasm will produce a bitemporal field loss. An optic radiation or occipital lobe problem will result in a contralateral visual field loss, either a hemianopia (loss of one-half of the visual field) or a quadrantanopia (loss of one-quarter of the visual field).

Methods of testing

The oculomotor (3rd) nerve

The trochlear (4th) nerve

The abducent (6th) nerve

Methods of examining eye movement

If the patient complains of double vision there are two methods of determining the cause.

The pupillary reflex

The pupil is innervated by parasympathetic fibres that constrict and sympathetic fibres that dilate the pupil. The parasympathetic fibres are on the surface of the 3rd nerve and a dilated pupil results when these are affected. A constricted pupil occurs when the sympathetic fibres are affected by a disease process. This may occur anywhere along the pathway from their origin in the sympathetic ganglion at the level of the 1st and 2nd thoracic nerve roots up through the neck, where the fibres are closely related to the internal carotid artery, into the cranium, where the fibres are adjacent to the carotid siphon of the internal carotid artery, into the orbit via the superior orbital fissure. Sympathetic fibres also innervate the eyelid. Impairment of the sympathetic pathway will result in mild ptosis and a constricted pupil, known as Horner’s syndrome (see Figure 4.9). Other signs of a Horner’s syndrome that are more subtle and more difficult to elicit are enophthalmos (the eye is partially withdrawn into the eye socket) and reduced or absent sweating (anhydrosis) on that side of the face.

Anatomy

The trigeminal nerve has motor fibres that supply the ipsilateral pterygoid and masseter muscles (the muscles that push open and pull closed the jaw, respectively) and sensory fibres that supply sensation to the anterior one-third to two-thirds of the scalp, forehead, cheek and jaw on the same side, but not the angle of the jaw (see Figure 4.11).The motor fibres are only rarely affected by disease whereas there are many causes of sensory loss on the face. It is important to differentiate between altered sensation on the face due to a trigeminal nerve lesion and a quintothalamic tract lesion (the pathway between the 5th cranial nerve nucleus and the thalamus). The angle of the jaw is spared and the sensory loss affects only the anterior one-third to two-thirds of the scalp with a trigeminal nerve (peripheral nervous system) sensory problem whereas, with a quintothalamic tract (central nervous system) lesion, the sensory loss will extend over the entire (contralateral) scalp and face including the angle of the jaw. As this is almost invariably associated with spinothalamic tract involvement, the sensory loss will also affect the neck, arm and leg on the same side.

Method of testing

Whether testing pain, light touch or temperature sensation, test the forehead, cheek and jaw on each side. If an area of abnormality is found, continue to test up over the forehead, across to the midline, back towards the ear and down to below the jaw until you find normal sensation, and the precise distribution of the sensory loss will be determined.

The corneal reflex is tested with cotton wool (not paper as it may abrade the cornea). Approach the eye from the side in order to avoid blinking due to the visual stimulus and touch the lateral aspect of the cornea (not the sclera) gently. The afferent pathway is the 1st division of the trigeminal nerve; the efferent pathway is the facial nerve resulting in eye closure. A similar reflex is the nasal tickle reflex. Tickle the inside of the nose with cotton wool: the afferent pathway is the 2nd division of the trigeminal nerve; the efferent pathway is the facial nerve. A normal corneal and nasal tickle response evokes forced eye closure bilaterally.

The motor component is tested by asking the patient to protrude the jaw (pterygoid muscles). If it is normal it will protrude in the midline; if there is an abnormal 5th motor nerve the jaw will deviate towards the side of the weak muscle – the side of the lesion. Another technique is to gently push the jaw to the right and then to the left; an inability to resist indicates a contralateral 5th motor nerve lesion. To test the masseter muscles the patient is requested to clench the jaw while the examiner places both hands over the muscles to feel for the contraction (see Figure 4.12). If there is an abnormality the muscle cannot be felt to contract beneath the fingers.

Anatomy

The facial nerve nucleus is in the lateral pons of the brainstem. The nerve fibres hook around the 6th nerve nucleus and exit the pons laterally close to the 8th nerve (see Figure 1.6). The nerve then passes across the subarachnoid space and exits the skull through the facial canal and passes through the parotid gland to supply the facial muscles of the forehead, around the eyes (orbicularis oculi), the cheek and around the mouth (orbicularis oris). A branch leaves the nerve before the canal and supplies the stapedius muscle in the middle ear (the nerve to stapedius); if it is affected, for example in a patient with a Bell’s palsy, it causes hyperacusis (increased sensitivity to noise) in the ear. Another branch, the lingual nerve, supplies the sensation of taste to the anterior two-thirds of the tongue; if affected it causes altered taste on the ipsilateral side of the tongue.

Method of testing

Facial movements are tested by asking the patient to show their teeth, close their eyes tightly and then open them wide. This latter command will cause elevation of the eyebrows and differentiates a lower motor from an upper motor facial weakness. The frontalis muscle controls movement of the forehead and has bilateral innervation from the facial nerve. Thus, with an upper motor problem, the patient can still wrinkle the forehead and raise the eyebrows whereas, with a lower motor problem, they cannot (Figure 4.13 shows the difference between upper and lower motor problems).

The most common lesion affecting the facial nerve is a Bell’s palsy. This typically produces a LMN weakness on the same side of the face and is often, but not invariably, associated with ipsilateral hyperacusis (increased hearing in that ear) and altered taste.

Anatomy

The 8th cranial nerve has two components, the auditory or cochlear (hearing) nerve and the vestibular (balance) nerve (see Figure 4.15). The nerve emerges from the ear canal in the cerebellar–pontine angle, pierces the dura mata (the thick membrane surrounding the brain, just beneath the bone) and transverses the subarachnoid space. It enters the brainstem in the lateral pons where the vestibular and auditory nuclei are situated.

Methods of testing

The external ear canal is examined first to ensure it is not occluded by wax, and to assess the eardrum. There are many ways to test hearing, including:

If there is hearing impairment, then it is useful to perform the Weber test and Rinne test using a 256-Hz tuning fork. In the Weber test, the ringing tuning fork is placed in the midline of the forehead. If there is a conduction defect (middle ear problem), the noise will be heard in that ear; on the other hand, if there is an 8th nerve deafness, the noise will be heard in the opposite ear. In the Rinne test, the tuning fork is placed next to the ear and subsequently on the mastoid process. Bone conduction (on the mastoid) is louder than air conduction (next to the ear) when deafness is due to a conduction problem. An easy way to remember the features of these two tests is for examiners to occlude their own external canal and test themselves. Occluding the external canal simulates a conduction defect (see Figure 4.14).

The vestibular pathway

If a patient has the sensation that the head or room is spinning (vertigo), the problem must affect the vestibular pathway (see Figure 4.15), which runs from the ear to the cerebellum or vestibular nuclei in the brainstem and up to the vestibular cortex in the temporal lobe. Practically speaking, most cases of vertigo relate to inner ear problems and, to a lesser extent, cerebellar or brainstem lesions; they are very rarely due to more central pathology. Associated deafness and tinnitus coinciding with the vertigo (and not preexisting, unrelated problems) indicate a problem in the ear while other neurological symptoms such as diplopia, dysarthria, dysphagia, weakness or sensory disturbance point to a problem in the brainstem. Vertigo due to problems in the ear or vestibular nerve is referred to as peripheral vertigo while vertigo related to brainstem or cerebellar problems is referred to as central vertigo. Vertigo is discussed in Chapter 7, ‘Episodic disturbances of neurological function’.

Anatomy

The glossopharyngeal nerve nucleus lies in the lateral part of the medulla; the nerve emerges from the medulla, traverses the subarachnoid space and exits through the jugular foramen in the base of the skull. It supplies sensation to the soft palate and pharynx, together with taste to the posterior two-thirds of the tongue.

Method of testing

The gag reflex is an unpleasant test and is only recommended as part of the examination when the patient complains of dysphagia. The phayrnx is touched with a blunt object such as a spatula. The patient experiences a choking sensation. The soft palate will rise to the opposite side if there is a 10th cranial nerve (the efferent arc of the gag reflex) problem. The reflex will be absent if there is a 9th cranial nerve (the afferent arc of the gag reflex) problem.

Anatomy

The vagus nerve nucleus lies in the lateral aspect of the medulla; the nerve emerges laterally and traverses the subarachnoid space to exit through the jugular foramen at the base of the skull. It then descends in the neck to the thorax and abdomen.

Method of testing

The patient is requested to say ‘ah’ and movement of the soft palate is observed. If there is a problem with the vagas nerve, the palate will deviate to the opposite side as it is pulled upwards by the normal muscle (see Figure 4.16). This can also be observed with the gag reflex, described above.

Anatomy

The accessory nerve nucleus also lies in the lateral medulla and the nerve exits adjacent to the lateral aspect of the pyramids of the motor pathway or corticospinal tract and traverses the subarachnoid space to exit through the jugular foramen in the base of the skull. After it exits the jugular foramen it is joined by the spinal nerve component that derives fibres from the C2–4 nerve roots to innervate the sternocleidomastoid and trapezius muscles. The spinal accessory nerve is prone to iatrogenic injury with posterior dissection of the neck.

Method of testing

The patient turns the head to one side and the examiner observes the sternocleidomastoid muscle on the opposite side (as the muscle pulls the sternum and mastoid closer together). To test the trapezius, the patient shrugs the shoulders against resistance (see Figure 4.17).

Anatomy

The hypoglossal nerve nucleus lies adjacent to the midline of the medulla and the nerve emerges from the anterior aspect of the medulla just laterally to the pyramids of the medulla and traverses the subarachnoid space to exit through the hypoglossal canal in the base of the skull to supply the muscle of the ipsilateral half of the tongue.

Method of testing

The patient is asked to protrude the tongue and, as the muscle pushes the tongue forward, the tongue will deviate towards the paralysed side (see Figure 4.18).

The accompanying DVD demonstrates the techniques for examination of the cranial nerves. The most common disorder to affect the brainstem is cerebrovascular disease. The following discussion entitled the ‘Rule of 4’ is a simplified method for understanding the brainstem and the various vascular syndromes. This has been previously published in the Internal Medicine Journal [1].

The 4 rules of the ‘Rule of 4’

Figure 4.19 depicts a cross-section of the brainstem at the level of the medulla, but the concept of 4 lateral and 4 medial structures also applies to the pons; only the 4 medial structures relate to the midbrain. Figure 4.20 shows the ventral aspect of the brainstem showing the emerging cranial nerves from the midbrain, pons and medulla.

The 4 medial structures and the associated deficit

The 4 lateral structures and the associated deficit

The 4 cranial nerves in the medulla and the associated deficit

The 12th cranial nerve is the motor nerve in the midline of the medulla. Although the 9th, 10th and 11th cranial nerves have motor components, using the simple rule that they do not divide evenly into 12, they are thus not motor nerves in the midline.

The 4 cranial nerves in the pons and the associated deficit

The 6th cranial nerve is the motor nerve in the pons. The 7th is a motor nerve but it also carries pathways of taste and, using the Rule of 4, it does not divide equally into 12 and thus it is not a motor nerve that is in the midline. The vestibular portion of the 8th nerve is not included in order to keep the concept simple and to avoid confusion. Nausea, vomiting and vertigo occur with involvement of the vestibular connections in the lateral medulla.

The 4 cranial nerves above the pons and the associated deficit

The 3rd and 4th cranial nerves are the motor nerves in the midbrain.

The motor and sensory pathways pass through the entire length of the brainstem and can be likened to ‘meridians of longitude’ while the various cranial nerves can be regarded as ‘parallels of latitude’. If you can establish where the meridians of longitude and parallels of latitude intersect, you have established the site of the lesion.

Thus a medial brainstem syndrome will consist of the 4 Ms + the relevant motor cranial nerve (3rd, 6th or 12th) and a lateral brainstem syndrome will consist of the 4 Ss + either the 9th, 10th and11th cranial nerves if in the medulla or the 5th, 7th and 8th cranial nerves if in the pons.

Medial (paramedian) brainstem syndromes

Let us assume that the patient you are examining has a brainstem problem (most often a stroke). If you find UMN signs in the arm and the leg on one side then you know the patient has a contralateral medial brainstem syndrome because the motor pathway is paramedian and crosses at the level of the foramen magnum (at the decussation of the pyramids, where the brainstem meets the spinal cord). The involvement of the motor pathway is the ‘meridian of longitude’. Refer to Figure 4.21 for a summary of the signs. So far the lesion could be anywhere in the medial aspect of the brainstem although, if the face is also affected, it has to be above the mid pons, the level of the 7th nerve nucleus.

The motor cranial nerve, ‘the parallel of latitude’, indicates whether the lesion is in the medulla (12th), pons (6th) or midbrain (3rd). Remember that cranial nerve palsy will be ipsilateral to the side of the lesion and contralateral to the hemiparesis. If the medial lemniscus is also affected, you will find a contralateral (the same side affected by the hemiparesis) loss of vibration and proprioception in the arm and leg as the posterior columns also cross at or just above the level of the foramen magnum.

The median longitudinal fasciculus (MLF) is usually not affected when there is a hemiparesis as the MLF is further back in the brainstem. The MLF can be affected in isolation, a ‘lacunar infarct’, and this results in an ipsilateral internuclear ophthalmoplegia, with failure of adduction (movement towards the nose) of the ipsilateral eye and leading eye nystagmus on looking laterally to the opposite side of the lesion in the contralateral eye. If the patient has involvement of the left MLF, when asked to look to the left the eye movements would be normal, but on looking to the right the left eye would not go past the midline while there would be nystagmus in the right eye as it looked to the right.

Lateral brainstem syndromes

Once again we are assuming that the patient you are seeing has a brainstem problem, most likely a vascular lesion. The 4 Ss or ‘meridians of longitude’ will indicate that you are dealing with a lateral brainstem problem, and the cranial nerves or ‘parallels of latitude’ will indicate whether the problem is in the lateral medulla or lateral pons. Refer to Figure 4.22 for a summary of the signs.

A lateral brainstem infarct will result in:

An ipsilateral Horner’s syndrome with partial ptosis and a small pupil (miosis) is due to involvement of the sympathetic pathway. The power, tone and reflexes should all be normal. So far all we have done is localise the problem to the lateral aspect of the brainstem. By adding the relevant 3 cranial nerves in the medulla or the pons we can localise the lesion to one of these regions of the brain.

The lowest 4 cranial nerves are in the medulla and the 12th nerve is in the midline so that the 9th, 10th and 11th will be in the lateral aspect of the medulla. When these are affected the result is dysarthria and dysphagia with an ipsilateral impairment of the gag reflex so that the palate will pull up to the opposite side. Occasionally, there may be weakness of the ipsilateral trapezius and/or sternocleidomastoid muscle. This is lateral medullary syndrome, usually due to occlusion of the ipsilateral vertebral or posterior inferior cerebellar arteries.

The next 4 cranial nerves are in the pons and the 6th nerve is the motor nerve in the midline, so that the 5th, 7th and 8th are in the lateral aspect of the pons. When these are affected there will be ipsilateral facial weakness, weakness of the ipsilateral masseter and pterygoid muscles (muscles that open and close the mouth) and occasionally ipsilateral deafness. A tumor such as an acoustic neuroma in the cerebellopontine angle will result in ipsilateral deafness, facial weakness and impairment of facial sensation; there may also be ipsilateral limb ataxia if it compresses the ipsilateral cerebellum or brainstem. The sympathetic pathway is usually too deep to be affected.

If there are signs of both a lateral and a medial brainstem syndrome, one needs to consider a basilar artery problem, possibly an occlusion.

In summary, if one can remember that:

1. Gates, P. The rule of 4 of the brainstem: A simplified method for understanding brainstem anatomy and brainstem vascular syndromes for the non-neurologist. Intern Med J. 2005;35(4):263–266.

2. Adams, R.D., Victor, M. Principles of neurology. New York: McGraw–Hill Book Company; 1985. [1186].

3. Gerling, J., Meyer, J.H., Kommerell, G. Visual field defects in optic neuritis and anterior ischemic optic neuropathy: Distinctive features. Graefes Arch Clin Exp Ophthalmol. 1998;236(3):188–192.

4. Williams, P.L., et al. Gray’s anatomy, 37th edn. London: Churchill Livingstone; 1989.

5. Moore, K.L., Dalley, A.F. Clinically oriented anatomy, 4th edn. Maryland: Lippincott Williams & Wilkins; 1999.

6. Davies, E., et al. Gray’s anatomy, 34th edn. Longmans; 1967. [1011, Figure 813].