Ординатура / Офтальмология / Английские материалы / Oxford American Handbook of Ophthalmology_Tsai, Denniston, Murray_2011

566 CHAPTER 16 Neuro-ophthalmology

Myotonic dystrophy

This uncommon autosomal dominant dystrophy results from an expanded CTG repeat in the dystrophica myotonica protein kinase (DMPK) gene (Ch19q). Anticipation occurs whereby the triplet expansion increases in successive generations, leading to earlier and more severe disease.

Prevalence is estimated at around 5/100,000, being highest among French-Canadians. It is characterized by a failure of muscle relaxation after contraction.

Clinical features

Ocular

•Bilateral ptosis, cataracts (polychromatic “Christmas tree cataracts” or posterior subcapsular), orbicularis oculi weakness; rarely, pigmentary retinopathy (“butterfly” pigmentation centrally, reticular at midperiphery, and atrophic far periphery), and myotonia of extraocular muscles.

Systemic

•Mournful facies, dysphasia, dysphagia, muscle weakness with delayed relaxation (“myotonic grip”), testicular atrophy, frontal baldness, dIQ, cardiac myopathy, and conduction abnormalities (may lead to fatal cardiac failure).

Investigations

•DNA analysis is used to confirm the diagnosis.

•ECG should be performed annually for conduction abnormalities; these may occur in otherwise minimally affected individuals.

Treatment

Multidisciplinary management may include neurology, cardiology, physiotherapy, occupational therapy, and speech therapy. Offer genetic counseling, annual influenza vaccination, and cataract surgery (when symptomatic). General anesthetics may unmask subclinical respiratory failure, leading to problems of ventilatory weaning.

BLEPHAROSPASM AND OTHER DYSTONIAS 567

Blepharospasm and other dystonias

Blepharospasm is a relatively common condition that, in its severe form, can be very disabling in terms of both vision and social function. It is more common in women (F:M 2:1) and increases with age. It is a type of focal dystonia in which there is tonic spasm of the orbicularis oculi.

The condition may be idiopathic (essential blepharospasm) or secondary to ocular or periocular disease (see Table 16.27). Blepharospasm may be associated with dystonias involving other facial muscles.

Essential blepharospasm

Clinical features

•Bilateral involuntary lid closure, increase frequency of lid closure (normal is around 10–20x/min); may be precipitated by stress, fatigue, social interactions; may be relieved by relaxation or distraction, e.g., touching face or whistling. There are often marked fluctuations from day to day, but the condition generally worsens over years.

•Associated ocular disease may include underlying precipitants (particularly lid and ocular surface) and secondary anatomical changes of the lid (ptosis or entropion) or brow (brow-ptosis or dermatochalasis).

Investigations

Typical isolated blepharospasm does not usually require investigation. If atypical (e.g., associated weakness or any other neurological abnormality), consult with a neurologist and consider imaging (e.g., MRI) and other tests (e.g., EMG).

Treatment

•Botulinum toxin (A) is usually given as multiple injections of the upper and lower lid; it has high rate of success in the short term (up to 98%) but generally only lasts for 3 months. Complications include ptosis, epiphora, keratitis, dry eyes, and ocular motility disorders (diplopia).

•Treat any underlying ocular disease.

•Other treatment options include medical (e.g., benzodiazepines) and surgical (myectomy or chemomyectomy with doxorubicin) ones.

Table 16.27 Causes of blepharospasm

568 CHAPTER 16 Neuro-ophthalmology

Other dystonias of the face and neck

•Meige’s syndrome: blepharospasm with midfacial spasm; regarded as a spillover of essential blepharospasm to involve the midfacial musculature. It may compromise speech and eating and drinking.

•Torticollis: tonic spasm of sternocleidomastoid causes sudden sustained movement of the head to one side.

Other involuntary facial movement disorders

•Hemifacial spasm: tonic-clonic spasm of facial musculature that, unlike blepharospasm or Meige’s syndrome, is unilateral, may occur during sleep, and typically affects a younger age group. It suggests irritation of the root of the CN VII by a compressive lesion (usually an abnormal vessel, but needs imaging to rule out a posterior fossa tumor).

•Facial myokymia: fleeting movements of facial musculature that may be associated with caffeine, stress, MS, or rarely tumors of the brainstem

•Facial tic: brief, repetitive stereotypic movements that are suppressible (at least initially). It may be associated with Gilles de la Tourette syndrome.

Lid apraxia

Normal blinking requires both the inhibition of levator palpebre superioris and the activation of orbicularis oculi. In lid-opening apraxia, there is total inhibition of LPS with no activation of orbicularis oculi (OO). This results in sustained lid closure with difficulty in initiating lid opening. It is associated with extrapyramidal diseases (e.g., Parkinson’s disease, progressive supranuclear palsy, Huntington’s disease, Wilson’s disease).

Lid retraction and poor initiation of lid closure may also be seen in Parkinson’s disease, progressive supranuclear palsy, and Parinaud’s syndrome.

FUNCTIONAL VISUAL LOSS 569

Functional visual loss

Functional visual loss (also called nonorganic visual loss, psychogenic visual impairment) is a diagnosis of exclusion. It can often coexist with genuine pathology.

Suspecting functional visual loss

Consider this diagnosis when the patient reports poor vision but some of the following features are present.

Visual function and history

•Visual functioning obviously does not correlate with history (e.g., patient reported blindness but was able to navigate around the hospital, waiting room, or examination room).

•Patient cannot perform tasks that he/she may consider to be visual but actually are not (e.g., signing name).

•Recent stressful event elicited in history (e.g., impending exams).

Normal examination

•No apparent pathology after detailed examination

•Absence of RAPD in the context of profound reported asymmetrical visual loss. Bilateral symmetrical pathology may give slow (sluggish) pupillary light responses but no RAPD.

•Retinoscopy and subjective refraction shows absence of uncorrected refractive error.

•Optokinetic nystagmus is demonstrable using field stimulus that patient reports not being able to discern.

Inconsistent abnormalities in the examination

•Goldman perimetry features: Spiraling isopters regress toward fixation as the test progresses; crossed isopters show that a dimmer or smaller target is surprisingly seen further in the periphery than a brighter or larger target. Crowded isopters show that targets of greatly differing size or brightness are suddenly seen when they reach about the same eccentricity within the visual field.

•Ishihara plates: patient may give inconsistent responses (e.g., recognize

“12” but no other numbers, yet repeatedly trace the plates correctly). It is important to exclude defective color vision in the normal eye to validate RAPD observations.

Diagnosing functional visual loss

Diagnose functional visual loss only when the patient has demonstrated normal vision. This requires an encouraging, empathic approach and an adroit examination. Consider the following methods.

Tests of stereoacuity

Normal stereoacuity implies normal visual acuity.

The crossed-cylinder technique

•Fog good eye with +6D lens in trial frame, +0.25 before “blind” eye.

•Rotate a crossed +3D cyl before a –3.0 cyl.

•See if the patient can be encouraged to read with the “blind” eye when the cylinders are superimposed to negate each other.

570 CHAPTER 16 Neuro-ophthalmology

Tests of reading vision

In some cases, normal reading vision can be demonstrated, proving normal visual potential despite apparently impaired Snellen acuities.

Tests of color vision

If the patient gives normal Ishihara plate responses, then his/her visual acuity is at least 20/80. For those with congenital red-green color blindness, the presence of a red filter should enable them to read the plates, provided they have an acuity of at least 6/24.

Etiologies

•Conversion disorder: visual loss may be a manifestation of psychological or social difficulties.

•Malingering: feigned visual loss for other (usually material) benefit.

Management

Patients suspected of functional visual loss will often need encouragement, reassurance, and follow-up. If the diagnosis remains uncertain, use a term such as visual loss of unknown cause in the notes.

Referral to an ophthalmologist familiar with unexplained visual loss (e.g., neuro-ophthalmologist or pediatric ophthalmologist) may avoid unnecessary investigations.

Investigations

Investigation is mandatory when there is diagnostic uncertainty. Consider the following:

•Electrodiagnostic testing (EDT): normal VEP results support reasonable vision but abnormal results can be found in the absence of genuine pathology. EDT may identify early Stargardt’s disease or cone dystrophy.

•Neuro-imaging, e.g., contrast-enhanced MRI of visual pathway.

•Investigation as a chronic optic neuropathy of unknown etiology (e.g., for Leber’s mutations).

•In exceptional circumstances (when cortical injury is suspected), positron emission tomography (PET) can reveal organic disease when other imaging techniques give normal results.

Treatment

When functional visual loss is diagnosed, the patient should be counseled carefully. The physician faces the unusual situation of contesting the patient’s symptoms. However, an adversarial scenario can be both disagreeable and entirely counterproductive. The patient can be reassured that he/she has healthy eyes and that the return of normal visual functioning is expected.

With support, patience, and reassurance, the patient can be allowed to resolve his/her visual functioning. The underlying problem may be far beyond the scope of most ophthalmologists’ expertise. In some cases, a clinical psychologist may be helpful.

Strabismus

Anatomy and physiology (1) 572

Anatomy and physiology (2) 574

Amblyopia 576

Binocular single vision 578

Strabismus: assessment 580

Strabismus: outline 582

Comitant strabismus: esotropia 584

Comitant strabismus: exotropia 586

Incomitant strabismus 588

Restriction syndromes 590

Alphabet patterns 593

Strabismus surgery: general 595

Strabismus surgery: horizontal 597

572 CHAPTER 17 Strabismus

Anatomy and physiology (1)

Extraocular muscles

The orbit forms a pyramid in which the lateral and medial walls are at 45* to each other, and the central axis is thus at 22.5*(approximated to 23*). The four rectus muscles originate from the annulus of Zinn (Table 17.1).

The superior oblique (SO; like the levator palpebrae superioris) originates from the orbital apex outside the annulus; in contrast, the inferior oblique (IO) arises from the nasal orbital floor. The obliques lie inferior to their corresponding rectus (R) muscle (i.e., SO lies inferior to SR and IO inferior to IR) (see Figs. 17.1 and 17.2).

The spiral of Tillaux describes the way the recti insert increasingly posterior to the limbus (MR, IR, LR, then SR). Innervation is by CN III for SR, MR, IR, IO; by CN IV for SO; and by CN VI for LR.

Table 17.1 Anatomy of extraocular muscles

574 CHAPTER 17 Strabismus

Anatomy and physiology (2)

Eye movements

Eye movements may be monocular (ductions) or binocular (versions and vergences). Versions are conjugate eye movements, i.e., both eyes move in the same direction, whereas vergences are disconjugate, i.e., both eyes move in opposite directions. Eye movements may be described as rotations of the globe around horizontal (x), anteroposterior (y), and vertical

(z) axes—the axes of Fick.

Ductions comprise abduction (outward), adduction (inward), supraduction (upward), infraduction (downward), intorsion (superior limbus moves inward), and extorsion (superior limbus moves outward) (Table 17.2).

Versions include dextroversion (right gaze), levoversion (left gaze), supraversion (upgaze), infraversion (downgaze), dextrocycloversion (superior limbus moves right), and levocycloversion (superior limbus moves left) (Fig. 17.3). Vergences include convergence (inward) or divergence (outward).

The extraocular muscles do not act in isolation. Each agonist (e.g., LR) has an antagonist that acts in the opposite direction in the same eye (i.e., ipsilateral MR). Increased innervation of the agonist is accompanied by decreased innervation of its antagonist (Sherrington’s law). Each agonist also has a yoke muscle that acts in the same direction in the other eye (i.e., contralateral MR in this example). During conjugate movement yoke muscles receive equal and simultaneous innervation (Hering’s law).

Table 17.2 Actions of extraocular muscles

ANATOMY AND PHYSIOLOGY (2) 575

Figure 17.3 The six cardinal positions of gaze (from observer’s perspective).