Pediatric Ophthalmology

Vision Measurement and Terminology in Children

• Snellen Chart Standards:

  • In Australia, vision is measured using the Snellen chart with a system based on meters.

  • In the United States, the system uses feet (e.g., $20/20$).

  • Standard vision in Australia is recorded as $6/6$.

  • The numerator (6) refers to the distance from the patient to the chart, which is $6\,m$.

• Vision Percentages and Legal Thresholds:

  • $6/6$: Normal vision.

  • $6/12$: Represents $80\%$ of normal vision. This is the legal driving limit. A person with vision worse than $6/12$ in both eyes cannot hold a license.

  • $6/24$: Represents approximately $50\%$ of normal vision.

  • $6/60$: Represents $10\%$ vision, equivalent to seeing only the top letter on the Snellen chart.

• Vision Testing in Babies and Toddlers:

  • Because very small children cannot read letters, picture-matching techniques are used.

  • Children are shown a chart at $6\,m$ with various pictures and asked to point to matching images.

  • For children under the age of three, clinical assessment relies on whether the baby can "fix and follow" an object. An interesting toy is moved in front of one eye to see if the baby locks onto and tracks it.

  • Clinicians also check if a baby objects to covering one eye; if a baby protests when a specific eye is covered, it indicates that the good eye is being blocked and the child is forced to look through a very weak eye.

Amblyopia (Lazy Eye)

• Definition and Neurophysiology:

  • Amblyopia is the medical term for lazy eye, though "lazy eye" is often used colloquially (and incorrectly) to describe a droopy eyelid (ptosis) or crossed eyes (strabismus).

  • It relates to poor neural connections from the eye to the occipital lobe caused by visual problems during childhood.

  • Hubel and Wiesel's Cat Experiment (1970 Nobel Prize): Scientists sewed the eyelids of a kitten shut. They found that if the lids were opened within eight weeks, the kitten regained perfect vision. If kept shut for more than eight weeks, the kitten became permanently blind in that eye because the occipital lobe never learned to achieve full resolution despite the eyeball being healthy.

• The Critical Period:

  • In humans, the critical period for visual development ends at approximately 10 years of age.

  • If a vision problem is not corrected by age 10, the child will be stuck with that level of vision permanently, as the brain can no longer "catch up."

• Causes of Amblyopia:

  • Uneven Spectacle Numbers (Anisometropia): This is the most common cause by a significant margin. If a child has a much stronger prescription in one eye and it goes uncorrected, the vision becomes weaker.

  • Strabismus (Crossed Eyes): The brain suppresses the image from the turning eye to avoid confusion, leading to amblyopia.

  • Cataracts: A cataract in one eye can block visual input.

  • Ptosis: A droopy eyelid that physically blocks the line of sight.

• Treatment Strategies:

  • Spectacles: Correcting the refractive error first.

  • Patching (Occlusion Therapy): The "good" eye is covered to force the brain to use the weaker eye. While once done full-time, current practice often involves patching for two to three hours after school, Monday through Friday.

  • Behavioral Techniques: Use of star charts and rewards to encourage compliance.

  • Atropine Drops: Used if patching fails. One drop in the good eye (e.g., on Wednesday and Sunday) dilates the pupil and paralyzes focusing power, blurring the vision for about three months to force the use of the weaker eye.

Refractive Errors and Retinoscopy

• Myopia (Short-sightedness): Rarely causes amblyopia because the child can still see clearly at close distances, allowing the occipital lobe to receive high-resolution signals.

• Hyperopia (Long-sightedness) and Astigmatism: Can cause amblyopia, especially if the prescription is uneven between the eyes.

• Retinoscopy: A technique used to determine spectacle prescriptions in small children or non-cooperative patients who cannot read a chart. It does not require the patient's active cooperation.

Strabismus (Crossed Eyes or Squint)

• Terminology:

  • Esotropia: Eyes turning inward (convergent).

  • Exotropia: Eyes turning outward (divergent).

  • Strabismus: The medical term for crossed eyes, sometimes referred to as a "squint" by doctors.

• Accommodative Esotropia:

  • This is the most common type of crossed eyes.

  • Mechanism: Children are naturally long-sighted (hyperopic). They can use their flexible lenses to focus ($10$ diopters or more). However, the reflex for accommodation (focusing) is linked to convergence (turning the eyes in).

  • Children with prescriptions of $+3$ or higher must focus so hard to see that their eyes turn in.

  • Treatment: Corrective glasses to relax the accommodation, which in turn relaxes the convergence and straights the eyes.

• Congenital Esotropia:

  • Characterized by a very large angle of inward turn.

  • These children usually have normal spectacle numbers (no glasses needed).

  • This condition causes dense amblyopia ($6/60$ vision if untreated).

  • Treatment: Surgery to shift muscles and straighten the eyes, usually performed around age two.

• Intermittent Exotropia:

  • Very common; the eye turns out when the child is tired or daydreaming.

  • Unique Trait: This does not cause amblyopia. The vision remains $6/6$ because the brain does not suppress the image in the same way; it may even provide a wider visual field.

  • Treatment: Surgery is primarily a cosmetic decision and is often done before the child starts school to prevent teasing.

Ocular Tumors in Children

• Retinoblastoma:

  • A primary malignant tumor of the retina involving the RB gene (a tumor suppressor gene).

  • It is potentially fatal as it can spread to the brain. Inheritance is autosomal dominant.

  • Sign: Leukocoria (a white pupil reflection), often noticed in flash photography.

  • Treatment: Frequent screening under anesthesia for at-risk families; the eye may need to be removed (enucleated).

• Optic Nerve Glioma:

  • A benign tumor associated with Neurofibromatosis Type 1 (NF1).

  • Found in $10\text{--}15\%$ of children with NF1.

  • Large gliomas can push the eyeball forward (proptosis) and stretch the optic nerve, leading to blindness and the need for an ocular prosthesis.

  • Lisch Nodules: Small brown lumps on the iris; a clinical sign of NF1.

• Rhabdomyosarcoma:

  • A very rare but highly malignant and deadly tumor of the orbit (eye socket).

Retinopathy of Prematurity (ROP)

• Pathophysiology:

  • Originally discovered in babies kept in oxygen incubators. High oxygen levels trick the retina into stopping blood vessel growth (suppressing VEGF).

  • When the baby is removed from the high-oxygen environment, there is a frantic regrowth of vessels. These vessels grow into the vitreous rather than along the retina.

  • This leads to bleeding, fibrosis, and tractional retinal detachment.

• Screening and Risk Factors:

  • Target Group: Babies born under $32$ weeks gestation or weighing less than $1250\,g$.

  • Screening involves weekly or two-weekly exams in the NICU (Neonatal Intensive Care Unit).

• Treatment:

  • Laser Ablation: Peripheral retina is destroyed with a laser to stop the production of VEGF. This preserves the macula (central vision) while sacrificing some peripheral vision.

  • Anti-VEGF Injections: Medications like Avastin are used in low-resource settings where laser technology is unavailable.

Common Ophthalmic Conditions

• Blocked Tear Ducts (Nasolacrimal Duct Obstruction):

  • Caused by a persistent membrane in the duct.

  • Symptoms include a "lake" of tears and recurrent sticky eyes (conjunctivitis).

  • $90\%$ of cases resolve spontaneously by the first birthday.

  • If persistent, a blunt probe is used under general anesthetic to pop the membrane.

• Uveitis and Iritis:

  • Common in patients with juvenile arthritis.

  • Signs: Posterior synechiae (adhesions between iris and lens creating a cloverleaf pupil) and keratic precipitates (white blobs of cells on the back of the cornea).

  • Treatment: Cortisone/Prednisolone drops.

• Childhood Cataracts:

  • Can be metabolic, infectious, or congenital. Treated via surgical removal.

• Orbital and Preseptal Cellulitis:

  • Preseptal (Periorbital): Infection anterior to the orbital septum. Often from a scratch or stye; treated with oral or IV antibiotics (e.g., flucloxacillin, Keflex).

  • Orbital: Dangerous infection inside the eye socket, usually spread from the ethmoid sinus through the thin lamina papyracea.

  • Warning Signs: Red eye, inability to move the eye full-range, abscess near the optic nerve. Requires CT scan and surgical drainage.

• Meibomian Cyst vs. Stye:

  • Stye: A small focal abscess of an eyelash follicle oil gland.

  • Meibomian Cyst (Chalazion): A blockage of one of the $25$ larger oil glands in the tarsal plate cartilage. It is a pea-sized lump. If it does not resolve, it is treated by curettage (scooping out the contents) under anesthetic.