Cyclic heterotropia is a rare and poorly understood form of strabismus that is characterized by alternating intervals of manifest strabismus and orthotropia. A 48-hour cycle is most common, with a 24-hour interval of constant esotropia followed by a 24-hour interval of normal binocular vision. Also called clock-mechanism, alternate-day, or circadian strabismus, other time cycles such as 72 and 96 hours may occur. Although technically an intermittent form of strabismus, cyclic strabismus differs from typical intermittent strabismus in that a significant heterophoria is not present during the orthotropic phase. Likewise, there are no defects of binocular vision or ocular motility at this time. Furthermore, the strabismic phase is not associated with mechanisms that are often responsible for latent or intermittent deviations becoming manifest, such as fatigue, accommodation, or the disruption of sensory fusion.1
The direction of strabismus is usually esotropic; most cases occur in children and are idiopathic.2–4 Adult-onset cyclic strabismus is much less frequent than the childhood variety and may be associated with acquired unilateral vision loss.5–8 The etiology and pathogenesis of this rare form of strabismus are debated on.2–8 The cyclic pattern may persist for months or years, with the expectation that it will become constant eventually. Treatment is often surgical and based on the magnitude of the tropia measured on the strabismic days.
This report describes the diagnosis and treatment of an adult patient who developed cyclic esotropia after undergoing a scleral buckling procedure for a retinal detachment. In an effort to gain insight into the etiology and pathogenesis of the condition, the patient underwent extensive testing. We provide the results of this testing in addition to a review of other case reports of adult-onset cyclic strabismus.
A 37-year-old man presented to our department with the diagnosis of cyclic esotropia of the left eye (LE). The patient had undergone successful scleral encirclement (using a 2.5-mm-wide buckle sutured 14 mm from the limbus) for a retinal detachment in the LE approximately 2 years previously. He reported that he developed the cyclic esotropia 2 months after the operation. This was confirmed by ophthalmologic evaluations carried out on several occasions. We verified the diagnosis of a cyclic esotropia and identified a periodicity of approximately 48 hours (24 hours of orthotropia followed by 24 hours of constant esotropia) (Fig. 1) during three different hospitalizations of at least 3 days each. The patient denied diplopia when the deviation was manifest presumably because of LE anisometropic amblyopia. He also denied that he experienced any alterations in his behavior, mood, sleeping habits, temperature, micturition frequency, libido, appetite, and sweating. Although his medical history was positive for cocaine addiction, he reported that the times of his drug use did not coincide to the presence or absence of his esotropia. On examination, the patient’s distance visual acuity was 20/20 with a plano correction in the right eye (RE) and 20/60 in the LE. Cycloplegic refraction was +0.50 diopters (D) in the RE and −18.00 D in the LE, with best-corrected visual acuity of 20/60 in the presumably anisometropic amblyopic LE.
On a “strabismic” day, a comitant constant left esotropia of 30 to 40 prism D with an associated left hypotropia of 4 prism D was present at distance and near. Versions were full, and head posture was normal; LE suppression was demonstrated on both the Worth 4-Dot and the Bagolini striated glass tests. Gross stereopsis of 3000 seconds was reported on the Titmus stereofly test. On a “straight” day, the patient’s eyes were orthotropic (nonstrabismic). Suppression was present on the Worth 4-Dot and Bagolini striated lenses, and stereopsis was still reduced (3000 seconds) despite the absence of a manifest esotropia.
The fundus evaluation was unremarkable in the RE; myopic chorioretinopathy with signs of a previous scleral buckle was apparent in the LE. The cyclicity of the esotropia remained unchanged during the follow-up period. The neurological assessment (interview, level of consciousness, pupillary assessment, cranial nerve testing, motor function, sensory function, tone, cognitive function, fatigue, and edrophonium testing) was unremarkable both on straight and strabismic days. Magnetic resonance imaging (MRI) of the brain failed to reveal any abnormality. Magnetic resonance imaging of the orbits showed an irregular contour of the posterior pole of the left eye caused by the scleral buckling. Angio-MRI showed hypoplasia of the P1 segment of the left posterior cerebral artery and ectasia of the left posterior communicant artery, without apparent compression of the III cranial nerve. Single-fiber electromyography (sfEMG) recorded from the left orbicularis muscle on the strabismic day did not show any abnormalities. Electroencephalography (EEG) was within normal limits both in the presence and absence of the esotropia.
Neither the periodicity nor the magnitude of the esotropia changed with a contact lens correction. After neurological and systemic medical conditions were ruled out, the patient underwent strabismus surgery for the magnitude of esotropia present on the strabismic day; surgery consisted of a recession of the medial rectus (5 mm) and resection of the lateral rectus (7 mm) of the LE. At surgery, it was noted that the ocular muscles did not show anatomical anomalies or adhesions resulting from the scleral buckle procedure.
At a 2-month postsurgical follow-up examination, it was determined that the cyclic esotropia was no longer present. Cover testing revealed a left esotropia of 8Δ. The ocular motility assessment revealed a mild elevation deficiency of the LE caused by the scleral buckling. No binocularity was demonstrated on the Worth 4-Dot test nor the Bagolini striated lens test. Gross stereopsis was demonstrated at the stereofly test. The patient was followed for 2 years, and there were no significant clinical changes.
We described an adult patient with acquired cyclic esotropia. In reviewing the literature, we found 16 case reports of adult-onset cyclic heterotropia (Table 1). The age of onset ranged from 21 to 67 years (median, 34 years), and a cyclic pattern of 48 hours was the most common. Esotropia was reported in eight cases, exotropia in four cases, exotropia associated with vertical deviations in two cases, and vertical deviations in two cases. When not interrupted by strabismus surgery, the condition persisted for as long as 7 years in some cases.
Adult-onset cyclic strabismus is usually associated with decreased visual acuity in one or both eyes because of retinal detachment, optic atrophy, traumatic aphakia, high myopia, or retinitis pigmentosa. Other related diseases or conditions that have been reported to precede the onset of acquired cyclic strabismus include surgery (eg, scleral buckling, craniofacial surgery, laser surgery), brachytherapy, and Graves disease (Table 1). When present in childhood, cyclic strabismus is typically spontaneous with an average age of onset of 3 to 4 years. During the orthotropic phase, children generally have a fully controlled heterophoria at all distances, with good fusion and stereopsis. In contrast, during the “squinting” period, generally, there is a constant manifest strabismus for all distances, with no demonstrable fusion or stereopsis. The cyclic nature of the strabismus may last from a few weeks to several years, after which the cycle3,5,6,8 becomes irregular and finally the deviation becomes constant. Surgery based on the magnitude of the heterotropia as measured on a strabismic day is often successful in permanently curing the condition (Table 1). A bilateral medial rectus recession, alone or in conjunction with a resection of the lateral rectus in the affected eye, is typically the procedure performed for cyclic esotropia. Botulinum toxin injections may or may not be beneficial.17
The pathogenesis of cyclic heterotropia is unknown. Some authors postulate that there is a dysfunction of the oculomotor nuclei and/or nerves.4,5 However, saccadic eye movements on both strabismic and straight days show normal saccadic velocity-amplitude relationships, suggesting that oculomotor dysfunction does not play a role in the pathogenesis of this condition.4 Other authors9 have hypothesized that the etiology is a central nervous system adaptation to a peripheral visual defect; however, cyclic strabismus also occurs in patients with normal visual acuity5,10 and can persist even when visual acuity is restored.4
Helveston3 suggested that even though patients with cyclic strabismus have normal fusion 50% of the time, they should be considered basically strabismic. His hypothesis is supported by reports of successful surgery based on the full amount of heterotropia on the squinting day and by the frequent presence of visual impairment in one eye (Table 1). However, a case of cyclic exotropia developing after the surgical correction of an acquired cyclic esotropia has been reported.11 Richter2 suggested that the trigger causing cyclic esotropia could be located in a number of anatomical sites, including within the oculomotor nuclei or their interconnections, or within the sensory pathways that control the oculomotor nuclei.2,6 Moreover, Richter2 postulated that the strabismus may be the only outward manifestation of a more basic disturbance, and that cycles such as those seen with this entity are not unique in medicine. Although many other periodic physiological phenomena (eg, body temperature, blood pressure, metabolism, hormone production) and behavioral rhythms (eg, sleep-wake, feeding) have been previously associated with this cyclic condition,6–12 concomitant variations of behavioral and biological parameters have not been demonstrated in cyclic strabismus,13 and an interaction with oculomotor nuclei or superior colliculi has not been yet established.
For this patient, we performed a complete neurological and ophthalmologic examination, an MRI of the brain and orbits, an angio-MRI, an sfEMG, and an EEG; none of these procedures uncovered an apparent cause for the cyclic strabismus. The fatigue and edrophonium tests and sfEMG ruled out myasthenia gravis. The angio-MRI findings uncovered a congenital malformation; however, not one that would not affect ocular motility or the cyclicity of strabismus. These negative findings are in accord with those from other reports of patients with cyclic esotropia (Table 1), as well as with other MRI,7,9,10 computed tomography,5 and EEG14 studies.
In conclusion, the etiology and pathogenesis of cyclic esotropia remains unknown. Surgical treatment based on the full magnitude of the strabismus on the squinting day is typically effective in eliminating the strabismus. When presented with a patient with intermittent strabismus that is present some days and not others, the eye care provider should consider the diagnosis of cyclic strabismus.
Department of Medicine and Health Sciences
University of Molise
via F. de Sanctis s.n.c.
None of the authors have conflicts of interest with the submission, and no financial support was received for this submission.
Received January 20, 2012; accepted November 14, 2012.
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