10.3.1 How OCT Can be Used during Examination Under Anesthesia

For children too young to cooperate with tabletop OCT, handheld or overhead-mounted OCT imaging can be completed during examination under anesthesia or during anesthesia for ocular surgery. We generally perform this step after intraocular pressure (IOP) has been measured and the child’s airway stabilized with either a laryngeal mask or endotracheal tube (as appropriate for the situation), but before B-scan ultrasound or photographic imaging, as the gel applied for these modalities can be hard to clear and may degrade the subsequent OCT images. We use an eyelid speculum during imaging. Anesthetic depth may need to be increased if the eyes are rotated upward or downward, as midline eyes are needed for optimal OCT imaging. Frequent topical lubricant drops are used as the ocular surface can dry quickly, which will degrade the OCT image. Knowledge of the patient’s refractive error helps to focus the camera. Lastly, the imager must be adept at monitoring the proximity of the OCT device to the patient’s cornea while simultaneously adjusting the direction and focus of the unit to capture the area of interest. In our institution, for clinical care we use the handheld Envisu (formally Bioptigen) OCT and for research we use the FLEX OCT.

We have found OCT performed during anesthesia to be helpful for infants, uncooperative children, or children with intellectual disability, as well as patients with poor visual fixation due to poor vision or nystagmus (which resolves under general anesthesia) (Fig. 10‑1).

10.3.2 Tabletop OCT for Children

Children over the age of 5 years are generally able to cooperate with tabletop OCT, though it is reasonable to attempt imaging on children as young as 3 years old, provided you have an experienced photographer, visual fixation is intact with minimal nystagmus, and there is a clear visual axis. To perform OCT imaging the child is seated in front of the imaging device. Depending on the child’s height it might be easier for them to kneel on the chair or stand with their head resting on the chin rest, similarly to how they would be examined at the slit lamp. Smaller children can sit on a parent’s or guardian’s lap. For glaucoma evaluation, we like to have the following protocols: a peripapillary RNFL scan (or circle scan), a 61-line volume scan over macula, as well as attempting a Glaucoma Module Premium Edition (GMPE, specific to Spectralis). Table 10‑1 summarizes practical strategies for successful image acquisition.

10.3.3 Structural Considerations for Image Acquisition

Any type of media opacity can make OCT imaging difficult. Corneal scarring, clouding or Haab striae, especially if in the central cornea, can preclude imaging. Anterior segment dysgenesis, iris irregularities, and cataract can also make imaging challenging (Fig. 10‑2). In glaucoma following cataract surgery, it can be very challenging to focus the camera in aphakic eyes with large refractive errors. We often find that the en-face view and the OCT image cannot be focused at the same time. In severe glaucoma, the long axial length and the high myopia that are often present can also make OCT device focus challenging. It is critical in these cases to review the imaging quality and assess for any segmentation failures before interpreting the study.

10.4.1 Optical and Anatomic Considerations for Image Acquisition and Interpretation

There are several anatomic changes that impact refraction and optics in very young eyes and must be considered when obtaining and interpreting OCT images. The greatest increase in axial length occurs after birth until about age 2 to 3 years, with an increase of about 2 mm every year. After age 5 or 6 years, the axial length increases by only about 1 mm more to reach its adult length. ³ There is minimal increase in axial length after age 10 to 15 years, in normal eyes. Corneal curvature also undergoes the largest change in the first few months of life. The neonatal cornea is steeper, with a mean keratometry value of 51.2 diopters at birth and 44.9 diopters by age 1 to 2 years. There is no significant change in corneal curvature after the age of 6 months. ³ Refraction and astigmatism also vary with age.

Due to the shorter axial length of the infant eye, the OCT image of the infant eye is magnified compared to that of an adult. In other words, there is an inverse relationship between axial length and retinal image size. ⁴ To summarize, due to these optical differences, the scan length in an infant is proportionally less (depending on age) than the scan length that is presumed for an adult. Work from Maldonado et al suggests age-specific imaging protocols to account for these optical changes ^{5 , 6} when using the handheld Bioptigen Envisu C-class. For the Spectralis FLEX, only the corneal curvature measurement can be adjusted.

There are small, but reproducible OCT measurement changes that occur with changing eye dimensions. Cross-sectional studies have suggested that the normative OCT values for pRNFL vary with axial length for white children, with the average pRNFL thickness decreasing by about 2.6 μm with every 1-mm increase in axial length. ⁷ Similarly, in very young children (ages 0 to 5 years) mean pRNFL was inversely related to axial length. ⁸ It is possible that the inverse relationship between axial length and pRNFL thickness may be, at least in part, be due to a magnification artifact of OCT imaging, the result of a relative increase in the diameter of the projected circle scan and smaller size of the optic disc image with increasing axial length. ^{4 , 9 , 10} Practically, however, a 1.0 mm change in axial length (less than what is expected as a change in axial length from age 3 to 18 years) engenders a 2 to 3 μm change in pRNFL, which is less than the inter-test reproducibility of the OCT machine.

The relationship between age and pRNFL thickness seems to vary by study, with some studies suggesting a positive linear correlation of the average RNFL thickness with age ¹¹ and others finding no relationship, ⁷ especially after controlling for variables such as refractive error. ^{8 , 12} Age, especially in very young children (less than 3 years), does seem to correlate with retinal layer thickness, especially near the fovea. ⁸ Specifically, the ganglion cell layer (GCL), inner nuclear layer (INL), and ganglion cell complex (GCC, which is the combined volumes of the macular nerve fiber layer, the GCL, and the inner plexiform layer) volumes showed an inverse relationship with age, while the photoreceptor layer’s volume (calculated by combining the volume of layers from the external limiting membrane to the Bruch’s membrane), increased logarithmically until about 2 years of age. ⁸ This corresponds histologically to the maturation of the foveal pit and increasing density of cones in the postnatal period. The published mean central macular thickness for very young children (0–5 years) is lower than that of older children (5–16 years) and should be taken into consideration when considering macular changes related to glaucoma. ⁸

In summary, we can safely assume that the average pRNFL should be reproducible for the normal eye of a healthy child over time. The pRNFL quadrants or clock hours are expected to change over time due to changes in the tilt of the optic nerve in the growing eye.

10.4.2 Comparing to a Normative Database

None of the available OCT machines and software has integrated a normalized pediatric database. Instead, these devices use adult normative data to set the measurement ranges and parameters. Since pRNFL measurements are expected to be thicker in children than in adults, ¹³ any results obtained must be interpreted against published normative OCT values for children. Normative pRNFL and macula values for very young children (ages 0 to 5 years) was recently published using the overhead-mounted Spectralis FLEX OCT device. ⁸ Across several studies the average pRNFL for older children was similar, though the measurements obtained by Cirrus, Spectralis, and Stratus OCT (OCT3, Carl Zeiss Meditec, Dublin, CA) devices are not interchangeable (Table 10‑1). ^{7 , 8 , 11 , 13 , 14 , 15 , 16 , 17} There are likely racial variations in pRNFL thickness, with one study showing that the pRNFL in black children is thicker than in white children of a similar age. ⁷ This difference, however, is smaller than the magnitude of thinning expected in pRNFL due to disease (such as glaucoma), especially when looking at the average values.

Table 10‑2 summarizes mean pRNFL thickness across multiple studies of normal eyes of children using SD-OCT.