Purpose To determine the incidence of isolated, presumed fourth nerve palsy in a defined population, and to report the frequency of each cause. Design Retrospective, population-based case-series. Methods A population-based database was used to identify all cases of isolated fourth nerve palsy in Olmsted County, MN, USA diagnosed over a 15-year period (January 1, 1978 to December 31, 1992). The most likely etiology was determined by review of the entire medical record by two ophthalmologists. A priori definitions were applied for assigning cause. The incidence of fourth nerve palsy and the frequency of each etiology were calculated. Decade of life at presentation was recorded. Results 73 patients, (74 episodes, 70 [95%] unilateral) were identified. Mean age at presentation was 41.8 (range 3.3 to 81.6) years. The age- and sex-adjusted annual incidence rate was 5.73 per 100,000 per year (95% Confidence Interval [CI] 4.31–7.14). The most common etiology was presumed congenital (49%) followed by hypertension (18%) and trauma (18%). One patient (1%) had fourth nerve palsy due to a known intracranial neoplasm. For 3 patients (4%) the cause of fourth nerve palsy was undetermined. The most common decade of presentation overall was the fourth decade, including for presumed congenital cases. Conclusions In this population-based study, the majority of isolated fourth nerve palsies were presumed congenital, even though they presented throughout adulthood. Other etiologies such as hypertension and trauma were less frequent, and in no case was an isolated fourth nerve palsy the presenting sign of an intracranial tumor.

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