Chapter 22 Neurological disease

Neurology is a large and diverse subject which covers many conditions that require long-term coordinated care and have serious effects on the daily lives of patients and their families. Neurology includes conditions as diverse as cognitive disorders involving higher level mental functioning through to disorders of peripheral nerve and skeletal muscle. It is a specialty requiring good clinical skills and examination technique which cannot be replaced with investigations or imaging techniques alone (Table 22.1).

Table 22.1 UK incidence of common neurological conditions

Conditions	Events per 100 000/year
Cerebrovascular events	210
Shingles (herpes zoster) and postherpetic neuralgia	150
Diabetic and other neuropathies	105
Epilepsy	46
Parkinson’s disease	19
Severe brain injury and subdural haematoma	13
All CNS tumours	9
Trigeminal neuralgia	8
Meningitis	7
Multiple sclerosis	7
Presenile dementia (below 65 years)	4
Myasthenia, all muscle and motor neurone disease	5

Common symptoms and signs

Pattern recognition in neurology – interpretation of history, symptoms and examination – is very reliable. Practical experience is vital. There are three critical questions in formulating a clinical diagnosis:

What is/are the site(s) of the lesion(s)?

What is the likely pathology?

Does a recognizable disease fit this pattern?

Difficulty walking and falls

Change in walking pattern is a common complaint (Box 22.1). Arthritis and muscle pain make walking painful and slow (antalgic gait). The pattern of gait is valuable diagnostically.

Box 22.1

Common gait abnormalities

Spasticity/hemiparesis

Parkinson’s disease

Cerebellar ataxia

Position sense loss (stamping)

Distal weakness (slapping)

Proximal weakness (waddling)

Apraxia of gait

Spasticity (p. 1082), more pronounced in extensor muscles, with or without weakness, causes stiff and jerky walking. Toes of shoes become scuffed, catching level ground. Pace shortens; a narrow base is maintained. Clonus – involuntary extensor rhythmic leg jerking – may occur.

In a hemiparesis when spasticity is unilateral and weakness marked, the stiff, weak leg is circumducted and drags.

Parkinson’s disease: shuffling gait

There is muscular rigidity (p. 1118) throughout extensors and flexors. Power is preserved; the pace shortens, and slows to a shuffle; its base remains narrow. A stoop and diminished arm swinging become apparent. Gait becomes festinant (hurried) with short rapid steps. There is difficulty turning quickly and initiating movement, sometimes with falls. Retropulsion means small backward steps, taken involuntarily when a patient is halted.

Cerebellar ataxia: broad-based gait

In lateral cerebellar lobe disease (p. 1083) stance becomes broad-based, unstable and tremulous. Ataxia describes this incoordination. When walking, the person tends to veer to the side of the affected cerebellar lobe.

In disease of midline structures (cerebellar vermis), the trunk becomes unsteady without limb ataxia, with a tendency to fall backwards or sideways – truncal ataxia.

Sensory ataxia: stamping gait

Peripheral sensory lesions (e.g. polyneuropathy, p. 1145) cause ataxia because of loss of proprioception (position sense). Broad-based, high-stepping, stamping gait develops. This form of ataxia is exacerbated by removal of sensory input (e.g. vision) and worse in the dark. Romberg’s test, first described in sensory ataxia of tabes dorsalis (p. 1129), becomes positive.

Vertigo (p. 1078) means the illusion of movement, a sensation of rotation or tipping. The patient feels the surroundings are spinning or moving. This is distressing and often accompanied by nausea or vomiting.

Blackout, like dizziness, is simply descriptive, implying either altered consciousness, visual disturbance or falling. Epilepsy (p. 1112) and syncope are mentioned in detail (p. 1116); hypoglycaemia and anaemia must be considered. Commonly no sinister cause is found. A careful history is essential.

Collapse is a vague term, but often used. Avoid it.

No serious disease is found in many patients (>20%) referred with symptoms suggestive of possible neurological conditions.

Fatigue is common: when it is an isolated symptom, neurological disease is rarely discovered. There is a borderland (sometimes contentious) between neurology and psychiatry.

Examination and formulation

Following a short or detailed examination, relevant findings are summarized in a brief formulation – the basis for investigation, transfer of information and management (Practical Boxes 22.1, 22.2 and Table 22.2).

Practical Box 22.1

Five-part short neurological examination

Posture of outstretched arms

Wasting, fasciculation

Practical Box 22.2

10-part neurological examination

1. State of consciousness, arousal, appearance

2. Mental state, attitude, insight (see Box 22.5, p. 1069)

3. Cognitive function – orientation, recall, level of intellect, language, other cortical problems, e.g. apraxia

4. Gait and balance tests

5. Skull – contour, bruits

6. Neck – stiffness, palpation and auscultation of carotids

7. Cranial nerves (Table 22.3)

8. Motor system

Upper limbs:

Wasting and fasciculation

Posture of arms: drift, rebound, tremor

Tone: spasticity, clonus, extrapyramidal rigidity

Power: 0–5 scale (Table 22.2)

Tendon reflexes: + or ++ normal; +++ pathological; 0 = absent with reinforcement

Thorax and abdomen:

Respiration

Thoracic and abdominal muscles

Abdominal reflexes

Cremasteric reflexes

Lower limbs:

Wasting and fasciculation

Tone, power and tendon reflexes

Plantar responses

9. Coordination and fine movements

10. Sensory system

First: is feeling in limbs, face and trunk normal?

Posterior columns:

Vibration (128 Hz tuning fork)

Joint position

Light touch

2-point discrimination (normal: 0.5 cm finger tips, 2 cm soles)

Spinothalamic tracts:

Pain: use a split orange-stick or a sterile pin

Temperature: warm and cold objects

If sensation is abnormal, chart areas involved

Table 22.2 Six grades of muscle power

Grade	Definition
5	Normal power
4	Active movement against gravity and resistance
3	Active movement against gravity
2	Active movement with gravity eliminated
1	Flicker of contraction
0	No contraction

Functional neuroanatomy

The neurone and synapse

The neurone is the functional unit of the entire nervous system (Fig. 22.1). Its cell body and axon terminate in a synapse. Size and type of each group of neurones vary. A thoracic spinal cord α-motor neurone has an axonal length of >1 metre and innervates between several hundred and 2000 muscle fibres in one leg – a motor unit. By contrast, some spinal or intracerebral interneurones have axons under 100 µm long, terminating on one neuronal cell body.

Figure 22.1 The functional unit: neurone and neurotransmitters. (a) The action potential, i.e. nerve impulse, travels down the axon. Microtubules carry neurotransmitters to nerve endings. (b) Action potential I depolarizes the synaptic membrane, opening voltage-gated calcium channels. (c) Influx of calcium ions cause vesicles to fuse with the membrane allowing neurotransmitter binding to receptors (i) and activation of secondary messengers that modulate gene transcription and also open ligand-gated channels (ii). This allows ions to enter, depolarize the membrane and initiate action potential II.

Neurotransmitters

Neurotransmitters are excitatory (acetylcholine, noradrenaline, adrenaline, 5-hydroxytryptamine, dopamine, glutamate and aspartate) or inhibitory (γ-aminobutyric acid (GABA), histamine and glycine). Neuropeptides, e.g. vasopressin, ACTH, substance P and opioid peptides, as well as the purines (ATP and AMP) are both excitatory and inhibitory.

Synaptic transmission is mediated by neurotransmitters released by action potentials passing down an axon. Neurotransmitters activate postsynaptic receptors and are removed by transporter proteins. The neurotransmitter-receptor reaction increases ionic permeability and propagates a further action potential. Axonal electrical activity and synaptic chemical release is the basis of neurological function.

Clinical features of focal brain lesions: general mechanisms

The symptoms and signs suggest the area of the brain that is malfunctioning (e.g. aphasia – the left frontal lobe, hemiparesis – internal capsule or a Bell’s palsy – Vllth cranial (facial) nerve).

Focal lesions of the cortex, and lesions throughout the nervous system, cause symptoms and signs by two processes:

Suppression or destruction of neurones and surrounding structures (Fig. 22.2). This is the commonest process – part of the system simply fails to work.

Synchronous discharge of neurones by irritative lesions (Fig. 22.3), e.g. cortical lesions, causes epilepsy, either partial or generalized.

Figure 22.2 Principal features of destructive cortical lesions in a right-handed individual.

Figure 22.3 Effects of irritative cortical lesions.

Localization within the cerebral cortex

This subject causes unnecessary difficulty. Work on neuronal networks, functional imaging and plasticity questions the traditional views of highly specific localization of cortical function. The following paragraphs summarize areas of clinical relevance.

The dominant hemisphere (usually left)

The concept of cerebral dominance arose from a simple observation: right-handed stroke patients with acquired language disorders had destructive lesions within the left hemisphere. Right-handed (and 70% of left-handed) people have language function on the left.

More specifically, destructive lesions within the left fronto-temporo-parietal region cause disorders of communication:

Spoken language – aphasia, also called dysphasia

Writing – agraphia

Reading – acquired alexia.

Developmental dyslexia describes delayed, disorganized reading and writing ability in children, usually with normal intelligence.

Aphasia

Aphasia is loss of or defective language from damage to the speech centres within the left hemisphere. Numerous varieties have been described.

Broca’s (expressive, anterior) aphasia

Damage in the left frontal lobe causes reduced speech fluency with relatively preserved comprehension. The patient makes great efforts to initiate language, which becomes reduced to a few disjointed words with failure to construct sentences. Patients who recover say they knew what they wanted to say, but could not get the words out.

Wernicke’s (receptive, posterior) aphasia

Left temporo-parietal damage leaves fluency of language but words are muddled. This varies from insertion of a few incorrect or non-existent words into speech to a profuse outpouring of jargon (i.e. rubbish with wholly non-existent words). Severe jargon aphasia is bizarre and often mistaken for psychotic behaviour.

Patients who recover from Wernicke’s aphasia say that they found speech, both their own and others’, like an unintelligible foreign language, i.e. incomprehensible, but they could neither stop speaking nor understand speech.

Nominal (anomic, amnestic) aphasia

This means difficulty naming familiar objects. Naming difficulty is an early feature in all types of aphasia.

Global (central) aphasia

This means the combination of the expressive problems of Broca’s aphasia and the loss of comprehension of Wernicke’s with loss of both language production and understanding. This is due to widespread damage to speech areas and is the commonest aphasia after a severe left hemisphere infarct. Writing and reading are also affected.

Dysarthria

Dysarthria is disordered articulation – slurred speech. Language is intact. Paralysis, slowing or incoordination of muscles of articulation or local discomfort causes various patterns of dysarthria. Examples are the gravelly speech of pseudobulbar palsy (p. 1080), the jerky, ataxic speech of cerebellar lesions, the monotone of Parkinson’s, and speech in myasthenia that fatigues and dies away. Many aphasic patients are also dysarthric.

The non-dominant hemisphere

Disorders in right-handed patients with right hemisphere lesions are often difficult to recognize. There are abnormalities of perception of internal and external space. Examples are losing the way in familiar surroundings, failing to put on clothing correctly (dressing apraxia), or failure to draw simple shapes – constructional apraxia.

Memory and its disorders

Disorders of memory follow damage to the medial surfaces of both temporal lobes and their brainstem connections – the hippocampi, fornices and mammillary bodies. Bilateral lesions are necessary to cause amnesia. In all organic memory disorders recent events are recalled poorly, in contrast to the relative preservation of distant memories.

Memory loss (the amnestic syndrome) is part of dementia (p. 1137) but also occurs as an isolated entity (Box 22.2).

Box 22.2

Causes of an amnestic syndrome

Dementia

Alcohol (Wernicke–Korsakoff syndrome)

Head injury (severe)

Anoxic brain damage and following carbon monoxide poisoning

Posterior cerebral artery occlusion (bilateral)

Herpes simplex encephalitis

Chronic sedative and solvent abuse

Bilateral invasive tumours

Following hypoglycaemia

Arsenic poisoning (very rare)

Neuroanatomy: essential elements

For clinical purposes the complexity of neuroanatomy must be reduced to its core elements:

Cranial nerves

Three systems of motor control:

– corticospinal or pyramidal system

– extrapyramidal system

– cerebellum

Motor unit

Reflex arc

Sensory pathways and pain

Control of the bladder and sexual function.

Cranial nerves (Table 22.3)

I: Olfactory nerve

This sensory nerve arises from olfactory (smell) receptors within nasal mucosa. Branches pierce the cribriform plate and synapse in the olfactory bulb. The olfactory tract passes to the olfactory cortex.

Table 22.3 Cranial nerves

Number	Name	Main clinical action
I	Olfactory	Smell
II	Optic	Vision, fields, afferent light reflex
III	Oculomotor	Eyelid elevation, eye elevation, ADduction, depression in ABduction, efferent (pupil)
IV	Trochlear	Eye intorsion, depression in ADduction
V	Trigeminal	Facial (and corneal) sensation, mastication muscles
VI	Abducens	Eye ABduction
VII	Facial	Facial movement, taste fibres
VIII	Vestibular Cochlear	Balance and hearing
	Cochlear
IX	Glossopharyngeal	Sensation – soft palate, taste fibres
X	Vagus	Cough, palatal and vocal cord movements
XI	Accessory	Head turning, shoulder shrugging
XII	Hypoglossal	Tongue movement

Anosmia (loss of sense of smell) is caused by head injury (shearing of olfactory neurones as they pass through the cribriform plate at the skull base) or tumours of the olfactory groove (e.g. meningioma). Olfaction is temporarily (occasionally permanently) lost or diminished after upper respiratory infections and with local disorders of the nose. Many patients with gradual onset anosmia over many years may be unaware of the deficit, e.g. in Parkinson’s disease where anosmia precedes motor symptoms by many years but is often not noticed by the patient.

Detailed smell testing is difficult in routine clinical practice and rarely performed. Adequate testing requires use of commercially available kits such as scratch and sniff cards or odour filled pens with forced multiple choice identification.

II: Optic nerve and visual system (Fig. 22.4)

Light regulated by the pupillary aperture is converted into action potentials by retinal rod, cone and ganglion cells (see page 1055). The lens, under control of the ciliary muscle, produces the image (inverted) on the retina. Axons in the optic nerve (1) decussate at the optic chiasm (2), fibres from the nasal retina cross and join with uncrossed fibres originating in the temporal retina to form the optic tract (3). Each optic tract thus carries information from the contralateral visual hemifield.

Figure 22.4 The visual pathway. 1. Mononuclear field loss – complete optic nerve lesion. 2. Bitemporal hemianopia – chiasmal lesion. 3. Homonymous hemianopia – optic tract lesion. 4. Homonymous quadrantanopia – temporal lesion. 5. Homonymous quadrantanopia – parietal lesion. 6. Homonymous hemianopia with macular sparing – occipital cortex or optic radiation. 7. Homonymous hemianopia (hemiscotoma) – occipital pole lesion.

From the lateral geniculate body, fibres pass in the optic radiation through the parietal and temporal lobes (4 and 5) to reach the visual cortex of the occipital lobe (6 and 7), which is somatotopically organized with macular vision located at the occipital pole (see Fig. 22.4).

Beyond the visual cortex visual information is further processed by neighbouring visual association areas to detect lines, orientation, shapes, movement, colour and depth; there is even a distinct area responsible for face recognition.

Visual acuity

This is assessed in each eye with a Snellen chart and/or Near Vision Reading Types, corrected for refractive errors with lenses or a pinhole. The patient should stand 6 metres from a well-lit chart. Acuity is recorded as distance in metres from the chart over distance at which the line should be legible, e.g. 6/6 indicates ‘normal’ acuity and 6/60 very poor acuity.

Visual field defects

Visual fields are assessed at the bedside by confrontation – comparing the examiner’s and patient’s fields, one eye at a time and quadrant by quadrant. Patience and good technique are required to get reliable results. White and red targets (traditionally hatpins) are used to assess peripheral and central fields respectively although in practice a fingertip is often substituted as a cruder screening test. More detailed quantification of fields may be obtained using Goldmann (manual) or Humphrey (automated) perimetry testing.

Field defects are described as hemianopic when half the field is affected and quadrantanopic when a quadrant is affected. Lesions posterior to the optic chiasm produce homonymous field defects, indicating involvement of the same part of the visual field in both eyes as information from the two visual hemifields is separated beyond this point. Lesions damaging decussating nasal fibres at the optic chiasm cause bitemporal defects.

Retinal and local eye lesions

See page 1063 in Chapter 21.

Optic nerve lesions

Unilateral visual loss, commencing with a central or paracentral (off-centre) scotoma, is the hallmark of an optic nerve lesion. Because most fibres in the optic nerve subserve macular vision, lesions within the nerve disproportionately affect central vision and colour vision. A total optic nerve lesion causes unilateral blindness with loss of pupillary light reflex. Examination findings in optic neuropathy:

Reduced acuity in affected eye

Scotoma (usually central)

Impaired colour vision (assess with Ishihara plates)

An afferent pupillary defect (see below)

Optic atrophy – pale disc (develops late)

Causes are listed in Box 22.3.

Box 22.3

Causes of optic neuropathy

Inflammatory (optic neuritis), e.g. demyelination, sarcoidosis, vasculitis

Optic nerve trauma or compression, e.g. glioma, meningioma, aneurysm, bone disorders affecting orbit

Toxic, e.g. tobacco-alcohol, ethambutol, methyl alcohol, quinine, hydroxy chloroquine, radiation

Ischaemic optic neuropathy, e.g. giant cell arteritis

Hereditary optic neuropathies, e.g. Leber’s

Nutritional deficiency, e.g. vitamin B₁ and B₁₂

Infection, e.g. orbital cellulitis, syphilis, TB

Neurodegenerative disorders, e.g. leucodystrophies

Papilloedema and its causes (Box 22.4)

Papilloedema

Papilloedema means swelling of the optic disc. Causes are shown in Box 22.4. The earliest signs of swelling are disc pinkness, with blurring and heaping up of disc margins, nasal first. There is loss of spontaneous pulsation of retinal veins within the disc. The physiological cup becomes obliterated, the disc engorged with dilated vessels. Small haemorrhages often surround the disc.

Box 22.4

Causes of optic disc swelling

Raised intracranial pressure (papilloedema)

Brain tumour, abscess, or haemorrhage. Idiopathic intracranial hypertension, hydrocephalus

Optic nerve disease

Optic neuritis, e.g. multiple sclerosis

Ischaemic optic neuropathy, e.g. giant cell arteritis

Toxic optic neuropathy, e.g. methanol

Venous occlusion

Venous sinus thrombosis

Central retinal vein thrombosis

Orbital mass lesions

Retinal vascular disease

Malignant hypertension

Vasculitis, e.g. systemic lupus erythematosus

Metabolic causes

Hypercapnia, chronic hypoxia, hypocalcaemia

Disc infiltration

Leukaemia, sarcoidosis, optic nerve glioma

Various conditions simulate true disc swelling. Marked hypermetropic (long-sighted) refractive errors make a disc appear pink, distant and ill-defined. Myelinated nerve fibres at disc margins and hyaline bodies (drusen, p. 1064) can be mistaken for disc swelling.

Disc infiltration also causes a swollen disc with raised margins (e.g. in leukaemia).

When there is doubt about disc oedema, i.v. fluorescein angiography is diagnostic; retinal leakage is seen with papilloedema.

Papilloedema produces few if any visual symptoms other than momentary visual obscurations with changes in posture. The underlying disease is the source of the patient’s symptoms. The blind spot is enlarged but this is not noticed by the patient. However, over time progressive and permanent constriction of visual fields occurs, ultimately culminating in optic atrophy.

Inflammatory optic neuropathy (optic neuritis)

Optic neuritis is one of the most common causes of subacute visual loss. Symptoms may vary from a mild fogging of central vision with colour desaturation to a dense central scotoma, but very rarely complete blindness. Pain on eye movements is almost universal. The optic disc usually appears normal despite severe visual loss (unless the inflammation is at the optic nerve head in which case the disc may appear swollen in the acute phase).

A plaque of demyelination within the optic nerve is the most common cause in Western populations. Dedicated MRI imaging of the optic nerves may show the inflammatory plaque and imaging of the brain may show additional inflammatory lesions which confer a higher risk of developing multiple sclerosis (MS). Approximately 50% of patients go on to develop MS with prolonged follow-up (see p. 1123). Recovery of visual acuity to 6/9 or better occurs in 95% of cases over months, with recovery time improved by high-dose i.v. steroids given acutely.

Optic neuritis may be caused by infective or other inflammatory disorders, e.g. sarcoidosis or vasculitides (see Box 22.3).

Anterior ischaemic optic neuropathy

The anterior part of the optic nerve is supplied by the posterior ciliary arteries, occlusion or hypoperfusion of which leads to infarction of all or part of the optic nerve head. There is sudden or stuttering altitudinal visual loss (typically the lower half of the visual field) with disc swelling, later replaced by optic atrophy. The other eye is later affected in one-third of cases.

Individuals with small hypermetropic discs seem to be predisposed, and often there are relatively few vascular risk factors. Less commonly arteritis is the cause (see p. 536).

Optic atrophy

Optic atrophy means disc pallor, from loss of axons, glial proliferation and decreased vascularity. This may eventually develop following any type of optic neuropathy of sufficient severity to extensively damage axons within the nerve, including chronic papilloedema.

Optic chiasm

Bitemporal hemianopia or quadrantanopia occurs with compression of the chiasm from above or below. Common causes are:

Pituitary tumours (p. 946)

Meningioma

Craniopharyngioma.

Optic tract and optic radiation

Optic tract lesions (rare) cause a homonymous hemianopia (loss of the contralateral visual field in both eyes). Optic radiation lesions cause homonymous quadrantanopic defects. Temporal lobe lesions (e.g. tumour, infarction) cause upper quadrantic defects, and parietal lobe, lower.

Occipital cortex

Homonymous hemianopic defects are produced by unilateral posterior cerebral artery infarction (see p. 1101 stroke). The macular cortex (at each occipital pole) may be spared.

Widespread bilateral occipital lobe damage by infarction (‘top of the basilar’ syndrome), trauma or coning causes cortical blindness (Anton’s syndrome). The patient cannot see but characteristically lacks insight into this; he or she may even deny it. Pupillary responses remain normal (p. 1101).

The pupils

A slight difference between the size of each pupil (up to 1 mm) is common (physiological anisocoria) and does not vary with differing light levels. The pupil tends to become smaller and irregular in old age (senile miosis); anisocoria is more pronounced. Convergence becomes sluggish with ageing.

Pupillary reactions to light and accommodation may be tested (Fig. 22.5). A bright torch (not an ophthalmoscope light!) should be used to test the pupillary light reaction.

Figure 22.5 Pupillary light reflex. Afferent pathway: (1) Light activates optic nerve axons. (2) Axons (some decussating at the chiasm) pass through each lateral geniculate body, and (3) synapse at pretectal nuclei. Efferent pathway: (4) Action potentials pass to Edinger–Westphal nuclei of IIIrd nerve, then, (5) via parasympathetic neurones in IIIrd nerves to cause (6) pupil constriction.

Afferent pupillary defect. A complete optic nerve lesion causes a dilated pupil and an afferent pupillary defect (APD). For a left APD:

The pupil is unreactive to light (i.e. the direct reflex is absent).

The consensual reflex (constriction of the right pupil when the left is illuminated) is absent. Conversely, the left pupil constricts when light is shone in the intact right eye, i.e. the consensual reflex of the right eye remains intact.

Relative afferent pupillary defect (RAPD). This occurs with incomplete damage to one optic nerve relative to the other. An RAPD is a sensitive sign of optic nerve pathology and can provide evidence of an optic nerve lesion even after recovery of vision. For a left RAPD:

Direct and indirect reflexes are intact in each eye but differ in relative strength.

When the light is swung from one eye to the other, the left pupil dilates slightly when illuminated and constricts slightly when the right eye is illuminated (the consensual reflex is stronger than the direct).

Horner’s syndrome (see Box 22.5)

The sympathetic nervous supply to the eye is a three neurone pathway originating in the hypothalamus and descending by way of the brainstem and cervical cord to T1 nerve root, paravertebral sympathetic chain and, on via the carotid artery wall, to the eye. Damage to any part of the pathway results in Horner’s syndrome. This is significant not only because it affects vision but also because it may indicate a serious underlying pathology.

Box 22.5

Causes of Horner’s syndrome

Hypothalamic, hemisphere and brainstem lesions

Massive cerebral infarction

Brainstem demyelination or lateral medullary infarction

Cervical cord

Syringomyelia and cord tumours

T1 root

Apical lung tumour or TB

Cervical rib

Brachial plexus trauma

Sympathetic chain and carotid artery in neck

Following thyroid/laryngeal/carotid surgery

Carotid artery dissection

Neoplastic infiltration

Cervical sympathectomy

Miscellaneous

Congenital

Cluster headache, usually transient

Idiopathic

The clinical features of Horner’s syndrome are:

Unilateral miosis (constricted pupil)

Partial ptosis

Loss of sweating on the same side (extent depending upon the level of the lesion)

Subtle conjunctival injection and enophthalmos may be present

The miosis is most easily seen in low light as the pupil cannot dilate, and may not be apparent in bright light.

Myotonic pupil (Holmes–Adie pupil)

This is a dilated, often irregular, pupil, more frequent in women; it is common and usually unilateral. There is no (or very slow) reaction to bright light and also incomplete constriction to convergence. This is due to denervation in the ciliary ganglion, of unknown cause, and has no other pathological significance. A myotonic pupil is sometimes associated with diminished or absent tendon reflexes.

Argyll Robertson pupil

Now rarely seen in clinical practice. A small, irregular pupil is fixed to light but constricts on convergence. The lesion is in the brainstem surrounding the aqueduct of Sylvius. Once considered diagnostic of neurosyphilis, it is now only occasionally seen in diabetes or MS.

III, IV, VI: Oculomotor, trochlear and abducens nerves

These cranial nerves supply the extraocular muscles and disorders commonly result in abnormal eye movements and diplopia (double vision) due to breakdown of conjugate (yoked) eye movements. Diplopia may also occur with local orbital lesions or myasthenia gravis.

Examining eye movements

Pursuit (slow) eye movements and saccadic eye movements are tested separately. The examiner assesses the range of eye movements in all directions and asks the patient to report double vision. Jerky pursuit movements with saccadic intrusion (i.e. brief fast saccades interspersed with slower pursuit movements), overshoot on saccadic movements and nystagmus may indicate cerebellar or brainstem pathology.

Control of eye movements

Fast voluntary eye movements originate in the frontal lobes. Fibres descend and cross in the pons to end in the centre for lateral gaze (paramedian pontine reticular formation – PPRF), close to the VIth nerve nucleus. Each PPRF also receives input from:

the ipsilateral occipital cortex – pathway concerned with tracking objects

the vestibular nuclei – pathways linking eye movements with position of the head and neck (vestibulo-ocular reflex, p. 1072).

Conjugate lateral eye movements are coordinated from each PPRF via the medial longitudinal fasciculus (MLF, Fig. 22.6). Fibres from the PPRF pass both to the ipsilateral VIth nerve nucleus (lateral rectus) and, having crossed the midline, to the opposite IIIrd nerve nucleus (medial rectus and other muscles) via the MLF, thus linking the eyes for lateral gaze.