Article Text

Keratoconus associated with chromosome 13 ring abnormality
  1. C J HEAVEN
  1. Royal Albert Edward Infirmary, Wigan WN1 2NN
  2. St Mary's Hospital, Manchester M13 0JH, UK
  1. F LALLOO,
  2. E McHALE
  1. Royal Albert Edward Infirmary, Wigan WN1 2NN
  2. St Mary's Hospital, Manchester M13 0JH, UK
  1. Mr Heaven

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Editor,—We wish to report a patient with a ring abnormality of chromosome 13 who developed keratoconus. As far as we are aware such an association has not previously been reported. This case may provide some further insight into the presumed genetic basis of keratoconus.

CASE REPORT

A 19 year old woman was referred by her optometrist because of impaired distance vision present for several months which could not be satisfactorily corrected with spectacles. There was no history of familial ocular problems. The patient did not suffer from atopy.

Unaided visions were: right 6/9 part, left 6/24. The optimal spectacle correction and visual acuities were: right +1.00/−1.00 × 90 = 6/7.5, left +1.50/−3.00 × 110 = 6/18. With a pinhole the visual acuities improved to: right 6/5 part, left 6/6. The retinoscopy reflex in the left eye showed an “oil droplet” appearance with distortions, and slit lamp assessment of this eye revealed a definite corneal cone. Conventional keratometry measurements were: right 7.08 mm × 123/7.05 mm × 33 (47.62/47.87D), left 6.99 mm × 150/6.67 mm × 60 (48.25/50.62D). Videokeratography (TMS) demonstrated corneal topographies typical of keratoconus, more marked on the left side, with the cone apices inferior to the corneal centre. The condition was managed by the fitting of rigid gas permeable contact lenses.

The patient was born at full term weighing 2.2 kg. There was delay in achieving her early childhood developmental milestones and, as a result, chromosomal investigations were performed at the age of 8 years. These revealed a chromosome 13 ring abnormality (46,XX,r(13)). She showed mild learning difficulties, particularly of her reading and writing skills. She has, however, remained within mainstream education. During childhood she suffered from persistent middle ear infections and required a hearing aid. Puberty was delayed with menarche occurring at age 15 years 2 months. At the age of 19 years, she has a height on the 9th centile, weight on the 75th centile, and occipitofrontal circumference on the 10th centile. She has mild dysmorphic features of epicanthic folds, depressed nasal bridge, and overfolded helices. She also has underdevelopment of the right breast and a hypoplastic toe nail on the right fifth toe.

Further cytogenetic analysis in 1999 demonstrated that the long arm subtelomeres are absent from the ring. The karyotype is, therefore, 46,XX,r (13),(p11q34).ish r(13) (p11q34)(85A10-).

COMMENT

A number of cases of ring 13 have been reported with the same breakpoints of p11 and q34.1 While microcephaly, mild dysmorphism, mental retardation, and foot anomalies have been described, there have been no reports of an association with keratoconus.

The aetiology of keratoconus remains unclear. The thinning of the corneal stroma and the resulting ectasia are presumed to be due to an abnormality of the corneal collagen or extracellular matrix. Although many cases occur sporadically, twin and family studies suggest that there is, at least in part, a genetic basis to the condition.2 Candidate genes include those that code for the components of collagen. In the case described above, the ring (13) has occurred following a deletion of the end of the long (q) arm of the chromosome. The association with keratoconus raises the possibility that a deleted gene may have a role in the maintenance of corneal integrity. It is interesting that the genes COL4A1 and COL4A2 which code for the α1 and α2 chains, respectively, of collagen IV are located on the long arm of chromosome 13 (13q33-q34).3

References

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