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.