TRAINEES Embryology in clinical practice

he fascinating world of embryology separated into complete or incomplete. is both beautiful and practical. It is This condition results in a failure of eyelid a home video of our evolutionary development which results in a fusion of Thistory through the ages from the the external skin to the anterior surface single cell through to the life aquatic, the of the globe which in advanced forms is development of gut, limbs and brain, and associated with corneal maldevelopment most interestingly of all; the eye. The eye is and trabecular meshwork disorders. particularly fascinating because it is used by supporters of so called ‘intelligent design’ Coloboma to advocate an inherently indivisible nature Coloboma is Greek for ‘defect’ and can to the most sensitive sense organ of them manifest itself in myriad forms. It can affect all and as proof for a divine creator being. Figure 1: A coloboma affecting the optic disc and one or many parts of the eye including the inferior retina. Understanding ocular embryology is one iris, optic disc and retina. The choroidal thing but the practical aspect arises when fissure is a tract in which blood vessels are a defect which runs through the macula can we begin to see how the process can go laid in order to nourish the developing eye. greatly affect the visual acuity. wrong and how disease can occur. To follow This might be considered akin to builders A retinocoele is a variant of a coloboma on from last issue’s introduction to the running temporary wiring and piping right in which retina herniates through the topic, let us explore some practical ocular through the middle of a kitchen they are gap formed through malclosure of the embryology. installing to help with the building process, choroidal fissure. which will be removed prior to the owners Basic disorders of the house returning to use their kitchen. Persistent foetal vasculature The eye continually develops throughout Indeed, while the wires and piping may Extrapolating our analogy above about the gestation as a rapidly changing structure. It have very much helped the builders install building of a kitchen, imagine a group of is important to have some knowledge of the the kitchen, were they to remain they builders complete the installation but leave details of this process to help understand would vastly reduce the owners enjoyment the now unneeded wires and drainage the pathological and clinical problems that of their new kitchen as they would get in pipes running right through the middle of may be associated with mal-development. the way, be a tripping hazard, and generally the revamped room. This amounts to the Once the tissues are differentiated they still look cosmetically out of place. A coloboma hyaloid artery and vein in the eye. If the need to simply get bigger. The diameter is a defect that results from an abnormality anterior portion remains intact then so of the eyeball grows by nearly 40% during of the closure of the choroidal fissure in called persistent anterior foetal vasculature the final 12 weeks of pregnancy and as such the inferonasal quadrant of the developing is said to exist, although the name until there are a number of ophthalmologic optic cup in the sixth week. Closure starts recently was persistent hyperplastic disorders associated with prematurity such at the equator and extends anteriorly and primary vitreous. This mass of tissue behind as retinopathy, amblyopia, strabismus and posteriorly. Incomplete closure creates a the lens can contract and pull the ciliary refractive error. Neonatal vision is tested defect of any size from the margin of the processes into full view, as well as causing in the 24 hours after birth and again at six pupil to the optic disc, which may vary from cataract and leukocoria. weeks. It is important that any defect is being very subtle to being massive and If the posterior portion of the hyaloid identified and corrected before the point at unmissable. Embryological appreciation system remains then persistent posterior which the patient can become permanently is therefore key in understanding this foetal vasculature is said to exist and visually impaired. disorder. is most often seen as a dense fold of The most basic disorder in the As seen in Figure 1, a coloboma that condensed vitreous extending forward development of the eye is that the eye affects the optic disc and retina can from the optic disc to the ora serrata and fails to develop at all. This is called be visualised on examination using an can result in a retinal detachment. anophthalmia. Complete anophthalmia ophthalmoscope. A persistent pupillary membrane is not is exceedingly rare and usually associated A coloboma of the iris, however, can be a defect of any part of the hyaloid artery with other craniofacial abnormalities. seen perfectly clearly with the naked eye system and is in fact located as the name Interestingly, in this case, the extraocular and appear as a ‘keyhole’ shaped pupil. If suggests in the pupillary region, although muscles form along with the eyelids the coloboma is limited to the iris then it it is derived from incomplete atrophy of even if the eye itself is entirely absent. is called ‘coloboma iridis’. Some people a vascularised blood supply to the foetal Microphthalmia occurs when the eye use the term ‘cats eyes’ to describe such a lens. It does not normally cause any issues, does form although it is smaller and condition. unlike its hyaloidal cousins, although lens associated with some degree of disordered Colobomas have varying degrees of opacities and cataracts are not unknown. development. This might include a impact on long-term vision and correlate coloboma. with the different structures which may be Leukocoria Cryptophthalmos is technically a affected. A coloboma of the iris may have Other conditions that relate specifically to disorder of eyelid development and can be no impact on the vision. On the other hand, the embryology of the eye include retinal

eye news | FEBRUARY/MARCH 2016 | VOL 22 NO 5 | www.eyenews.uk.com TRAINEES

cataracta meaning waterfall or cascade. know this fact. The River Nile has six famous cataracts By far the most important gene from Aswan in the north to Bagrahwiyah that regulates eye development is the in the south, although the river itself flows intriguingly named paired box protein from south to north. Looking through 6 gene, more commonly called PAX-6, a waterfall is similar to the vision that found on chromosome 11. Gene mutations Figure 2: This child’s right eye has a white pupil on photography, a sign known as leukocoria. results from having a cataract, or so it was here produce a very disordered eye and apparently the case to those physicians of classically aniridia and Peter’s anomaly classical antiquity. are due to mutations in this gene. Anterior Cataracts have many causes and are segment dysgenesis has also been shown associated with various chromosomal to be related to defects in the FOXC1 gene, anomalies, genetic syndromes, infections, on chromosome 6, and the PITX2 gene, on trauma and metabolic diseases. chromosome 4. There are masses of other Figure 3: This baby has bilaterally enlarged eyes due to Approximately one third are genetic, one genes that regulate the development of congenital glaucoma. This is known as buphthalmos, or ‘ox eye’. third associated with systemic disease and the eye but if I mention more it will become one third idiopathic. Congenital cataracts massively confusing as remembering these detachment, congenital glaucoma and may be unilateral or bilateral and can genes is difficult as they do not relate to congenital cataract. All three of these cover a broad range of severity. Some lens anything sensible that we see in the world conditions can give rise to leukocoria opacities are visually insignificant and do around us. These ones mentioned here, or white reflex of the pupils during not progress whilst others can produce however, are the ones you really need to ophthalmoscopy. Leukocoria is Greek for profound visual impairment. Cataract is know. ‘white pupil’ (see Figure 2). one of the leading causes of blindness in Retinal detachment relates to the poor children and one of the important causes Conclusion fusion between the outer and inner layers of of amblyopia. Early detection of cataract is The embryology of the eye is both the optic cup and the intra-retinal potential therefore important as this is a potentially fascinating and daunting, but knowledge space remains. The separation of the neural treatable condition. of the basic principles outlined above is and pigmented layers may be partial or Visual development begins at birth and essential in understanding a great deal of complete and may result from unequal continues to eight years of age, with the subsequent pathology an ophthalmologist rates of growth which would put strain on majority of the process happening within may encounter in their clinic. For this the knitting and may lead to separation. the first three years. During these first three reason postgraduate exams almost always Congenital glaucoma is an abnormal years any reduction of retinal stimulation have at least a few embryology questions elevation of intraocular pressure in the can result in amblyopia or ‘lazy eye’, and its present, which makes understanding these newborn and is usually caused by a defect severity is proportionate to the density of principles essential to exam success. in the mechanism by which fluid leaves the cataract and the amount of time it remains aqueous humour. This may have a genetic untreated. or infective aetiology and it is associated with rubella infection in early pregnancy. It Germ layers and genes is a serious and sight-threatening condition Before ending the embryology series we where prompt diagnosis and treatment can should consolidate the building blocks of help optimise a child’s vision. It is therefore the embryo, the germ layers, and the genes important to be aware of the signs of that regulate their behaviour. The outside congenital glaucoma in a baby. structures such as the cornea, conjunctiva, The prominent hallmark signs of this lids and lacrimal passages, as well as some condition are buphthalmos, Greek for inside structures such as the lens (via the ‘ox’s eye’ meaning an unusually large eye, lens vesicle) and a portion of the vitreous and lacrimation. The corneas may appear are derived from surface ectoderm. The cloudy with associated Haab striae, and the neuroectoderm is responsible for the SECTION EDITOR child may be photophobic. Haab striae are neural structures and their close associates, Gwyn Samuel such as the retina, optic nerve, retinal horizontal breaks in Descemet’s membrane Williams, and are best remembered as both Haab pigment epithelium, iris epithelium and Medical Retina Fellow, and Horizontal start with the letter ‘H’. In dilator and sphincter pupillae. The neural Moorfields Eye Hospital, any newborn with excessively watering crest cells keep uveitis specialists in work London, UK. or tearing eyes a diagnosis of congenital by developing into the choroid, the ciliary E: gwynwilliams@ doctors.org.uk glaucoma should always be considered muscles, the corneal endothelium, a (see Figure 3). If congenital glaucoma is portion of the sclera, the optic nerve sheath suspected, urgent referral to a glaucoma and the orbital bones. Lastly the mesoderm, AUTHOR specialist should be sought without delay to sandwiched in the middle, develops into the Naser Ali, action prompt treatment. extraocular muscle, a portion of the sclera Author, and a portion of the vitreous. The portion Retinal Fellow, A congenital cataract is a lens opacity Moorfields Eye Hospital, which is present at birth. Since the lens of the sclera derived from the mesoderm London, UK. is the focusing mechanism for directing is classically the temporal portion with the E: [email protected] light onto the retina, having a cataract is remainder being derived from neural crest like looking through a frosted glass. The cells. Quite why this comes up in exams so word cataract comes from the Latin word often is beyond me, but it does, so you must

eye news | FEBRUARY/MARCH 2016 | VOL 22 NO 5 | www.eyenews.uk.com