Monocular elevation deficit (MED) is a condition in which elevation of the affected eye is equally limited during both adduction and abduction. It is also called double elevator palsy, but pathophysiologically, simultaneous palsy of the superior rectus and inferior oblique muscles is unlikely, and currently the descriptive term congenital monocular elevation deficiency has been proposed.
MED is divided into congenital and acquired types. Congenital MED occurs sporadically and is caused by supranuclear palsy, primary superior rectus palsy, or primary inferior rectus restriction. Acquired MED is caused by trauma, cerebrovascular disorders (hypertension, thromboembolism, etc.), sarcoidosis, syphilis, or tumors (pinealocytoma, acoustic neuroma, metastatic tumors, etc.). Acquired cases are often due to midbrain infarction or tumors, and the first eye position is orthotropic with dissociation of conjugacy only on upward gaze.
Recently, a novel mutation in the TUBB3 gene has been reported in association with congenital MED4). There are also several reports on the clinical features and surgical outcomes of double elevator palsy in children1).
Congenital cases account for the majority. Acquired cases are caused by trauma, cerebrovascular disorders, tumors, etc., and are sometimes called monocular elevation paresis.
The pseudoptosis component improves with correction of vertical strabismus (strabismus surgery). If true ptosis remains, ptosis surgery is considered after confirming that pseudoptosis has been eliminated following strabismus surgery.
The pathology of MED is classified into the following three types.
Type 1
Inferior rectus restriction type
Etiology: Restriction of the inferior rectus (e.g., primary fibrosis)
Forced duction test (FDT): Positive (tightness of the inferior rectus)
Saccades: Normal until restriction occurs
Bell’s phenomenon: Usually poor
Type 2
Levator muscle dysinnervation type
Etiology: Dysinnervation of the levator muscle (including primary superior rectus palsy)
Forced duction test (FDT): Negative (no restriction)
Saccades: Slow both above and below the midline
Bell’s phenomenon: Usually absent
Type 3
Supranuclear type
Etiology: Supranuclear disorder (usually congenital)
Forced duction test (FDT): No restriction
Saccades: Normal to mildly decreased below the midline, abnormal to absent above the midline
Bell’s phenomenon: Usually preserved
Congenital MED occurs sporadically and is caused by supranuclear palsy, primary superior rectus palsy, or primary inferior rectus restriction. As a genetic factor, MED associated with a novel mutation in the TUBB3 gene has been reported in siblings 4).
Acquired MED occurs due to the following causes:
This is the most important test for classifying MED subtypes. It passively evaluates the presence or absence of restriction.
Under topical anesthesia, the patient is asked to look in the direction of the tested muscle to evaluate residual muscle strength.
Using red-green glasses to separate the images of both eyes, binocular visual function (fusion and suppression) is assessed at distance and near.
Under topical or general anesthesia, the conjunctiva near the limbus is grasped with forceps and the eye is passively rotated to evaluate the presence of extraocular muscle restriction (tightness). This test is the basis for MED subtype classification and surgical approach selection.
The main differential diagnoses for MED are listed below.
| Differential Diagnosis | Key Points for Differentiation |
|---|---|
| Brown syndrome | Limited elevation in adduction, Y-shaped exotropia |
| Duane syndrome | Globe retraction on downgaze |
| CFEOM | Bilateral, multiple extraocular muscles involved |
| Partial oculomotor nerve palsy | Similar especially with superior branch involvement |
| Progressive external ophthalmoplegia | Multiple extraocular muscles progressively affected |
In cases of congenital superior oblique palsy where the patient habitually fixates with the paretic eye, contralateral monocular elevation deficiency may occur due to contracture of the contralateral inferior rectus muscle. Differentiate using the Parks-Bielschowsky three-step test.
Correction of underlying refractive errors and treatment of amblyopia are fundamental. Especially in children, early detection and treatment of amblyopia are important.
Surgery is considered when any of the following criteria are met.
The surgical procedure is selected based on the results of the FDT.
FDT positive
Inferior rectus recession (IR recession)
Indication: Cases with confirmed restriction of the inferior rectus muscle
Principle: First-choice procedure for restrictive hypotropia
FDT negative
Knapp procedure (horizontal rectus transposition)
Indication: Cases without restriction and due to levator muscle insufficiency
Procedure: Transposition of the medial and lateral rectus muscles to the insertion of the superior rectus muscle.
Effect: Corrects hypotropia of 21–55 PD (mean 38 PD). Variability in effect.
Note: If some elevation remains, contralateral superior rectus recession is also an option.
This may be selected as an additional surgery when hypotropia persists after inferior rectus recession.
Ptosis surgery is performed after vertical strabismus has been corrected. Often the eyelid position improves once the vertical deviation is corrected, but if residual ptosis remains, surgical intervention should be considered after adequate alignment.
Awadein et al. (2015) reported surgical outcomes in cases of MED with inferior rectus restriction. Preoperative head tilt was present in 70% of the inferior rectus-only surgery group (IR group) and 54% of the inferior rectus plus contralateral superior rectus group (IR+SR group) 2).
Struck et al. (2015) reviewed 5 surgical cases of supranuclear MED. Cases included a patient with 25 PD of hypertropia and 20 degrees of chin-up posture, and a patient with 30 PD of hypotropia and 25 degrees of chin-up posture 3).
The result of the forced duction test (FDT) forms the basis for selecting the surgical procedure. If FDT is positive (restriction present), inferior rectus recession is performed. If FDT is negative (no restriction), the Knapp procedure (transposition of the horizontal rectus muscles to the insertion of the superior rectus) is the first choice.
The efferent pathway for upward gaze originates from the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). Signals from the riMLF cross the midline at the posterior commissure (PC), pass through the pretectum, and reach the superior rectus subnucleus of the oculomotor nucleus. Fibers exiting the superior rectus subnucleus cross the midline again to innervate the contralateral superior rectus muscle.
As a result of this double decussation, the superior rectus muscle receives innervation not only from the ipsilateral riMLF but also from the contralateral pretectum and superior rectus subnucleus.
In the supranuclear type of MED, input from the riMLF to the oculomotor nucleus is thought to be blocked. Since the riMLF projects bilaterally to the elevator muscles, a unilateral riMLF lesion alone does not cause bilateral upward gaze palsy. Bilateral upward gaze palsy requires a lesion of the PC.
In the peripheral oculomotor nerve, fibers destined for different extraocular muscles run in separate fascicles within the brainstem. Selective damage to the laterally running fibers for the inferior oblique (IO) and superior rectus (SR) muscles results in monocular elevation palsy, with absence of Bell’s phenomenon on the affected side.
Thomas et al. (2020) reported an association between a novel mutation in the TUBB3 gene and congenital MED in two siblings aged 7 and 12 years. The older brother presented with chin-up posture, decreased visual acuity, and MED in the right eye, while the younger brother similarly showed chin-up posture, head tilt, and limited elevation of the right eye 4).
TUBB3 encodes β-tubulin, a component of neuronal microtubules, and its mutations are associated with congenital cranial dysinnervation disorders. Elucidating the genetic basis of MED may contribute to future diagnosis and genetic counseling.
Priglinger et al. (2014) reported computer-assisted diagnosis using a biomechanical eye model. Clinical findings such as chin-up posture, partial binocular vision, and preservation of Bell’s phenomenon supported the diagnosis of supranuclear MED, and the vertical deviation varied from −14 PD to −4 PD depending on the direction of head tilt 5).
