Convergence Weakness with Intact Medial Rectus Function in Proptosis due to Orbital Tumors
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.293479
Source of Support: None, Conflict of Interest: None
Keywords: Convergence, convergence insufficiency, convergence weakness, orbital tumor, proptosis
The pathways regulating ocular convergence are less understood., Convergence weakness is considered as a central phenomenon and usually a feature of dorsal midbrain lesions., On the other hand, convergence insufficiency (CI) that clinically manifests as eye strain after near work is possibly due to insufficient accommodation. The etiopathological factors causing CI is multiple and less understood.
Orbital tumors usually present with proptosis. The lesions may involve muscles causing impaired ocular movements. However, in the presence of intact movements with proptosis, convergence weakness is unexpected. The literature addressing convergence weakness/insufficiency in proptosis due to orbital tumors is sparse. In the present report, we describe a short series of six patients with proptosis that showed clinical signs of convergence weakness/insufficiency.
The study was carried out after obtaining informed consent from the patients. Six patients presenting with unilateral proptosis were studied. The proptosis was gradual in onset, painless, progressive, and axial in nature. Other than proptosis, two patients had history of decrease in visual acuity. None complained of diplopia. There was no extraocular muscle paresis in any of the patient. Corneal sensations were normal. A heterophoria was excluded by performing cover-uncover test. Fundoscopy revealed no abnormalities.
Magnetic resonance imaging (MRI) of orbit revealed benign neoplasm in all except one who showed sphenoid dysplasia. Two patients had schwannoma and two had meningioma [Figure 1] and [Figure 2]. In one, the lesion was hemangioma. MRI of brain revealed no abnormalities. The clinical and radiological characteristics of patients are summarized in [Table 1].
During routine ophthalmological evaluation, all were noted to have varying degrees of deficiency in convergence. In 2 patients, there was a failure of convergence despite intact adduction [Video 1]. In the remaining 4, the affected eye showed inadequate convergence. When the eyes are focused at a near point, initially convergence (adduction of bilateral medial recti) was seen. However, it was ill sustained and one of the eyeballs quickly drifted away to neutral position [Video 2]. In three patients, the proptosed eye ball drifted away, whereas, in one, this phenomenon was observed in the nonproptosed eye. The aforementioned findings were reproducible in all.
The space-occupying lesions of orbit usually present with proptosis. Proptosis can cause secondary restriction of extraocular muscle movements and ocular malalignment. However, associated findings of convergence deficiency/CI in patients with proptosis are unknown.
Orbital lesions may involve extraocular muscles or the nerve supplying them. In the event of medial rectus or the third nerve involvement, the extraocular movements would be affected and convergence cannot be tested in such patients. However, such neuromuscular involvement is uncommonly seen, and most of the patients have normal extraocular movements. Convergence weakness in these patients is unexpected.
Normally, the presence of intact adduction with convergence weakness points toward dorsal midbrain lesion., It can be a part of spectrum of ocular manifestations in lesions of dorsal midbrain (Parinaud's syndrome) that include upgaze paresis, light near dissociation, and convergence retraction nystagmus. In 3 of our patients, we observed obvious convergence failure and the rest had varying degrees of convergence weakness. MRI excluded any brainstem etiology in all.
Convergence weakness may not be obvious and may be brought about by prolonged near work, termed as convergence insufficiency. Working at a near point requires a fine balance between the convergence and accommodation. If this synchrony is lost, it can lead to muscular fatigue, eye strain, and CI. In CI, there is reduced or inability to maintain the required convergence on a near object and is characterized by exophoria at near point, remote near point of convergence, and reduced positive fusional convergence at near. The possible causes include poor accommodation or convergence, wide interpupillary distance, and presbyopia. Systemic disorders and anxiety neurosis have also been implicated. Weak eye muscles as a cause of CI is less evident.
Near vision consists of three physiological components (near reflex triad) which include convergence, accommodation, and miosis. When an object is brought near in front of eye, the bilateral medial recti muscles contract to bring convergence of eyeball. There is an associated contraction of ciliary muscles (accommodation) that relaxes the zonular lens fibers to increase its convexity. At the same time, the pupils constrict to increase the depth of focus to adjust to near vision.
The exact mechanism of one of the near reflex components (convergence) is not well understood., It is implicated that the cortical neurons arise from area 19 and 22 of occipital cortex, descend down to decussate in thalamo tectal area (para median thalamus). In the midbrain, the vergence premotor neurons (near response cells) are located dorsal and dorsolateral to oculomotor nucleus. From midbrain, the near response cells neurons project to medial rectus muscles.
The patients we report had orbital tumors and showed convergence deficiency/weakness on examination. Brain imaging excluded a central cause in all. Normally, it requires a great effort of extraocular muscles to achieve convergence and more importantly to sustain it. The phenomenon observed in our patients is possibly due to the increased force and endurance that is required to maintain the convergence/adducted state of medial recti muscles in a proptosed eye. Interestingly, one of the patients showed deficient convergence in the contralateral nonproptosed eye. However, such an observation could not be explained.
The article reports an interesting observation of deficient convergence in the presence of intact medial rectus function in patients with proptosis. Currently, the mechanism (central or peripheral) underlying this phenomenon is unclear. The symmetry of orbital contents possibly plays a significant role in the convergence mechanism.
Compliance with Ethical Requirements
Informed consent was obtained from the patients.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2]