Ординатура / Офтальмология / Английские материалы / Clinical Ophthalmology A Systematic Approach 7th Edition_Kanski, Bowling_2011

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2 Evaluation of the extraocular muscles in thyroid eye disease (Fig. 19.1B).

3Bony involvement of orbital tumours is better assessed using CT than MR.

4 Orbital cellulitis for assessment of intraorbital extension and subperiosteal abscess formation.

5Detection of intraorbital calcification as in meningioma and retinoblastoma.

6Detection of acute cerebral (Fig. 19.1C) or subarachnoid (Fig. 19.1D) haemorrhage because this is harder to visualize on MR in the first few hours.

7When MR is contraindicated (e.g. patients with ferrous foreign bodies).

Fig. 19.1 CTscans. (A) Coronal image shows blow-out fractures of the left orbital floor and medial wall with orbital emphysema; (B) axial image shows bilateral enlargement of extraocular muscles and right proptosis; (C) axial image shows an acute parenchymal haematoma in the right temporal lobe; (D) axial image shows extensive subarachnoid blood in the basilar cisterns, and the Sylvian and interhemispheric fissures

(Courtesy of N Sibtain – figs A, C and D; A Pearson – fig. B)

Magnetic resonance imaging

Physics

Magnetic resonance imaging (MR) depends on the rearrangement of positively-charged hydrogen nuclei (protons) when a tissue is exposed to a short electromagnetic pulse. When the pulse subsides, the nuclei return to their normal position re-radiating some of the absorbed energy. Sensitive receivers pick up this electromagnetic echo. Unlike CT, it does not subject the patient to ionizing radiation. The signals are analyzed and displayed as a cross-sectional image which may be: (a) axial, (b) coronal or (c) sagittal.

Weighting

Weighting refers to two methods of measuring the relaxation times of the excited protons after the magnetic field has been switched off. Various body tissues have different relaxation times so that a given tissue may be T1or T2-weighted, (i.e. best visualized on that particular type of image). In practice, both types of scans are usually performed. It is easy to tell the difference between CT and MR because bone appears white on CT but is not clearly demonstrated on MR.

1T1-weighted images are best for normal anatomy. Hypointense (dark) structures include CSF and vitreous. Hyperintense (bright) structures include fat, blood, contrast agents and melanin (Fig. 19.2A, C and E).

2T2-weighted images, in which water is shown as hyperintense are useful for viewing pathological changes because oedematous tissue (e.g. inflammation) will be of brighter signal than normal surrounding tissue. CSF and vitreous are hyperintense as they have high water content. Blood vessels appear black on T2 imaging unless they are occluded (Fig.19.2B, D and F).

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Fig. 19.2 MRscans. (A) T1-weighted coronal image through the globe in which vitreous is hypointense (dark) and orbital fat is hyperintense (bright); (B) T2weighted scan in which vitreous is hyperintense and orbital fat is hypointense; (C) T1-weighted axial image through the globes and optic nerves in which vitreous and CSFaround the optic nerves are hypointense and orbital fat is hyperintense; (D) T2-weighted axial image in which vitreous and CSFare hyperintense; (E) T1weighted midline sagittal image through the brain in which the CSFin the third ventricle is hypointense; (F) T2-weighted axial image through the brain in which the CSF in the lateral ventricles is hyperintense

Contrast enhancement

1Gadolinium acquires magnetic moment when placed in an electromagnetic field. Administered intravenously, it remains intravascular unless there is a breakdown of the blood–brain barrier. It is only visualized on T1-weighted images, and enhancing lesions such as tumours and areas of inflammation will appear bright. Ideally MR is performed both before (Fig. 19.3A) and after (Fig. 19.3B) administration of gadolinium. Special head or surface coils can also be used to improve spatial definition of the image. Adverse effects with gadolinium are uncommon and usually relatively innocuous.

2Fat-suppression techniques are applied for imaging the orbit because the bright signal of orbital fat on conventional T1-weighted imaging frequently obscures other orbital contents. Fat-suppression eliminates this bright signal and better delineates normal structures (optic nerve and extraocular muscles) as well as tumours, inflammatory lesions and vascular malformations. The two types of fat suppression sequence used for orbital imaging are:

aT1 fat saturation used with gadolinium allows areas of abnormal enhancement (e.g. optic nerve sheath) to be visualized as the T1 high signal of the surrounding orbital fat is suppressed (Fig. 19.3C and D).

bSTIR (Short T1 Inversion Recovery) is the optimal sequence for detecting intrinsic lesions of the intraorbital optic nerve (e.g. optic neuritis – Fig. 19.3E). STIR images have very low signal from fat but still have high signal from water.

3FLAIR (fluid-attenuated inversion recovery) sequences suppress the bright CSF on T2-weighted images to allow better visualization of adjacent pathological tissue such as periventricular plaques of demyelination (Fig. 19.3F).

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Fig. 19.3 Enhancement techniques. (A) Pre-contrast sagittal T1-weighted image of a meningioma; (B) post-contrast image shows enhancement of the tumour; (C) coronal STIRimage shows an intermediate signal intensity mass surrounding the left optic nerve consistent with an optic nerve sheath meningioma; (D) T1-weighted fat saturated coronal image of the same patient shows avid homogeneous enhancement of the meningioma; (E) coronal STIRimage of right retrobulbar neuritis shows a high signal within the optic nerve with enlargement of the optic nerve sheath complex; (F) sagittal FLAIRimage shows multiple periventricular plaques of demyelination

(Courtesy of D Thomas – figs A and B; N Sibtain – figs C–F)

Limitations

•It does not image bone (which appears black), although this is not necessarily a disadvantage.

•It does not detect recent haemorrhage and is therefore inappropriate in patients with acute intracranial bleeding.

•It cannot be used in patients with magnetic foreign objects (e.g. cardiac pacemakers, intraocular foreign bodies and ferromagnetic aneurysm clips).

•It requires the patient to cooperate and remain motionless.

•It is difficult to perform on claustrophobic patients.

Neuro-ophthalmic indications

MR is the technique of choice for lesions of the intracranial pathways.

1The optic nerve is best visualized on coronal STIR images in conjunction with coronal and axial T1 fat saturation post-gadolinium images (see Fig. 19.3E). Axial T1 images are useful for displaying normal anatomy. MR can detect lesions of the intraorbital part of the optic nerve (e.g. neuritis, gliomas) as well as intracranial extension of optic nerve tumours.

2Optic nerve sheath lesions (e.g. meningiomas) are of similar signal intensity on T1and T2-weighted images but enhance avidly with gadolinium (see Fig. 19.3D).

3Sellar masses (e.g. pituitary tumours) are best visualized by T1-weighted contrast-enhanced studies (see Fig. 19.3B). Coronal images optimally demonstrate the contents of the sella turcica as well as the suprasellar and parasellar regions and are usually supplemented by sagittal images.

4Cavernous sinus pathology is best demonstrated on coronal images that may require contrast.

5Intracranial lesions of the visual pathways (e.g. inflammatory, demyelinating, neoplastic and vascular). MR allows further characterization of these lesions as well as better anatomical localization, and may be indicated in situations where CT is normal but the clinical picture suggests otherwise.

Angiography

Magnetic resonance angiography

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Magnetic resonance angiography (MRA) is a non-invasive method of imaging the intraand extracranial carotids and vertebrobasilar circulations (Fig. 19.4A) to demonstrate stenosis, dissection, occlusion, arteriovenous malformations and aneurysms. This technique uses the motion sensitivity of MR to visualize blood flow within vessels and does not require contrast. However, because it relies on flow, thrombosed aneurysms may be missed and turbulent flow may lead to difficulties in interpretation. Furthermore MRA is unreliable in detecting very small aneurysms.

Fig. 19.4 Cerebral angiography. (A) Normal MRA of the external carotid and vertebral circulation; (B) CTangiogramshows a left posterior communicating aneurysm (arrows); (C) normal CTvenogram; (D) conventional catheter angiogramwith subtraction shows an aneurysmarising fromthe internal carotid artery at its junction with the posterior communicating artery (arrow)

(Courtesy of N Sibtain – figs A–C; JD Trobe, from Neuro-ophthalmology, in Rapid Diagnosis in Ophthalmology, Mosby 2008)

Computed tomographic angiography

Computed tomographic angiography (CTA) is emerging as the method of choice in the investigation of intracranial aneurysms (Fig. 19.4B). It enables acquisition of extremely thin slice images of the brain following intravenous contrast. Images of the vessels can be reconstructed in three dimensions and viewed from any direction. The investigation is safe and quick and without the 1% risk of stroke associated with conventional catheter angiography.

Computed tomographic venography

Computed tomographic venography (CTV) may be useful when MRA is contraindicated or there are difficulties in distinguishing slow flow from thrombus on MR. The technique is similar to CTA, except that images are acquired in the venous phase of contrast enhancement (Fig. 19.4C). However, it is not as sensitive as MRA in detecting associated parenchymal changes and this usually limits its use to equivocal cases.

Conventional catheter angiography

Conventional intra-arterial catheter angiography is usually performed under local anaesthetic. A catheter is passed via the femoral artery into the internal carotid and vertebral arteries in the neck under fluoroscopic guidance. Following contrast injection images are taken in rapid succession. Digital subtraction results in images of the contrast-filled vessels without any background structure such as bone (Fig. 19.4D). Until recently, this technique was the first-line investigation in the diagnosis of intracranial aneurysms but is now mainly reserved for cases where CTA is equivocal or negative.

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Optic nerve

Anatomy

General structure

1Afferent fibres. The optic nerve carries approximately 1.2 million afferent nerve fibres which originate in the retinal ganglion cells. Most of these synapse in the lateral geniculate body, although some reach other centres, notably the pretectal nuclei in the midbrain. Nearly one-third of the fibres subserve the central 5° of the visual field. Within the optic nerve itself the nerve fibres are divided into about 600 bundles, each containing 2000 fibres, by fibrous septae derived from the pia mater (Fig. 19.5).

2Surrounding sheaths

aThe innermost sheath is the delicate vascular pia mater.

bThe outer sheath comprises the arachnoid mater and the tougher dura mater which is continuous with the sclera; optic nerve fenestration involves incision of this outer sheath. The subarachnoid space is continuous with the cerebral subarachnoid space and contains cerebrospinal fluid (CSF).

Fig. 19.5 Structure of the optic nerve. (A) Clinical appearance; (B) longitudinal section, LC= lamina cribrosa; arrow points to a fibrous septum; (C) transverse section, P= pia, A = arachnoid, D= dura; (D) surrounding sheaths and pial blood vessels

(Courtesy of Wilmer Institute – figs A, B and C)

Anatomical subdivisions

The optic nerve is approximately 50 mm long from globe to chiasm. It can be subdivided into four segments:

1Intraocular segment (optic disc, optic nerve head) is the shortest, being 1 mm deep and 1.5 mm in vertical diameter.

2Intraorbital segment is 25–30 mm long and extends from the globe to the optic foramen at the orbital apex. Its diameter is 3–4 mm because of the addition of the myelin sheaths to the nerve fibres. At the orbital apex the nerve is surrounded by the tough fibrous annulus of Zinn, from which originate the four rectus muscles.

3Intracanalicular segment traverses the optic canal and measures about 6 mm. Unlike the intraorbital portion it is fixed to the canal, since the dura mater fuses with the periosteum.

4Intracranial segment joins the chiasm and varies in length from 5–16 mm (average 10 mm). Long intracranial segments are particularly vulnerable to damage by adjacent lesions such as pituitary adenomas and aneurysms.

Visual evoked potential

1Principle (Fig. 19.6). The visual evoked potential (VEP) test is a recording of electrical activity of the visual cortex created by stimulation of the retina. The main indications are the monitoring of visual function in babies and the investigation of optic neuropathy, particularly when associated with demyelination. It can also be used to monitor macular pathway function.

2Technique. The stimulus is either a flash of light (flash VEP) or a black-and-white checkerboard pattern on a screen which periodically reverses polarity (pattern VEP). Several tests are performed and the average potential is calculated by a computer.

3Interpretation. Both latency (delay) and amplitude of the VEP are assessed. In optic neuropathy both parameters are affected, with prolongation of latency and decrease in amplitude. Threshold VEP (by using different sized checks) can detect early or

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subclinical dysfunction as the smaller check size responses may become abnormal earlier while larger check size responses remain within normal limits.

Fig. 19.6 Principles of the visually evoked potential

Signs of optic nerve dysfunction

1 Reduced visual acuity for distance and near is common but may also occur with a great variety of other disorders.

2Afferent pupillary defect (see below).

3Dyschromatopsia (impairment of colour vision), which mainly affects red and green. A simple way of detecting a uniocular colour vision defect is to ask the patient to compare the colour of a red object.

4Diminished light brightness sensitivity, often persisting after visual acuity returns to normal, as for instance following an attack of optic neuritis.

5Diminished contrast sensitivity (see Ch. 14).

6Visual field defects, which vary with the underlying pathology, include diffuse depression of the central visual field, central scotomas, centrocaecal scotomas, nerve fibre bundle and altitudinal (Table 19.1).

Table 19.1 -- Focal visual field defects in optic neuropathies

1Central scotoma

•Demyelination

•Toxic and nutritional

•Leber hereditary optic neuropathy

•Compression

2Enlarged blind spot

•Papilloedema

•Congenital anomalies

3Respecting horizontal meridian

•Anterior ischaemic optic neuropathy

•Glaucoma

•Disc drusen

4Upper temporal defects not respecting vertical meridian

•Tilted discs

Optic atrophy

Primary optic atrophy

Primary optic atrophy occurs without antecedent swelling of the optic nerve head. It may be caused by lesions affecting the visual pathways from the retrolaminar portion of the optic nerve to the lateral geniculate body. Lesions anterior to the optic chiasm result in unilateral optic atrophy, whereas those involving the chiasm and optic tract will cause bilateral changes.

1Signs

•White, flat disc with clearly delineated margins (Fig. 19.7A).

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•Reduction in the number of small blood vessels on the disc surface (Kestenbaum sign).

•Attenuation of peripapillary blood vessels and thinning of the retinal nerve fibre layer.

•The atrophy may be diffuse or sectoral depending on the cause and level of the lesion.

•Temporal pallor may indicate atrophy of fibres from the papillomacular bundle, which enters the optic nerve head on the temporal side.

•Band atrophy caused by involvement of the fibres entering the optic disc nasally and temporally with sparing of the superior and inferior portions occurs in lesions of the optic chiasm or tract.

2Causes

•Optic neuritis.

•Compression by tumours and aneurysms.

•Hereditary optic neuropathies.

•Toxic and nutritional optic neuropathies.

•Trauma.

Fig. 19.7 Optic atrophy. (A) Primary due to compression; (B) secondary due to chronic papilloedema; (C) consecutive due to vasculitis

(Courtesy of P Gili – fig. B).

Secondary optic atrophy

Secondary optic atrophy is preceded by long-standing swelling of the optic nerve head.

1Signs vary according to the cause. The main features are (Fig. 19.7B):

•White or dirty grey, slightly raised disc with poorly delineated margins due to gliosis.

•Reduction in the number of small blood vessels on the disc surface.

•Surrounding ‘water marks’.

2 Causes include chronic papilloedema, anterior ischaemic optic neuropathy and papillitis.

Consecutive optic atrophy

Consecutive optic atrophy is caused by diseases of the inner retina or its blood supply. The cause is usually obvious on fundus examination such as retinitis pigmentosa, old vasculitis (Fig. 19.7C), retinal necrosis and excessive retinal photocoagulation.

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Classification of optic neuritis

Optic neuritis is an inflammatory, infective or demyelinating process affecting the optic nerve. It can be classified both ophthalmoscopically and aetiologically as follows.

Ophthalmoscopical classification

1Retrobulbar neuritis, in which the optic nerve head is normal, at least initially, because the optic nerve head is not involved. It is the most frequent type in adults and is frequently associated with multiple sclerosis (MS).

2Papillitis is characterized by hyperaemia and oedema of the optic disc, which may be associated with peripapillary flame-shaped haemorrhages (Fig. 19.8). Cells may be seen in the posterior vitreous. Papillitis is the most common type of optic neuritis in children, although it can also affect adults.

3Neuroretinitis is characterized by papillitis in association with inflammation of the retinal nerve fibre layer and a macular star figure (see below). It is the least common type and is only rarely a manifestation of demyelination.

Fig. 19.8 Severe papillitis

(Courtesy of R Bates)

Aetiological classification

1Demyelinating, which is by far the most common cause.

2Parainfectious, which may follow a viral infection or immunization.

3Infectious, which may be sinus-related, or associated with cat-scratch fever, syphilis, Lyme disease, cryptococcal meningitis in patients with AIDS and herpes zoster.

4Non-infectious causes include sarcoidosis and systemic autoimmune diseases such as systemic lupus erythematosus, polyarteritis nodosa and other vasculitides.

Demyelinating optic neuritis

Overview

Demyelination is a pathological process by which normally myelinated nerve fibres lose their insulating myelin layer. The myelin is phagocytosed by microglia and macrophages, subsequent to which astrocytes lay down fibrous tissue in plaques. Demyelinating disease disrupts nervous conduction within the white matter tracts of the brain, brainstem and spinal cord. Diseases which may cause ocular problems are the following:

1Isolated optic neuritis, with no clinical evidence of generalized demyelination, although in a high proportion of cases this subsequently develops.

2Multiple sclerosis (MS), which is by far the most common (see below).

3Devic disease (neuromyelitis optica), which is a very rare disease that may occur at any age. It is characterized by bilateral optic neuritis and the subsequent development of transverse myelitis (demyelination of the spinal cord) within days or weeks.

4Schilder disease, which is a very rare, relentlessly progressive, generalized disease with an onset prior to the age of 10 years and death within 1–2 years. Bilateral optic neuritis without subsequent improvement may occur.

Multiple sclerosis

Multiple sclerosis (MS) is an remitting idiopathic demyelinating disease involving white matter within the CNS (Fig. 19.9A and B). 1 Presentation is typically in the 3rd–4th decades in one of two ways:

aRelapsing/remitting episodes of demyelination with complete or incomplete recovery is by far the most common pattern. After 10 years about 50% of patients develop continuously progressive disease with occasional remissions.

bProgressive disease from the onset, without remissions, affects about 10% of patients and is very difficult to treat.

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2Signs

aSpinal cord involvement may cause weakness, stiffness, sphincter disturbance and sensory loss with a ‘trouser-like’ distribution.

b Brainstem disease may result in diplopia, nystagmus, dysarthria and dysphagia.

cCerebral hemisphere lesions may give hemiparesis, hemianopia and dysphasia.

dPsychological problems include intellectual decline, depression, euphoria and dementia.

eTransient features include Lhermitte sign (electrical sensation on neck flexion), dysarthria-dysequilibrium-diplopia syndrome and Uhthoff phenomenon (sudden worsening of vision or other symptoms on exercise or increase in body temperature).

4Investigations

aLumbar puncture shows leucocytosis, IgG level >15% of total protein and oligoclonal bands on protein electrophoresis.

bMR shows ovoid periventricular and corpus callosum plaques with their long axes perpendicular to the ventricular margins (see Fig. 19.9C). Plaques of acute demyelination may be highlighted with gadolinium on T1-weighted scans.

5 Treatment options in selected cases include systemic steroids and interferon beta-1a.

6Ophthalmic features

aCommon are optic neuritis (usually retrobulbar), internuclear ophthalmoplegia and nystagmus.

b Uncommon are skew deviation, ocular motor nerve palsies and hemianopia.

cRare are intermediate uveitis and retinal periphlebitis.

Fig. 19.9 Multiple sclerosis. (A) Histology shows a plaque with perivascular cuffing; (B) pathological specimen shows plaques of demyelination in the periventricular white matter; (C) T1-weighted axial MRimage shows periventricular plaques

Association between optic neuritis and multiple sclerosis

Although some patients with optic neuritis have no clinically demonstrable associated systemic disease, the following close association exists between optic neuritis and MS.

•Approximately 15–20% of MS patients present with optic neuritis.

•Optic neuritis occurs at some point in 50% of patients with established MS.

•The overall 10 year risk of developing MS following an acute episode of optic neuritis is 38%.

•At the first episode of optic neuritis, patients who also show T2-signal lesions on MR (see Fig. 19.9C) but no clinical evidence of

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MS, have a 56% risk of developing clinical MS within 10 years; those with no lesions have a 22% risk.

•Even when MR lesions are present, clinical MS does not develop within 10 years in 44% of cases.

•In a patient with optic neuritis, the subsequent risk of MS is increased with winter onset, HLA-DR2 positivity and Uhthoff phenomenon.

Clinical features of demyelinating optic neuritis

1Presentation is usually between the ages of 20 and 50 years (mean around 30 years) with subacute monocular visual impairment. Some patients experience positive visual phenomena (phosphenes) characterized by tiny white or coloured flashes or sparkles. Discomfort or pain in or around the eye is common and frequently exacerbated by ocular movements. This may precede or accompany the visual loss and usually lasts a few days. Frontal headache and tenderness of the globe may also be present.

2Signs

•VA is usually 6/18–6/60 although rarely it may be worse.

•Other signs of optic nerve dysfunction (see above).

•The optic disc is normal in the majority of cases (retrobulbar neuritis); the remainder show papillitis.

•Temporal disc pallor may be seen in the fellow eye, indicative of previous optic neuritis.

3Visual field defects (Fig. 19.10)

•The most common is diffuse depression of sensitivity in the entire central 30°.

•This is followed in frequency by altitudinal/arcuate defects and then by focal central/centrocaecal scotomas.

•Focal defects are frequently accompanied by an element of superimposed generalized depression.

4Course. Vision worsens over several days to 2 weeks and then begins to improve. Initial recovery is fairly rapid and then slowly improves over 6–12 months.

5Prognosis. Approximately 75% of patients recover visual acuity to 6/9 or better. However, despite return of visual acuity other parameters of visual function, such as colour vision, contrast sensitivity and light brightness appreciation often remain abnormal. A mild relative afferent pupillary defect may persist and optic atrophy may ensue, particularly following recurrent attacks. About 10% of patients develop chronic optic neuritis characterized by slowly progressive or stepwise visual loss, unassociated with periods of recovery.

Fig. 19.10 Visual field defects in optic neuritis. (A) Central scotoma; (B) centrocaecal scotoma; (C) nerve fibre bundle; (D) altitudinal

Treatment of demyelinating optic neuritis

1Indications. When visual acuity within the first week of onset is worse than 6/12, treatment may speed up recovery by 2–3 weeks. This may be relevant in the patients with poor vision in the fellow eye or those with occupational requirements, but this small benefit must be balanced against the risks of using high dose steroids. However, therapy does not influence the eventual visual outcome and the great majority of patients do not require treatment.

2Regimen

aIntravenous methylprednisolone sodium succinate 1g daily for 3 days followed by oral prednisolone (1 mg/kg daily) for 11 days and then tapered over 3 days.

bIntramuscular interferon beta-1a at the first episode of optic neuritis is beneficial in reducing the development of clinical MS over the following 3 years in patients at high risk based on the presence of subclinical brain lesions on MR. However, the benefit is small and most patients do not commence interferon until they have had a second episode of clinical

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