CLINICALLY ORIENTED NEUROANATOMY: ‘MERIDIANS OF LONGITUDE AND PARALLELS OF LATITUDE’

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chapter 1

Clinically Oriented Neuroanatomy

‘MERIDIANS OF LONGITUDE AND PARALLELS OF LATITUDE’

Although most textbooks on clinical neurology begin with a chapter on history taking, there is a very good reason for placing neuroanatomy as the initial chapter. It is because clinical neurologists use their detailed knowledge of neuroanatomy not only when examining a patient but also when obtaining a neurological history in order to determine the site of the problem within the nervous system. This chapter not only describes the neuroanatomy but attempts to place it in a clinical context.

The ‘student of neurology’ cannot be expected to remember all of the detail but needs to understand the basic concepts. This understanding, combined with the correct technique when taking the neurological history (see Chapter 2, ‘The neurological history’) and performing the neurological examination (see Chapter 3, ‘Neurological examination of the limbs’, Chapter 4, ‘The cranial nerves and understanding the brainstem’, and Chapter 5, ‘The cerebral hemispheres and cerebellum’), together with the illustrations in this chapter will enable the ‘non-neurologist’ to localise the site of the problem in most patients almost as well as the neurologist. It is intended that this chapter serve as a resource to be kept on the desk or next to the examination couch.

To help simplify neuroanatomy the concept of the meridians of longitude and parallels of latitude is introduced to liken the nervous system to a map grid. The site of the problem is where the meridian of longitude meets the parallel of latitude. Examples will be given to explain this concept.

It is also crucial to understand the difference between upper and lower motor neurons. The terms are more often (and not unreasonably) used to refer to the central and peripheral nervous systems, CNS and PNS, respectively. More specifically, upper motor neuron refers to motor signs that result from disorders affecting the motor pathway above the level of the anterior horn cell, i.e. within the CNS, while lower motor neuron refers to motor symptoms and signs that relate

• The hallmark of clinical neurology is to evaluate each and every symptom in terms of its nature (i.e. weakness, sensory, visual etc) and its distribution in order to decide where the lesion is.

• The nature and distribution of the symptoms and signs (if present), i.e. the neuroanatomy, point to the site of the pathology in the nervous system. They rarely if ever indicate the nature of the underlying pathology.

to disorders of the PNS, the anterior horn cell, motor nerve root, brachial or lumbrosacral plexus or peripheral nerve (see Table 1.1). The alterations in strength, tone, reflexes and plantar responses (scratching the lateral aspect of the sole of the foot to see which way the big toe points) are different in upper and lower motor neuron problems.

TABLE 1.1

Upper and lower motor neuron signs

	Upper motor neuron signs	Lower motor neuron signs
Weakness	The UMN pattern^∗	Specific to a nerve or nerve root
Tone	Increased	Decreased
Reflexes	Increased	Decreased or absent
Plantar response	Up-going	Down-going

^∗The muscles that abduct the shoulder joint and extend the elbow and wrist joints are weak in the arms while the muscles that flex the hip and knee joints and the muscles that dorsiflex the ankle joint (bend the foot upwards) are weak in the legs.

The reason why this is so important is highlighted in Case 1.1.

The pathology is always at the level of the lower motor neuron signs.

CASE 1.1 A patient presents with difficulty walking

Often the non-neurologist directs imaging at the region of the lumbosacral spine, but this will only detect problems affecting the peripheral nervous system (including the cauda equina) between the 3rd lumbar nerve root (L3) and the first sacral (S1) nerve root. The patient is then referred for a specialist opinion and, when the patient is examined, there are signs of an upper motor neuron lesion that indicate involvement of the motor pathway (meridian of longitude) in the central and not the peripheral nervous system.

The lesion has to be above the level of the 1st lumbar vertebrae, the level at which the spinal cord ends. Imaging has been performed below this level and thus missed the problem. The appropriate investigation is to look at the spinal cord above this level.

CONCEPT OF THE MERIDIANS OF LONGITUDE AND PARALLELS OF LATITUDE

The meridians of longitude

The nervous system can be likened to a map grid (see Figure 1.1). Establishing from the history and examination the ‘meridians of longitude’ and ‘parallels of latitude’ will localise the pathological process.

FIGURE 1.1 Meridians of longitude and parallels of latitude Note: The motor pathway and the pathway for vibration and proprioception cross the midline at the level of the foramen magnum while the spinothalamic tract crosses immediately it enters the spinal cord.

• The descending motor pathway from the cortex to the muscle

• The ascending sensory pathway for pain and temperature

• The ascending sensory pathway for vibration and proprioception

The ascending sensory pathways extend from the peripheral nerves to the cortex.¹

The parallels of latitude

CENTRAL NERVOUS SYSTEM

• Cerebral cortex

• Cranial nerves of the brainstem

PERIPHERAL NERVOUS SYSTEM

• Nerve roots

• Peripheral nerves

If the patient has weakness the pathological process must be affecting the motor pathway somewhere between the cortex and the muscle while, if there are sensory symptoms, the pathology must be somewhere between the sensory nerves in the periphery and the cortical sensory structures. The presence of motor and sensory symptoms/signs together immediately rules out conditions that are confined to muscle, the neuromuscular junction, the motor nerve root and anterior horn cell.

It is the pattern of weakness and sensory symptoms and/or signs together with the parallels of latitude that are used to determine the site of the pathology.

The following examples combine weakness with various parallels of latitude to help explain this concept. The parallels of latitude follow the + sign.

• Weakness + marked wasting – the peripheral nervous system, as marked wasting does not occur with central nervous system problems

• Weakness + cranial nerve involvement – brainstem

• Weakness + visual field disturbance (not diplopia) or speech disturbance (i.e. dysphasia) – cortex

• Weakness in both legs + loss of pain and temperature sensation on the torso – spinal cord

• Weakness in a limb + sensory loss in a single nerve (mononeuritis) or nerve root (radiculopathy) distribution – peripheral nervous system

THE MERIDIANS OF LONGITUDE: LOCALISING THE PROBLEM ACCORDING TO THE DESCENDING MOTOR AND ASCENDING SENSORY PATHWAYS

The descending motor pathway (also referred to as the corticospinal tract) and the ascending sensory pathways represent the meridians of longitude. The dermatomes, myotomes, reflexes, brainstem cranial nerves, basal ganglia and the cortical signs represent the parallels of latitude. The motor pathways and dorsal columns both cross at the level of the foramen magnum, the junction between the lower end of the brainstem and the spinal cord, while the spinothalamic tracts cross soon after entering the spinal cord.

If there are left-sided upper motor neuron signs or impairment of vibration and proprioception, the lesion is either on the left side of the spinal cord below the level of the foramen magnum or on the right side of the brain above the level of the foramen magnum. If there is impairment of pain and temperature sensation affecting the left side of the body, the lesion is on the opposite side either in the spinal cord or brain. If the face is also weak the problem has to be above the mid pons.

Cases 1.2 and 1.3 illustrate how to use the meridians of longitude.

CASE 1.2 A patient with upper motor neuron signs

A patient has weakness affecting the right arm and leg, associated with increased tone and reflexes (upper motor neuron signs).

• This indicates a problem along the motor pathway in the CNS, either in the upper cervical spinal cord on the same side above the level of C5 or on the left side of the brain above the level of the foramen magnum (where the motor pathway crosses).

• If the right side of the face is also affected, the lesion cannot be in the spinal cord and must be in the upper pons or higher on the left side because the facial nerve nucleus is at the level of the mid pons. In the absence of any other symptoms or signs this is as close as we can localise the problem. It could be in the midbrain, internal capsule, corona radiata or cortex.

• The presence of a left 3rd nerve palsy is the parallel of latitude and would indicate a left midbrain lesion (this is known as Weber’s syndrome). Dysphasia (speech disturbance) or cortical sensory signs (see Chapter 5) are other parallels of latitude and would indicate a cortical lesion.

CASE 1.3 A patient with weakness in the right hand without sensory symptoms or signs

A patient has weakness in the right hand in the absence of any sensory symptoms or signs. In addition to the weakness the patient has noticed marked wasting of the muscle between the thumb and index finger.

• Weakness indicates involvement of the motor system and the lesion has to be somewhere along the ‘pathway’ between the muscles of the hand and the contralateral motor cortex. The absence of sensory symptoms suggests the problem may be in a muscle, neuromuscular junction, motor nerve root or anterior horn cell, the more common sites that cause weakness in the absence of sensory symptoms or signs. Motor weakness without sensory symptoms can also occur with peripheral lesions.

• Wasting is a lower motor neuron sign, a parallel of latitude, and clearly indicates that the problem is in the PNS (marked wasting does not occur with problems in the neuromuscular junction or with disorders of muscle; it usually points to a problem in the anterior horn cell, motor nerve root, brachial plexus or peripheral nerve). Plexus or peripheral nerve lesions are usually, but not always, associated with sensory symptoms or signs.

• The examination demonstrates weakness of all the interosseous muscles, the abductor digiti minimi muscle and flexor digitorum profundus muscle with weakness flexing the distal phalanx of the 2nd, 3rd, 4th and 5th digits, which are referred to as the long flexors. All these muscles are innervated by the C8–T1 nerve roots, but the long flexors of the 2nd and 3rd digits are innervated by the median nerve while the long flexors of the 4th and 5th digits are innervated by the ulnar nerve. The parallel of latitude is the wasting and weakness in the distribution of the C8–T1 nerve roots.

The motor pathway

The motor pathway (see Figure 1.2) refers to the corticospinal tract within the central nervous system that descends from the motor cortex to lower motor neurons in the ventral horn of the spinal cord and the corticobulbar tract that descends from the motor cortex to several cranial nerve nuclei in the pons and medulla that innervate muscles plus the motor nerve roots, plexuses, peripheral nerves, neuromuscular junction and muscle in the peripheral nervous system.

FIGURE 1.2 The motor pathway Note: The pathway crosses at the level of the foramen magnum where the spinal cord meets the lower end of the medulla.

The motor pathway:

The motor pathway crosses the midline at the level of the foramen magnum (the junction of the spinal cord and the lower end of the medulla).

• arises in the motor cortex in the pre-central gyrus (see Figure 1.5) of the frontal lobe

• descends in the cerebral hemispheres through the corona radiata and internal capsule

• passes into the brainstem via the crus cerebri (level of midbrain) and descends in the ventral and medial aspect of the pons and medulla

• descends in the lateral column of the spinal cord to the anterior horn cell where it synapses with the lower motor neuron

• leaves the spinal cord through the anterior (motor) nerve root

• passes through the brachial plexus to the arm or through the lumbosacral plexus to the leg and via the peripheral nerves to the neuromuscular junction and muscle.

If a patient has symptoms or signs of weakness, the problem must be somewhere between the muscle and the motor cortex in the contralateral frontal lobe (see Figure 1.2).

Case 1.4 illustrates a patient with both motor and sensory pathways affected.

CASE 1.4 A woman with difficulty walking

A 70-year-old woman presents with difficulty walking due to weakness and stiffness in both legs. There is no weakness in her upper limbs. She has also noticed some instability in the dark and a sensation of tight stockings around her legs. The examination reveals weakness of hip flexion associated with increased tone and reflexes and upgoing plantar responses. There is impairment of vibration and proprioception in the legs and there is decreased pain sensation in both legs and on both sides of the abdomen up to the level of the umbilicus on the front of the abdomen and several centimetres higher than this on the back.

• The weakness in both legs indicates that the motor pathway (meridian of longitude) is affected.

• The alteration of vibration and proprioception also indicates that the relevant pathway (another meridian of longitude) is involved.

• The increased tone and reflexes are upper motor neuron signs and, therefore, the problem must be in the CNS not the PNS, either the spinal cord or brain.

• The fact that the signs are bilateral indicates that the motor pathways on both sides of the nervous system are affected and the most likely place for this to occur is in the spinal cord, although it can also occur in the brainstem and in the medial aspect of the cerebral hemispheres. (For more information on the cortical representation of the legs, not illustrated in this book, look up the term ‘cortical homunculus’ which is a physical representation of the primary motor cortex.)

• The impairment of pain sensation is the 3rd meridian of longitude and indicates that the spinothalamic tract is involved.

• The upper motor neuron pattern of weakness and involvement of the pathway conveying vibration and proprioception simply indicate that the problem is above the level of L1, but the sensory level on the trunk at the level of the umbilicus is the parallel of latitude and localises the lesion to the 10th thoracic spinal cord level (see Figures 1.12 and 1.13).

This is not an uncommon presentation, and it is often taught that a thoracic cord lesion in a middle-aged or elderly female is due to a meningioma until proven otherwise.

THE PARALLELS OF LATITUDE: FINDING THE SITE OF PATHOLOGY WITHIN THE STRUCTURES OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

The parallels of latitude refer to the structures within the CNS and PNS that indicate the site of the pathology. For example, if the patient has a hemiparesis and a non-fluent dysphasia, it is the dysphasia that indicates that the weakness must be related to a problem in the dominant frontal cortex.

In the CNS the parallels of latitude consist of:

• the cortex – vision, memory, personality, speech and specific cortical sensory and visual phenomena such as visual and sensory inattention, graphaesthesia (see Chapter 5, ‘The cerebral hemispheres and cerebellum’)

• the cranial nerves of the brainstem (see Chapter 4, ‘The cranial nerves and understanding the brainstem’) – each cranial nerve is at a different level in the brainstem and thus represents a parallel of latitude. For example, if the patient has a 7th nerve palsy the problem either has to be in the 7th nerve or in the brainstem at the level of the pons.

The nerve roots and peripheral nerves are the parallels of latitude in the PNS:

• The motor and sensory nerves when affected will result in a very focal pattern of weakness and/or sensory loss that clearly indicates that the problem is in the peripheral nerve (see Chapter 11, ‘Common neck, arm and upper back problems’ and Chapter 12, ‘Back pain and common leg problems with or without difficulty walking’).

• The nerve roots include

• myotomes – motor nerve roots supplying muscles produce a classic pattern of weakness of several muscles supplied by that nerve root; specific nerve roots are part of the reflex arc and if, for example, the patient has an absent biceps reflex the lesion is at the level of C5–6

• dermatomes – areas of abnormal sensation from involvement of sensory nerve roots (see Figures 1.12, 1.13 and 1.22).

Parallels of latitude in the central nervous system

If a patient has a problem within the CNS, involvement of either the cortex or the brainstem will produce symptoms and signs that will enable accurate localisation. For example, the patient who presents with weakness involving the right face, arm and leg clearly has a problem affecting the motor pathway (the meridian of longitude) on the left side of the brain above the mid pons. The presence of a left 3rd nerve palsy (the parallel of latitude) would indicate the lesion is on the left side of the mid-brain while the presence of a non-fluent dysphasia (another parallel of latitude) would localise the problem to the left frontal cortex. Case 1.5 illustrates how to use the parallels of latitude in the CNS.

CASE 1.5 A man with right facial and arm weakness, vision and speech impairment

A 65-year-old man presents with weakness of his face and arm on the right side together with an inability to see to the right and, although he knows what he wants to say, he is having difficulty expressing the words. He also has, when examined, impairment of vibration and proprioception sensation in the right hand.

• The weakness of his face and arm indicate a lesion affecting the motor pathway or meridian of longitude on the left side of the brain above the mid pons.

• The difficulty expressing his words indicates the presence of a non-fluent dysphasia (the parallel of latitude), accurately localising the problem to the left frontal cortex.

• The inability to see to the right is another parallel of latitude and it reflects involvement of the visual pathways from behind the optic chiasm to the left occipital lobe in the left hemisphere, resulting in a right homonymous hemianopia (this is discussed in Chapter 5, ‘The cerebral hemispheres and cerebellum’).

• The impairment of vibration and proprioception in the right hand indicates that the parallel of latitude conveying this sensation is affected. Since the other symptoms and signs point to a left hemisphere lesion, the abnormality of vibration and proprioception indicates involvement of the parietal lobe, and this also would indicate that the visual disturbance is almost certainly in the left parietal lobe and not the occipital lobe. This sort of presentation is very typical of a cerebral infarct affecting the middle cerebral artery territory (see Chapter 10, ‘Cerebrovascular disease’).

THE HEMISPHERES

Figure 1.5 is a simplified diagram showing the main lobes of the brain and the cortical function associated with those areas. If the patient has cortical hemisphere symptoms and signs this clearly establishes the site of the pathology in the cortex of a particular region of the brain.

FIGURE 1.5 The left lateral aspect of the cerebral hemisphere Note: The central sulcus separates the frontal lobe from the parietal lobe (see Chapter 5, ‘The cerebral hemispheres and cerebellum’).

THE BRAINSTEM

Figure 1.6 shows the site of the cranial nerves in the brainstem with the numbers added: the 9th, 10th, 11th and 12th cranial nerves at the level of the medulla; the 5th, 6th, 7th and 8th at the level of the pons; and the 3rd and 4th at the level of the midbrain (see Chapter 4 for a detailed discussion of the brainstem and cranial nerves). Also note that the 3rd, 6th and 12th cranial nerves exit the brainstem close to the midline while the other cranial nerves exit the lateral aspect of the brainstem.

FIGURE 1.6 The ventral aspect of the brainstem showing the cranial nerves Reproduced and modified from Gray’s Anatomy, 37th edn, edited by PL Williams et al, 1989, Churchill Livingstone, Figure 7.81 [2]

This view is looking up from underneath the hemispheres. The cerebellum can be seen either side of the medulla and pons, the olfactory nerves (labelled 1) lie beneath the frontal lobes, and the under surface of the temporal lobes can be seen lateral to the 3rd and 4th cranial nerves.

1 = Olfactory, 2 = ophthalmic, 3 = oculomotor, 4 = trochlear, 5 = trigeminal, 6 = abducent, 7 = facial, 8 = auditory, 9 = glossopharyngeal, 10 = vagus, 11 = spinal accessory and 12 = hypoglossal

The presence of cranial nerve signs, except for a 6th or a 3rd nerve palsy in the setting of downward transtentorial herniation of the brain due to a mass above the tentorium, which can sometimes be a false localising sign (where the pathology is not in the nerve or brainstem), means the pathology MUST be at the level of that cranial nerve either in the nerve itself or in the brainstem.

Parallels of latitude in the peripheral nervous system

CRANIAL NERVES

In Figure 1.7 the important points to note are:

FIGURE 1.7 Dermatomes and nerves of the head and neck Reproduced from Gray’s Anatomy, 37th edn, edited by PL Williams et al, 1989, Churchill Livingstone, Figure 7.242 [2]

Note: The ophthalmic, maxillary and mandibular nerves are the three components of the 5th cranial nerve, the trigeminal nerve.

• The 1st division of the trigeminal nerve extends over the scalp to somewhere between the vertex and two-thirds of the way back towards the occipital region where it meets the greater occipital nerve supplied by the 2nd cervical nerve root. In a trigeminal nerve lesion the sensory loss will not extend to the occipital region, whereas with a spinothalamic tract problem it will.

• The 2nd and 3rd (predominantly the 3rd) cervical sensory nerve root supplies the angle of the jaw helping to differentiate trigeminal nerve sensory loss from involvement of the spinothalamic/quintothalamic tract. The angle of the jaw and neck are affected with lesions of the quinto/spinothalamic tract. Sensory loss on the face without affecting the angle of the jaw indicates the lesion is involving the 5th cranial nerve.

• The upper lip is supplied by the 2nd division and the lower lip by the 3rd division of the trigeminal nerve.

• The trigeminal nerve ends in front of the ear lobe.

The corneal reflex afferent arc is the 1st division of the trigeminal nerve; the nasal tickle reflex is the 2nd division (see Chapter 4, ‘The cranial nerves and understanding the brainstem’).

The anatomies of the muscles to the eye, the visual pathway and the vestibular pathway are discussed in Chapter 4, ‘The cranial nerves and understanding the brainstem’.

The purpose of the illustrations in the remainder of this chapter is to serve as a reference point for future use, and it is not anticipated that the reader will remember them all. With this textbook at the bedside the clinician can quickly refer to the illustrations to work out the anatomical basis of the pattern of weakness or of sensory loss.

THE UPPER LIMBS

Brachial plexus: The most important aspects to note in Figure 1.8 are:

FIGURE 1.8 Brachial plexus Reproduced and modified from Gray’s Anatomy, 37th edn, edited by PL Williams et al, 1989, Churchill Livingstone, Figure 7.243 [2]

Note: For simplification, the names of a number of nerves have been removed as they are rarely examined in clinical practice.

• The suprascapular nerve and the long thoracic nerve arise from the nerve roots (C5, C6 and C7) proximal to the junction of C5 and C6, helping to differentiate between a brachial plexus lesion at the level of C5–C6 and a nerve root lesion.

• The radial nerve arises from C5, C6 and C7.

• The median nerve arises from C5, C6, C7, C8 and T1.

• The ulnar nerve arises from the 8th cervical and 1st thoracic nerve roots. The clinical features of ulnar nerve lesions and pathology affecting the C8–T1 nerve roots are very similar, and a detailed examination is usually required to differentiate between these two entities.

Motor nerves and muscles of the upper limb:

AXILLARY AND RADIAL NERVES: Figure 1.9 shows the muscles innervated by the axillary and radial nerves. The important points to note are:

FIGURE 1.9 The axillary nerve and radial nerves Reproduced from Aids to the Examination of the Peripheral Nervous System, 4th edn, Brain, 2000, Saunders, Figure 15, p 12 [3]

• The axillary only supplies the deltoid muscle, not the other muscles (supraspinatus, infraspinatus and subscapularis) around the shoulder that collectively form the rotator cuff. Weakness isolated to the deltoid will indicate an axillary nerve lesion (occasionally seen with a dislocated shoulder).

• The branches of the radial nerve to the triceps muscle arise from above the spiral groove of the humerus. The spiral groove is a common site for compression and thus the triceps is not affected.

• The branches to the brachioradialis, extensor carpi radialis longus and extensor carpi radialis brevis arise proximal to the posterior interosseous nerve, these will be affected by a radial nerve palsy but spared if the problem is confined to the posterior interosseous nerve.

MEDIAN NERVE: Figure 1.10 shows the muscles supplied by the median nerve. The points to note are:

FIGURE 1.10 The median nerve Reproduced from Aids to the Examination of the Peripheral Nervous System, 4th edn, Brain, 2000, Saunders, Figure 27, p 20 [3]

• The branches to the flexor carpi radialis (flexes the wrist) and flexor digitorum superficialis (flexes the medial four fingers at the proximal interphalangeal joint) arise near the elbow, proximal to the anterior interosseous nerve.

• The anterior interosseous nerve supplies the flexor pollicis longus (flexes the distal phalanx of the thumb), the pronator quadratus (turns the wrist over so that the palm is facing downwards) and the lateral aspect of the flexor digitorum profundus (flexes the distal phalanges of the 2nd and 3rd digits).

• The nerve to abductor pollicis brevis, the muscle that elevates the thumb when the hand is fully supinated (palm facing the ceiling), arises at or just distal to the wrist in the region of the carpal tunnel.

ULNAR NERVE: Figure 1.11 shows the muscles innervated by the ulnar nerve. The points to note are:

FIGURE 1.11 The ulnar nerve Reproduced from Aids to the Examination of the Peripheral Nervous System, 4th edn, Brain, 2000, Saunders, Figure 36, p 26 [3]

• The branches to the flexor carpi ulnaris (flexes the wrist) and medial aspect of the flexor digitorum profundus (flexes the distal phalanges of the 4th and 5th digits) arise just distal to the medial epicondyle and will be affected by lesions at the elbow, a common sight of compression of the ulnar nerve. The lateral aspect of flexor digitorum profundus is innervated by the median nerve and, thus, flexion of the 2nd and 3rd distal phalanges will be normal with an ulnar nerve lesion at the elbow while they will be affected by C8–T1 nerve root or lower cord brachial plexus lesions.

• All other branches arise in the hand.

Case 1.6 illustrates a problem with the ulnar nerve.

CASE 1.6 A man with weakness in his right hand

A 35-year-old man presents with weakness confined to his right hand. This has been present for some time and he has noted thinning of the muscle between his thumb and index finger. He has also noticed pins and needles affecting his little finger, the medial half of his 4th finger and the medial aspect of his palm and the back of the hand to the wrist. The examination reveals reduced pain sensation in the distribution of the symptoms and weakness of abduction of the medial four digits and also bending the fingertips of the 4th and 5th but not the 2nd or 3rd fingers. The thumb is not affected.

• The presence of weakness indicates that the motor pathway (meridian of longitude) is affected and the altered sensation to pain indicates that either the peripheral nerve, nerve root or spinothalamic pathway to that part of the hand is affected.

• The presence of wasting is the parallel of latitude and indicates that the problem is in the PNS.

• The pattern of weakness clearly represents involvement of the ulnar nerve at the elbow (see Figure 1.11).

• The sensory loss affecting the medial 1½ digits and the medial aspect of both the palm and the dorsal aspect of the hand up to the wrist is also a parallel of latitude as the sensory loss is within the distribution of a peripheral nerve and indicates an ulnar nerve lesion (see Figures 1.14 and 1.15).

The remaining illustrations show the areas supplied by the various sensory nerves that detect light touch, pain and temperature. Neurologists remember these but ‘students of neurology’ do not need to remember them, although by remembering a few landmarks it is not hard to fill in the gaps. They are included in this chapter to provide a reference source for the clinician.

Cutaneous sensation of the upper limbs and trunk: Below each illustration are the one or two important features that are most useful at the bedside.

The dermatomes are higher on the back than they are on the front of the trunk. Sensory loss that is at the same horizontal level on both the anterior and posterior aspects of the trunk is not related to organic pathology and is more likely to be of functional origin.

The areas of sensation in the upper limb and trunk supplied by the sensory nerve roots, the dermatomes, are shown in Figures 1.12 and 1.13. A simple method of remembering the dermatomes is:

FIGURE 1.12 Dermatomes of the trunk and upper limb (anterior aspect) Reproduced from Aids to the Examination of the Peripheral Nervous System, 4th edn, Brain, 2000, Saunders, Figure 87, p 56 [3]

FIGURE 1.13 Dermatomes of the trunk and upper limb (posterior aspect) Reproduced from Aids to the Examination of the Peripheral Nervous System, 4th edn, Brain, 2000, Saunders, Figure 88, p 57) [3]

FIGURE 1.14 Sensory nerves of the upper limbs (anterior aspect) Reproduced from Gray’s Anatomy, 37th edn, edited by PL Williams et al, 1989, Churchill Livingstone, Figure 7.244 [2]

Note: The nerve root origin of the nerves is also shown. Note that the median nerve supplies the lateral 3 ½ digits while the ulnar nerve supplies the medial 1½ digits.

FIGURE 1.15 Sensory nerves of the upper limbs (posterior aspect) Reproduced from Gray’s Anatomy, 37th edn, edited by PL Williams et al, 1989, Churchill Livingstone, Figure 7.245) [2]

Note: The nerve root origin of the nerves is also shown.

FIGURE 1.16 A Lumbar plexus

Note: The main nerve to arise from the lumbar plexus is the femoral nerve. It is formed from the 2nd, 3rd and 4th lumbrical nerve roots.

Reproduced from Gray’s Anatomy, 37th edn, edited by PL Williams et al, 1989, Churchill Livingstone, Figures 7.252 and 7.256 [2]

• The arm–C7 affects the 3rd finger, C6 is lateral to this and C8 is medial to this.

• The trunk–T2 is at the level of the clavicle and meets C4; T8 is at the level of the xiphisternum (the lower end of the sternum), T10 is at the level of the umbilicus and T12 is at the level of the groin.

If you remember these landmarks, you can work out the areas in between supplied by the other dermatomes.

Figure 1.13 shows why it is very important to roll the patient over or sit them up to examine any sensory loss on the trunk. The dermatomes are higher on the back than they are on the front of the trunk. Sensory loss that is at the same horizontal level on both the anterior and posterior aspects of the trunk is not related to organic pathology and is more likely to be of functional origin.

THE LOWER LIMBS

Lumbar and sacral plexuses: Unlike the brachial plexus, there is little in the lumbosacral plexus (Figure 1.16) that helps localise whether the problem is in the lumbosacral plexus or the nerve roots. It is important to note that the sciatic nerve arises from predominantly the 5th lumbrical at the 1st and 2nd sacral nerve roots.

The motor nerves and muscles of the lower limb:

FEMORAL NERVE: The point to note (see Figure 1.17) is that the common peroneal nerve arises from the sciatic nerve above the popliteal fossa and above the level of the neck of the fibula, a common site for compression, and that there are no branches between where it arises and the neck of the fibula.