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An 11-year-old boy was referred to the inherited eye disease clinic at the University of Iowa for progressive vision loss OS greater than OD over the past year. The patient denied any peripheral visual field loss, nyctalopia, or photophobia. Multiple family members of both sexes had molecularly confirmed autosomal dominant retinitis pigmentosa (RP) caused by a heterozygous Val87Asp mutation in the rhodopsin gene (Fig. 1A).
Fig. 1(A) Pedigree, demonstrating autosomal dominant inheritance of retinitis pigmentosa (RP) in the proband’s family. Shaded boxes represent individuals affected with RP. Arrow indicates proband. (B) Colour fundus photographs of the right and left eye, significant for temporal pallor of both optic nerve heads. (C) Goldmann kinetic perimetry of the left and right eye, displaying the left homonymous visual field defects in both eyes. (D) Montage colour fundus photographs of the right and left eye of the patient’s father, characteristic of RP. (E) Goldmann kinetic perimetry of the patient’s father’s left and right eye, displaying symmetric (nonhomonymous) visual field defects from RP.
His best corrected visual acuity was 20/63 OD and 20/200 OS. No relative afferent pupillary defect (RAPD) was detected at this initial visit, although a small RAPD OS was identified at a subsequent clinic visit. Colour vision was not assessed. Slit-lamp examination of the anterior segment revealed no Lisch nodules. Dilated funduscopic examination revealed moderate pallor of the optic nerve OU (Fig. 1B). The retina appeared normal OU. There was no vascular attenuation or bone-spicule-like pigmentation. Goldmann kinetic perimetry revealed a relatively incongruous left homonymous quadrantanopsia (Fig. 1C).
Although his strong family history of RP was suggestive and led to his referral to the inherited eye disease clinic, the patient’s fundus examination findings and visual fields were different from those of other family members, including those seen in Figures 1D and 1E. These discrepancies prompted further work-up, including magnetic resonance imaging (MRI) of the brain with and without contrast. This revealed increased signal at the optic chiasm and bilateral proximal optic nerves, which extended posteriorly into the optic tracts, lateral geniculate bodies, and the right Meyer’s loop (Fig. 2). This lesion was felt to be consistent with an optic pathway glioma, and the patient was referred to paediatric oncology and neurosurgery for further evaluation and management. He was also referred to the paediatric genetics clinic for evaluation of possible neurofibromatosis type-1 (NF1) given his optic glioma. The geneticists found that he did not meet diagnostic criteria for NF1, as no other clinical features of this condition were identified. As such, and because optic glioma has no known association with RP, the oncologists felt this was a spontaneous neoplasm.
Fig. 2Axial FLAIR (fluid attenuation inversion recovery) (A) and T2 sequence (B) MRI images of the brain demonstrating optic pathway glioma of optic tracts and lateral geniculate nuclei (white arrows).
RP is a broad term used to refer to a heterogeneous collection of diseases caused by mutations in genes that encode proteins expressed in photoreceptors, the retinal pigment epithelium, and, in some cases, the choroid. RP may be inherited in an autosomal dominant, autosomal recessive, X-linked, or oligogenic manner.
Rod photoreceptor cells are often affected first, and the resulting low-grade inflammation, increased oxygen concentration, and loss of rod-derived cone viability factors lead to secondary loss of the cone photoreceptors.
Patients typically present with nyctalopia and symmetrical visual field defects (classically, a ring scotoma with preserved central vision and far peripheral vision).
Later examination findings classically include attenuation of the retinal arterioles, waxy pallor of the optic nerve head, bone-spicule-like pigmentation in the peripheral retina, and posterior subcapsular cataract.
Visual field defects caused by inherited eye diseases such as RP are characteristically symmetrical, and the visual field of one eye is the mirror image of the other.
By contrast, homonymous defects are usually caused by lesions posterior to the optic chiasm.
The presence of new visual field defects in a patient with a strong family history of molecularly confirmed autosomal dominant RP could easily lead one to suspect the disease, especially because the fundus may appear normal in early RP. The key clinical finding in this case was the homonymous visual field defect. If one had a high index of suspicion, such a defect could be detected by careful confrontation or static perimetry of the central 24 or 30 degrees. However, in patients suspected to have RP, the use of a Goldmann perimeter allows the clinician to evaluate the retinal periphery, which can make it easier to recognize ring scotomas. For this patient, the systematic evaluation of the entire visual field with Goldmann perimetry made the homonymous nature of the field defect very apparent. Had the perimetry been less suggestive and not immediately prompted the MRI, electrophysiologic testing such as electroretinography and visual evoked potential could have been helpful in correctly identifying the true cause of the patient’s vision loss.
Disclosure
The authors have no proprietary or commercial interest in any materials discussed in this article.
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