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

STRABISMUS SURGERY: HORIZONTAL 597

Strabismus surgery: horizontal

The most common deviations (esotropia and exotropia) are horizontal and are therefore generally amenable to surgery on the horizontal recti (Table 17.14). The most common procedure is a unilateral “recess/resect,” although the options range from single-muscle procedures to bilateral (simultaneous or staged) surgery involving multiple muscles.

Recess/resects

An MR recession/LR resection will reduce convergence, whereas an LR recession/MR resection will reduce divergence. Estimation of the amount of surgical correction (in mm) required for the size of strabismus (in ) may be assisted by use of surgical tables (e.g., Table 17.15).

However, such tables are only a guide and should be modified by each surgeon according to their own individualized outcomes.

Table 17.14 Outline of horizontal muscle surgery

Recession • Local conjunctival peritomy

•Identify and expose muscle

•Free muscle from Tenon’s layer

•Place two locking bites of an absorbable suture through the outer quarters of the muscle

•Disinsert tendon and measure recession

•Suture in new position: either directly to adjacent sclera or to the insertion (hang back technique)

•Close conjunctiva

Resection • Local conjunctival peritomy

•Identify and expose muscle

•Free muscle from Tenon’s layer

•Measure and place two locking bites of an absorbable suture posterior to intended resection

•Resect desired length of muscle

•Suture remaining muscle to insertion

•Close conjunctiva

Table 17.15 Absolute maximum surgical adjustments for rectus muscles

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Pediatric ophthalmology

Embryology (1) 600

Embryology (2) 602 Genetics 604

Pediatric examination 606

The child who does not see 608 Child abuse 610

Common clinical presentations: vision and movement 611 Common clinical presentations: red eye, watery eye, and

photophobia 613

Common clinical presentations: proptosis and globe size 615 Common clinical presentations: cloudy cornea and

leukocoria 617 Intrauterine infections 619 Ophthalmia neonatorum 621

Orbital and preseptal cellulitis 623 Congenital cataract: assessment 625 Congenital cataract: management 627 Uveitis in children 628

Glaucoma in children 630 Retinopathy of prematurity 632 Other retinal disorders 634 Developmental abnormalities 635 Chromosomal syndromes 638

Metabolic and storage diseases (1) 640 Metabolic and storage diseases (2) 642 Phakomatoses 644

Related pages:

Amblyopia, p. 576

Binocular single vision, p. 578 Strabismus, p. 580

Intraocular tumors: retinoblastoma, p. 505

Medical retina: retinitis pigmentosa, p. 456; congenital stationary night blindness, p. 458; macular dystrophies, p. 459; choroidal dystrophies, p. 462; albinism, p. 464; Coats’ disease, p. 452.

600 CHAPTER 18 Pediatric ophthalmology

Embryology (1)

The normal eye forms from an outpouching of the embryonic forebrain (neuroectoderm) with contributions from neural crest cells, surface ectoderm, and, to a lesser extent, mesoderm. The interactions between these layers are complex; failure may result in serious developmental abnormalities (p. 635).

General

The developing embryo comprises three germinal layers: ectoderm, mesoderm, and endoderm. The ectoderm differentiates into outer surface ectoderm and inner neuroectoderm.

The neuroectoderm continues to develop, forming first a ridge (neural crest), then a cylinder (neural tube), and finally vesicles within the cranial part of the tube to form the fore-, mid-, and hindbrain (prosencephalon, mesencephalon, telencephalon). The neural crest cells also migrate to contribute widely to ocular and orbital structures.

The globe

The optic vesicle develops as a neuroectodermal protrusion of the prosencephalon. It induces the overlying surface ectoderm to thicken into the lens placode. Then (week 4) both these structures invaginate to form a double-layered optic cup and lens vesicle, respectively. The cup is not complete but retains a deep inferior groove (optic fissure) in which mesodermal elements develop into the hyaloid and other vessels.

Closure starts at the equator (week 5) and proceeds anteroposteriorly; failure of closure results in colobomata (p. 635).

Anterior segment

Lens

The lens placode forms from surface ectoderm and invaginates to form the lens vesicle (week 5). At this point, the anterior lens epithelium is a unicellular layer surrounded by a basement membrane (the future capsule). This layer continues to divide throughout life.

The posterior cells elongate and differentiate into primary lens fibers. The anterior cells migrate to the equator and elongate forming the secondary lens fibers. These meet at the lens sutures.

Cornea

After separation of the lens vesicle, the surface ectoderm reforms as a epithelial bilayer with basement membrane. It is joined by three waves of migrating neural crest cells: the first wave (week 6) forms the corneal and trabecular endothelium; the second (week 7) forms the stroma; the third (also week 7) forms the iridopupillary membrane.

Sclera

The sclera develops from a condensation of mesenchymal tissue situated at the anterior rim of the optic cup. This begins at the limbus (week 7) and proceeds posteriorly to surround the optic nerve (week 12).

602 CHAPTER 18 Pediatric ophthalmology

Embryology (2)

Posterior segment

Retina

All retinal tissues develop from the optic cup (neuroectoderm). The inner layer of the cup divides into two zones: a superficial non-nucleated marginal zone and a deeper nucleated primitive zone. Mitosis and migration from the primitive zone leads to the formation of an inner neuroblastic layer (in which Müller cells, ganglion cells, bipolar cells, horizontal cells, and amacrine cells develop) and an outer neuroblastic layer (giving rise to primitive photoreceptor cells).

Familiar retinal organization starts with the formation of the ganglion cell layer and continues at the deeper levels with both cellular and acellular zones (nuclear and plexiform layers). This wave of retinal development starts at the posterior pole and proceeds anteriorly.

The photoreceptors arise from the outermost cells of the inner layer. Originally ciliated, these are replaced by distinctive outer segments. Cones develop first (months 4–6), rods later (month 7 on). These photoreceptor cells project toward the outer layer of the cup. The outer layer (the retinal pigment epithelium) thins to become one cell thick and becomes pigmented, the first structure in the body to do so.

Retinal vasculature develops from the hyaloid circulation and spreads in an anterior wave, reaching the nasal periphery before the temporal periphery (month 9); it may not be fully developed in premature infants.

Choroid

This vascular layer arises from endothelial blood spaces around the optic cup; the extension of posterior ciliary arteries to join the primitive choroidal vasculature; and the consolidation of venous networks to form the four vortex veins.

Optic nerve

Vacuolization of cells within the optic stalk allows ganglion cell axons to grow through from the retina. The appearance of crossed and uncrossed fibers results in the formation of the chiasm (months 2–4).

Myelination proceeds anteriorly from the lateral geniculate nucleus (LGN, month 5) to the lamina cribrosa (month 1 postnatal). The inner layer of the stalk develops supportive glial cells, which separate the nerve fibers into bundles; the outer layer gives rise to the lamina cribrosa.

Vitreous

The primary vitreous (week 5) forms in the retrolental space. It contains collagen fibrils, mesenchymal elements, and the hyaloid vasculature (which forms the tunica vasculosa lentis). Later (week 6) this is surrounded by the secondary vitreous and effectively forms Cloquet’s canal.

The secondary vitreous is avascular, transparent, and composed of very fine organized fibers. Failure of the vascular system to regress causes Mittendorf’s dot, Bergmeister’s papilla, persistent hyaloid artery, and persistent fetal vasculature (PFV; formerly known as persistent hyperplastic primary vitreous, PHPV).

EMBRYOLOGY (2) 603

Traditionally, tertiary vitreous was used to describe a relatively anterior condensation associated with the formation of lens zonules (which arise from the ciliary body).

Nasolacrimal drainage system

This develops from a cord of surface ectoderm, which is met by proliferating cords of cells from the lids above and from the nasal fossa below (see Table 18.1). Cannulation of the cord may be delayed distally, causing congenital obstruction. More commonly there is simply an imperforate mucus membrane at the valve of Hasner, which most often resolves spontaneously within the first year (year 1 postnatal).

Table 18.1 Summary of germinal layers

604 CHAPTER 18 Pediatric ophthalmology

Genetics

Genetic disorders may result from an abnormal karyotype (abnormal number of chromosomes, e.g., trisomies), an abnormal region of the chromosome (e.g., deletions, duplications), abnormal gene(s) at a single locus (autosomal or X-linked), abnormal mitochondrial DNA, or the interaction of a number of genes with the environment.

Single-gene autosomal disorders obey the laws of segregation and independent assortment noted by Mendel. This results in predictable patterns of inheritance (Table 18.2). More complex patterns arise from X-linked and mitochondrial disease. Most common conditions appear to be polygenic with additional contributions from environmental factors.

Even for single-gene disorders, the pattern of disease may be unpredictable. Such conditions may have incomplete penetrance (i.e., genotype present without the phenotype) or variable expressivity (i.e., wide range within the phenotype). In some conditions, anticipation may occur, where succeeding generations develop earlier and more severe disease. This is due to triplet repeats in which the number of repeats of a particular codon (e.g., GCT in the myotonic dystrophy gene) increases from generation to generation.

Inheritance patterns

Table 18.2 Inheritance patterns for single-gene defect with 100% penetrance

GENETICS 605

Table 18.3 Chromosomal locations of genes involved in ophthalmic disease (selected)

1Schnyder dystrophy

Stargardt/fundus flavimaculatus (ABCR4)

2Oguchi disease (arrestin) Waardenburg syndrome (PAX3)

3VHL (VHL gene) CSNB1(transducin (A))

4Anterior segment dysgenesis (PITX2)

5Reis–Bücklers, Thiel–Behnke, granular, lattice I (keratoepithelin, BIGH3)

6Tritanopia (S opsin)

Anterior segment dysgenesis (FOXC1)

7

8Retinitis pigmentosa (RP1, and numerous others)

9Tuberous sclerosis (TSC1, harmartin) Oculocutaneous albinism (OCA III, TRP1)

10Gyrate atrophy (OAT)

11Best’s disease (bestrophin) Aniridia, Peter’s anomaly (PAX6)

Oculocutaneous albinism (OCA1, tyrosinase)

12Meesmann (K3, keratin)

Chronic fibrosis of extraocular muscles (CFEOM1)

13Wilson disease

14

15Marfan syndrome (FBN1, fibrillin) Oculocutaneous albinism (OCAII, p)

16Tuberous sclerosis (TSC2, tuberin)

17Neurofibromatosis-1 (NF1, neurofibromin) Meesmann (K12, keratin)

18

19 Myotonic dystrophy (DMPK)

20

21Homocystinuria type 1 (cystathionine synthetase)

22Neurofibromatosis-2 (NF2, merlin) Sorsby fundus dystrophy (TIMP)

XOcular albinism (OA1) X-linked RP (RP2)

X-linked juvenile retinoschisis (RS1) Choroideremia (REP1)