Methods

The medical records of 107 patients younger than 19 years with childhood ptosis who were residents of Olmsted County, Minnesota, and were diagnosed between January 1, 1965, and December 31, 2004, were reviewed retrospectively. The inclusion criteria for study, incidence, and demographic data of these 107 patients have been reported previously. Institutional review board approval was obtained for this study. The cases were identified using the resources of the Rochester Epidemiology Project, a medical record linkage system designed to capture data on any patient–physician encounter in Olmsted County, Minnesota. The population of this county is relatively isolated from other urban areas, and virtually all medical care is provided to its residents by Mayo Clinic, Olmsted Medical Group, and their affiliated hospitals. Patients not residing in Olmsted County at the time of their diagnosis with childhood ptosis were excluded from the study.

Amblyopia was defined as 2 lines or more difference between the 2 eyes while wearing the proper prescription or lack of central, steady, and maintained fixation in the ptotic eye with presence of normal central, steady, and maintained fixation in the nonptotic eye. Each patient diagnosed with amblyopia was managed by an ophthalmologist (pediatric, oculoplastic, or comprehensive ophthalmologist). The initial and subsequent refractions were determined in most patients after the topical administration of 1% cyclopentolate in younger patients and by a manifest refraction for older patients. All refractions were converted into their spherical equivalent. Significant refractive error was defined as the presence of anisometropia of at least a 1-diopter difference between the spherical equivalents of each eye, hyperopia of 3 diopters or more, or astigmatism of 1 diopter or more. Amblyopia as a result of strabismus was diagnosed in those with an intermittent or constant horizontal deviation of at least 10 prism diopters or a vertical deviation of at least 2 prism diopters and in whom no other cause for decreased vision was apparent. If the examination results met the above criteria for both significant refractive error and strabismus, the cause of amblyopia was listed as “combination.” Cases of amblyopia secondary to eyelid occlusion of the visual axis alone, without frontalis muscle recruitment or chin-up posture, were determined to have neither significant refractive error nor strabismus.

Results

Sixteen (14.9%) of the 107 study patients were diagnosed with amblyopia in Olmsted County, Minnesota, during the 40-year period at a median age of 4.0 years (range, 1 month to 10.2 years; Table ). Fifteen cases of amblyopia were diagnosed in patients with unilateral ptosis, whereas 1 case was diagnosed in a childhood myasthenia gravis patient with bilateral ptosis. Of the 96 patients with a congenital form of ptosis, 14 (14.6%) patients were diagnosed with amblyopia, whereas 2 (14.3%) of the 11 patients with an acquired form of ptosis had amblyopia. All cases of amblyopia occurred in patients with unilateral ptosis, with the exception of 1 patient with childhood myasthenia gravis. Further information concerning the forms of ptosis and causes of amblyopia are shown in the Table . There were no cases of amblyopia resulting from a combination of significant refractive error and strabismus. All of the patients had unilateral ptosis with the exception of the patient with childhood myasthenia gravis.

Twelve (14.8%) of the 81 patients with simple congenital ptosis were diagnosed with amblyopia, of which 7 (8.6%) cases were the result of occlusion of the visual axis from the ptotic eyelid. Of the remaining 5 (6.2%) patients with amblyopia, 3 (3.7%) patients had a significant refractive error, including anisometropia in 2 (2.5%) patients and astigmatism in 1 (1.2%) patient, whereas the remaining 2 patients had an associated exotropic deviation.

Discussion

Amblyopia occurred in 1 in 7 patients with any form of childhood ptosis in this population-based cohort diagnosed over a 40-year period. Fourteen (14.6%) of 96 cases were diagnosed in patients with a congenital form of ptosis, and amblyopia as a result of visual axis occlusion alone occurred in less than 10% of the study patients. These rates are at the low end of the range of previous non–population-based estimates (14% to 48%) of amblyopia for all forms of congenital ptosis.

Simple congenital ptosis is the most common form of childhood ptosis. Within the group of 96 patients with any congenital form of ptosis, 81 patients were diagnosed with simple congenital ptosis, of which 12 (14.8%) were diagnosed with amblyopia. This rate similarly falls at the low end of the range of previous non–population-based estimates, although it is comparable with some recent reports including those from Lin and associates (21.5% of 130 patients) and Srinagesh and associates (25.3% of 87 patients).

Amblyopia has an estimated prevalence of 3.0% to 3.2% in the general population. The rate among patients with congenital ptosis has been reported to be higher than that of the general population. Previous clinical studies have examined the causes of amblyopia in the general population and have shown that approximately one third are the result of anisometropia, one third are the result of strabismus, and the remaining third are the result of a combination of both disorders or a form of occlusive stimulus deprivation. Occlusive stimulus deprivation amblyopia seems to be the least frequent subtype based on the relative rarity of the primary causative factors, such as infantile cataract (2 to 4.5 of every 10 000 births) and childhood ptosis (7.9 per 100 000 younger than 19 years).

The precise cause of the increased prevalence of amblyopia among patients with congenital ptosis is controversial. Although some authors have argued that the occlusive effect of the ptotic eyelid(s) does not interfere with visual development, subsequent reports have demonstrated that between 1.6% and 12.3% of patients with a diagnosis of congenital ptosis will have amblyopia solely because of occlusive stimulus deprivation. In the current report, within the group of 96 congenital ptosis patients, 7 (7.3%) cases of amblyopia were the result of occlusion of the pupillary axis by the ptotic eyelid. These 7 cases of occlusion amblyopia accounted for half of all the cases (n = 14) of amblyopia diagnosed in patients with congenital ptosis. All 7 patients also were diagnosed with simple congenital ptosis, the most prevalent subtype of congenital ptosis. Two of the 7 patients were recorded as having occlusion as the cause of ptosis, despite not having their refractive error measured. Each child was seen only once in the clinic. It is possible that each child had significant refractive error, although both patients were reported in the charts as having severe ptosis in the absence of a compensatory head tilt. One patient had a margin reflex distance of 0.5 mm in the ptotic eyelid, whereas the other patient had complete occlusion of the pupil because of the ptotic eyelid. Of the remaining 5 cases of amblyopia in patients with simple congenital ptosis, 3 were the result of significant refractive error and 2 were the result of strabismus.

Occlusion of the visual axis was the leading cause of amblyopia in patients with congenital ptosis in this report. This finding is in contrast to large referral-based retrospective studies of congenital ptosis in which the leading causes of amblyopia were strabismus or significant refractive error. In a study by Harrad and associates of 216 cases of simple congenital ptosis referred for oculoplastics evaluation, amblyopia developed in 37 (17%) patients, 20 (9.3%) cases of which were the result of strabismus and 5 (2.3%) cases of which were the result of stimulus deprivation of the visual axis from the ptotic eyelid. Likewise, in a study by Dray and Leibovitch of 130 patients who underwent surgical correction of ptosis, 30 (23%) patients were diagnosed with amblyopia, 16 (12.3%) cases of which were the result of strabismus and 9 (6.9%) cases of which were the result of occlusion. More recently, in a study of 92 patients with congenital ptosis, 22 (23.9%) of patients were diagnosed with amblyopia, with almost every case occurring in the context of coexisting anisometropia or strabismus. The same authors note that congenital ptosis patients are at risk of developing anisometropic and strabismic amblyopia even if not originally detected, and routine monitoring that includes regular cycloplegic refractions is recommended.

All cases of amblyopia occurred in patients with unilateral ptosis, with the exception of 1 patient with childhood myasthenia gravis. The child had bilateral ptosis with symmetric palpebral fissure heights (4 mm) and 30 prism diopters of exotropia. Because of the significant degree of exotropia, strabismus was recorded as the cause of amblyopia, although it has been speculated elsewhere that ptosis may be the initial cause of amblyopia that leads to strabismus.

Three of the 96 patients with congenital ptosis in the current study were diagnosed with blepharophimosis-ptosis-epicanthus inversus syndrome. Although amblyopia was not noted in this small group, the rate of amblyopia in patients with blepharophimosis-ptosis-epicanthus inversus syndrome has been reported previously to be as high as 56.4%, and early surgery is recommended. Two of the 3 patients underwent surgery at a mean age of 54.4 months (range, 44.4 to 64.4 months), whereas 1 patient diagnosed with blepharophimosis-ptosis-epicanthus inversus syndrome at birth with mild unilateral ptosis had no signs of amblyopia by age 14 years.

There are several limitations to the findings in this study. Although relatively isolated, some residents of Olmsted County with ptosis may have sought care outside the region, potentially underestimating the incidence of amblyopia in this population. Further, the presence of a compensatory head tilt has been correlated with a high incidence of amblyopia in patients with congenital ptosis in the absence of significant anisometropia and strabismus. In the current study, there were recording inconsistencies regarding the presence or absence of this pertinent finding during the 40-year period. Because of the young age of patients in the study (median age at diagnosis for simple congenital ptosis, 1.3 years; range, 32 days to 16.7 years), we were unable to assess stereoacuity to identify amblyopia indirectly, thus potentially underestimating the overall prevalence of amblyopia. Finally, the ability to generalize the findings of the current report is limited by the demographics of Olmsted County, a relatively homogeneous semiurban white population.

The findings of this study provide population-based prevalence rates for amblyopia in childhood ptosis diagnosed over a 40-year period. Amblyopia affected approximately 1 in 7 patients diagnosed with childhood ptosis, of which nearly half solely were the result of eyelid occlusion of the visual axis.