138 2 Hereditary Retinal and Allied Diseases
Blue cone monochromacy, a rare color vision disorder, was first described by Blackwell in 1957 [1]. It shares many characteristics with rod monochromacy. Blue cone monochromats differ from rod monochromats in their inheri- tance pattern; blue cone monochromats have an X-linked recessive pattern, whereas rod monochromats have an autosomal recessive inheritance. The molecular genetic study indi- cated that mutations exist in the red and green opsin in blue cone monochromats [2].
A pedigree of a Japanese family with blue cone monochromacy with three affected members and one female carrier is shown in Fig. 2.110 [3]. The fundus of one of these patients is shown in Fig. 2.111, which is essen- tially normal, although in the late stage some
atrophic changes may develop in the macula.
The visual acuity is approximately 0.2–0.3, which is slightly better than that of the com- plete form of rod monochromacy. Unlike rod monochromats, the blue cone function is selec- tively preserved. The results of the Farnsworth dichotomous Panel D-15 test shows several crossing lines perpendicular to the tritan axis (Fig. 2.112).
Examination with a Nagel anomaloscope yielded results similar to those seen with com- plete rod monochromatism, although most of the values are below those measured in a group of patients with complete rod monochromatism.
The spectral sensitivity curve determined by increment thresholds of 1° steps showed a narrow curve with a peak around 440 nm (Fig. 2.113).
2.15 Blue Cone Monochromacy
Fig. 2.110. Pedigree of family with blue cone monochromacy. Cross mark, examined; arrow, proband;
black squares, affected members.
(From Terasaki and Miyake [3], with permission)
Fig. 2.111. Fundus of a 10-year-old boy with blue cone monochromacy (case 1). (From Terasaki and Miyake [3], with permission)
RDI2(104%) 9/9/05 7:54 PM Page 138
2.15 Blue Cone Monochromacy 139
The full-field ERGs recorded from members of this family were nearly normal for the rod ERGs, with absence of the photopic ERG in the three affected members (Fig. 2.114). Although the blue cone ERG is present in blue cone monochromats (Fig. 2.115), the amplitude of the blue cone ERG is too small to be detected in
the full-field cone ERGs, and the implicit time is too long to follow 30-Hz flicker stimuli.
The diagnosis of this disorder is based on the presence of severely affected color vision with preserved blue function, nearly unrecord- able photopic ERGs, and a family pedigree com- patible with an X-linked inheritance pattern.
Fig. 2.112. Results of Farnsworth dichotomous Panel D-15 test from two patients with blue cone monochromacy. Several crossing lines were perpendicular to the tritan axis. (From Teraski and Miyake [3], with permission)
Fig. 2.113. Spectral sensitivity curves determined by increment thresholds on a white background for case 1 (circles) and two age- matched normal subjects (triangles). (From Terasaki and Miyake [3], with permission)
RDI2(104%) 9/9/05 7:54 PM Page 139
140 2 Hereditary Retinal and Allied Diseases
References
1. Blackwell HR, Blackwell OM (1957) “Blue mono- cone monochromacy”: a new color vision defect. J Opt Soc Am 47:338
2. Nathans J, Davenport CM, Maumenee IH, Lewis RA, Hijtmancik JF, Litt M, et al. (1989) Molecular
genetics of human blue cone monochromacy.
Science 245:831–838
3. Terasaki H, Miyake Y (1992) Japanese family with blue cone monochromatism. Jpn J Ophthalmol 36:
132–141
Fig. 2.114. Full-field ERGs recorded from a family with blue cone monochromacy (carrier mother, two sons, and mother’s brother), showing normal rod components and nearly absent cone components.
(From Terasaki and Miyake [3], with permission)
Fig. 2.115. Full-field ERGs elicited by photopically matched red and blue stimuli recorded from a normal subject, a rod monochromat, and two blue cone monochromats (cases 1 and 3). The patient with rod monochromacy shows absent red and blue responses, but the patients with blue cone mono- chromacy show small responses elicited by only the blue stimu- lus. The ERGs are characterized by an absent a-wave and a small b-wave with delayed implicit time. These are the properties of blue (S)-cone ERGs
RDI2(104%) 9/9/05 7:54 PM Page 140
