Monocular Elevation Deficit
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Monocular elevation deficit ( Double elevator palsy)
Monocular Elevation Deficiency (MED) is the limitation of elevation of the affected eye from any position of gaze with normal ductions in all other gazes. MED is a frequent cause of hypotropia and can be associated with ptosis/pseudoptosis. Superior rectus palsy, inferior rectus restriction and supranuclear lesions have been found to be contributory to MED.
MED can be congenital or acquired . Congenital cases of MED occur sporadically.
Congenital causes of MED include, supranuclear palsy, primary superior rectus (SR) paresis and primary inferior rectus(IR) restriction as in cases of congenital fibrosis of inferior rectus.
Acquired MED is usually caused by trauma, cerebrovascular diseases like hypertension, thromboembolism, sarcoidosis, syphilis, and midbrain tumors like pineocytomas, acoustic neuromas and metastatic tumors affecting supranuclear pathways.
MED has been classified into three types by Ziffer et al. 
Type 1 includes patients with primary IR restriction or fibrosis.
Forced duction test (FDT) demonstrates a tight IR. The upward saccades are usually normal until stopped by tight IR , preventing further upgaze. Bells phenomenon is usually poor.
In Type 2 MED, cases of primary SR palsy are included. FDT is free and upward saccades are slow both below and above midline. Bell’s phenomenon is usually absent.
Type 3 MED is supranuclear type and is usually congenital. It is characterized by intact or mildly reduced vertical saccadic velocity below midline but abnormal or absent velocity above midline and FDT is free.
There are different school of thoughts regarding the pathogenesis of MED. It has been established that superior rectus is the main muscle responsible for elevation of the eye, be it in abduction, adduction or primary position. Thus, the defective elevation can be explained by presence of a superior rectus palsy alone, inferior rectus restriction can also be considered as a cause of MED in presence of a normal superior rectus.
Efferent tracts for upgaze leave the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), cross the midline in the posterior commissure, course through the pretectum and enter the superior rectus (SR) subnucleus of the oculomotor nucleus, leaving the subnucleus, upgaze fibres to the SR immediately cross the midline again. As a result of double decussation of upgaze fibers, the SR receives innervation from ipsilateral riMLF as well as contralateral pretectum and SR subnucleus. In cases of MED, it is presumed that supranuclear input from riMLF into third cranial nerve nucleus is interrupted. Supranuclear deficiency affects upgaze, therefore causes monocular elevation deficiency.
- Age: Average age of presentation is 14 years as reported in previous studies.
- Gender: No gender predisposition
Patients usually present with the following complaints:
- Vertical misalignment of the eyes
- Abnormal head posture
- Inability to elevate the affected eye
- Drooping of eyelid
- Decrease vision
- Ptosis/Pseudoptosis: Ptosis can be associated with hypotropia because of fascial attachments between levator palpebrae superioris and superior rectus muscle. Pseudoptosis can be present that disappears when hypotropic eye takes up fixation in primary position. Pseudoptosis disappears after the successful vertical alignment of the globe . Pseudoptosis and true ptosis both the components can be present in the patient. In these cases when the patient is made to fix with the affected eye, ptosis partly recovers. True ptosis when present is usually complex type and is associated with Marcus Gunn Jaw Winking phenomenon (MGJWP).
- Deviation: There is hypotropia of the affected eye when the normal eye is fixing , but as the affected eye takes up fixation there is hypertropia of the normal eye. MED cases may also have co-existent horizontal deviations.
- Bell’s phenomenon is usually absent in cases of IR restriction and SR palsy as they limit the upward rotation of globe, but Bell's is usually present in cases of MED resulting from supranuclear defects.
- Amblyopia may be present if there is a constant deviation of affected eye or associated ptosis, or anisometropia.
- Abnormal Head Posture : Most commonly patient has a chin elevation if binocular fusion is present. Abnormal head posture may not be present if the affected eye is amblyopic.
- Upward Saccades: Upward saccades are slow/floating in cases of SR palsy, absent above the midline in supranuclear palsy , and intact (coming to an abrupt halt) in cases associated with IR restriction.
Forced duction test
Forced duction test is a clinical test which should be done pre-operatively to reveal any restriction. The test is typically performed under topical anaesthesia, conjunctiva is held at limbus with Pierce Hoskin forceps or globe holding forceps, and patient is asked to look in opposite direction of testing muscle to relax the concerned muscle. A positive Forced Duction Test (FDT) is seen in patients of IR restriction.
Active Force Generation Test
Active force generation test is done under topical anaesthesia by asking the patient to look in the direction of muscle being tested, presence of a tug is suggestive of residual SR action.
Red and green glasses over right and left eyes respectively, dissociate the two images and would be seen double. Test can be done both for distance and near, with the help of illuminated slit target. The slit is used vertically for charting horizontal deviations and horizontally for charting vertical deviations. The seperation between two images is recorded and is useful in diagnosis and follow-up.
Hess Screen And Lees Screen
Hess Screen and Lees Screen is used to document relative incomitance and underactions and is based on haploscopic principle. Two test objects are shown to the patient which is seen as one localisation.
Hess Screen consist of a three and a half feet black cloth marked with a series of red lines forming a square of 5 degree each. From the central zero point three squares 15 degree marks the inner square and outer square has excursion of 30 degree. The outer square is used for mild incomitances not detected on inner square charting. Patient wears a red green glass, with red glass in front of right eye and green glass in front of left eye, patient is given a green pointer which he aligns on the red dots and the observer records the points.
Lees Screen is also similar but does not require red green glass instead uses a mirror septa to cause dissociation of two eye.
The main differential diagnosis of monocular elevation deficit includes-
- Brown’s Syndrome is characterised by limitation of elevation in adduction, with divergence in upgaze caused by slippage of globe resulting in characterstic Y pattern
- Vertical Duane Syndrome can be diagnosed by the presence of globe retraction in downgaze along with limitation of elevation in upgaze.
- Congenital fibrosis of extra ocular muscles initially affecting inferior recti can be misdiagnosed as MED. Bilateral presentation and involvement of other extraocular muscles are features that help distinguish it.
- Partial third nerve palsy involving the superior division of oculomotor nerve may simulate the findings of MED.
- Progressive external ophthalmoplegia involves multiple extra ocular muscles.
- Orbital inflammatory disease and orbital cellulitis present with a tender inflamed globe.
Management of MED includes both non-surgical and surgical components.
The correction of underlying refractive error is to be done and amblyopia therapy is given if needed, followed by surgery whenever indicated.
Indications for surgery include
- Significant vertical deviation in primary gaze
- Significant abnormal head posture
- Deviation-induced amblyopia
- Diplopia in primary gaze, and restricted binocular fields.
The goal of surgery is to improve the position of the affected eye in primary gaze and to increase binocular field of vision.
The surgery of choice in the management of monocular elevation deficit depends on results of forced duction test. In patients with positive forced duction test, inferior rectus recession is the first choice of management.
In patients with negative forced duction test but a positive active force generation test, superior rectus resection can be done.
In patients with negative forced duction test and a negative active force generation test Knapp's procedure is preferred.
- Knapp's procedure 
Knapp’s procedure is commonly practiced and is now a well-established treatment for monocular elevation deficiency. In the Knapp’s procedure, the tendons of the medial and lateral rectus muscles are transposed to the insertion of the superior rectus muscle. Knapp’s in his work studied 15 patients over a period of eight years. The procedure showed marked variability correcting 21-55 PD (average 38 PD) hypotropia.
- Modifications of Knapp's procedure
For management of MED cases with associated horizontal deviation partial tendon Knapp’s (Modifed Knapp’s) procedure has been described, in which the superior part of equally divided horizontal muscles is placed near the superior rectus muscle insertion, allowing the correction of horizontal deviation with the untouched inferior half.
- Augmented Knapp's procedure, Knapp's procedure can be combined with posterior fixation sutures on the horizontal recti, known as Augmented Knapp's.
Following the surgical correction of hypotropia, and improvement in Bell’s phenomenon, associated ptosis and/or Marcus Gunn phenomenon be addressed.
- Modified Nishida's for monocular elevation deficit
No split no tenotomy technique by Nishida et al has originally been described for abducen nerve palsy, the procedure has been found to be useful in the management of monocular elevation deficit by Murthy et al and can help in correction of upto 30 pd of vertical deviation with low risk of anterior segment ischemia.
- Contralateral superior rectus recession 
Contralateral superior rectus recession can be an option of management in the patients with residual hypotropia after inferior rectus recession.
Management of Acquired MED
As there are wide variety of causes of acquired MED, thorough investigations (general, ophthalmologic, neurologic, neuro-ophthalmologic and orthoptic (ocular motility work up) should be undertaken, and management varies according to the underlying cause.
Ptosis can lead to refractive error and amblyopia, also if ptosis surgery is done before the strabismus surgery it can lead to exposure keratopathy as most of the patients have poor Bell's phenomenon.
Surgical complications seen commonly in a case of monocular elevation deficit include:
- Anterior segment ischemia, is a serious complication presenting with corneal edema, thinning, non pigmented keratin precipitates, iris atrophy and anterior chamber flare. Anterior segment ischemia can result if three or more adjacent recti surgery has been done in one eye, adults with atherosclerosis, previous radiation therapy, and retinal detachment surgery are more vulnerable. The complication can be avoided if two vertical recti along with one horizontal recti are not operated in the same sitting. Management includes, systemic and local corticosteroids
- Overcorrections and undercorrections.
- Tenon's prolapse and conjunctival cyst can result due to improper apposition of the conjunctiva which allows Tenon's to prolapse, and interferes with healing process.
- Corneal dellen formation can occur due to disruption of tear film and local dehydration.
- Other intraoperative complications like surgery on wrong eye, wrong muscle, hemorrhages, scleral perforation,slipped muscle, bradycardia, infections may also occur in patients undergoing surgery.
The average correction resulting from various surgical procedures vary, and has been summarised in Table 1.
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- Kamlesh, Dadeya S. Surgical management of unilateral elevator deficiency associated with horizontal deviation using a modified Knapp’s procedure. Ophthalmic Surg Lasers Imaging. 2003; 34:230–5.
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