Congenital stationary night blindness (CSNB) is a condition that is suspected in young patients with nonprogressive reduced vision, and nyctalopia with a normal appearing fundus. Visual acuity is variable, but usually ranges from 20/30 to 20/200, although worse vision can be found. Refraction is usually myopic, although in some cases it can be hyperopic, with some genotype–phenotype correlation. For example, NYX mutations are usually associated with high myopia. Other findings include nystagmus and strabismus. Color vision is usually normal though some genetic forms may have mild color defects. Slit-lamp and fundus examination are within normal limits, although there can be tigroid fundus or myopic degeneration in the presence of high myopia. The condition has complete penetrance with variable expressivity.

CSNB can be classified as complete or incomplete. Clinically, the most striking feature is the constant finding of severe nyctalopia in the complete form, as compared to the incomplete form, where night blindness might not always be present. CSNB can also be classified based on full-field electroretinogram (ff-ERG) features. The Schubert–Bornschein pattern is characterized by a b-wave that is smaller than the a-wave (electronegative ERG), whereas the Riggs subtype shows proportionally reduced a- and b-waves.

27.2 Molecular Genetics

The molecular causes of CSNB are described in ▶ Table 27.1. Approximately 55% of patients with typical X-linked CSNB have mutations in CACNA1F, whereas the other 45% have mutations in NYX.

There are allelic disorders associated with some of these genes. For example, CACNA1F mutations may also result in Åland Island eye disease (AIED), an X-linked recessive (XLr) condition characterized by decreased vision, nystagmus, red–green color deficiency, fundus hypopigmentation, progressive myopia, iris transillumination, and abnormal dark adaptation with abnormal photopic and scotopic responses on ERG. CACNA1F mutations have also been associated with CORD (cone–rod dystrophy) and optic atrophy.

27.3 Differential Diagnosis

27.3.1 Blue Cone Monochromacy (OMIM 303700)

This is an XLr disorder characterized by poor vision and nystagmus. It can be distinguished from CSNB because of abnormal color vision, a severely affected photopic ERG with minimally altered rod function, and, in some cases, macular atrophy. It is associated with mutations in OPN1LW and OPN1MW, the red–green photopigment genes.

27.3.2 Idiopathic Congenital Motor Nystagmus

Patients have a normal ERG and no nyctalopia.

27.3.3 Ocular and Oculocutaneous Albinism (OMIM 300500).

Although these individuals have nystagmus, clinical findings of iris transillumination, foveal hypoplasia, fundus hypopigmentation, and small gray optic nerves as well as the cutaneous and hair hypopigmentation when present allow for distinction from CSNB although there may be some confusion with AIED. The ff-ERG in albinism is supranormal .

27.3.4 Oguchi Disease

Oguchi disease is a rare disorder, sometimes classified as a form of CSNB. It is more common in the Japanese population. It is caused by mutations in either the SAG or the GRK1 gene. These patients characteristically show the Mizuo phenomenon, which is a golden-brown fundus with a yellow–gray metallic sheen in the light-adapted state that normalizes after prolonged dark adaptation.

27.4 Uncommon Manifestations

Rarely, individuals with CSNB may have paradoxical pupillary response, which is a miotic effect when lights are turned off or dilations response to light.

27.5 Clinical Testing

27.5.1 Full-Field Electroretinogram

Patients with CSNB have reduced scotopic b-wave amplitudes in response to bright flashes after dark adaptation, resulting in a larger amplitude a-wave compared to b-wave (electronegative b-wave; ▶ Fig. 27.1). In cases of complete CSNB, the b-wave is severely affected or absent. Incomplete CSNB shows a b-wave that is reduced but it can be measured. Obligate female carriers of XLr CSNB usually have normal ff-ERG, although reduced oscillatory potentials or reduced photopic b-wave have been described. Mild cases of CSNB may be missed if ERG is not performed.

Stay updated, free articles. Join our Telegram channel

Join