(S. A. Patney: Strabismology Desk Reference, chapter 48, JKA Publications)

PARALYTIC STRABISMUS: DOUBLE DEPRESSOR PALSY

One of the rarest conditions in ocular motility disorders, double depressor palsy is also known by the name of "Monocular depression Deficiency (MDD)". Double depressor palsy (DDP) is an intriguing condition. This is because its etiology is still not clear.

In this condition there is a unilateral or monocular paralysis of downgaze, which means that there is an absence or gross limitation of the affected eye in dextrodepression, direct depression and levodepression. That is to say that all the ductions of the affected eye in downgaze are absent or severely restricted.

Double depressor palsy is a very rare condition. The reports in the ophthalmic literature suggest that usually it is congenital and sporadic^{1, 2 and 16}. I have seen only one case in more than four decades and that was a congenital DDP. However, literature in neurology has quite a few reports of DDP, several of them acquired^{3, 4, 5, 6, 7, 8, 9 and 10}. The reason for the rare occurrence of this disorder lies in the fact that one of the depressors (inferior rectus) is supplied by the inferior branch of the 3^rd cranial nerve and the other depressor (superior oblique) by the 4^th cranial nerve (CN).

Some of the cases appearing to be those of double depressor palsy are in reality cases of partial 3^rd CN palsy. The depression by superior oblique (SO in adduction) can not be demonstrated because the eye can not be adducted, the medial rectus being affected too). In these cases presence of intorsion on attempted depression will show that SO function is intact.

(*A blowout fracture of the orbit may cause inferior rectus palsy by direct injury to the muscle or by entrapment of the inferior rectus at the fracture site. ** Scarring in the superior rectus is either due to injury or surgery)

The double depressor palsy is usually the result of one of the following:

1. A primary paralysis of inferior rectus muscle, congenital or acquired
2. A primary supranuclear palsy of depression (downgaze)
3. A secondary dysfunction of the inferior rectus due to ipsilateral superior rectus contracture

As indicated earlier, the etiology of DD palsy is baffling to say the least. The following anatomical facts have to be taken into account before trying to find the cause of DDP in a patient.

1. The two depressor-muscles involved are supplied by two separate cranial nerves, i.e., the inferior rectus by the inferior branch of the 3^rd CN and the superior oblique by the 4^th CN.
2. Unlike elevation (upgaze) in which the supranuclear fibres decussate through the posterior commissure, the depression (downgaze) fibres do not.
3. Supranuclear fibres of depression (downgaze) are present bilaterally in the rostral interstitial nucleus (riMLF) in midbrain. That means that for the double depressor palsy (DDP) or monocular depression deficiency (MDD) to occur the lesion has to be bilateral, making it a rare disorder.
4. The blood supply to riMLF and the tract below between the midbrain and the oculomotor nuclei comes from the paired posterior thalamic paramedian arteries^{11 and 12}. Bilateral blockage is required to cause a DDP except when there is a vascular anomaly¹² as detailed below. There are three variations of paired posterior thalamic paramedian arteries. One, each artery arises from 2 separate basilar communicating arteries. Two, each artery arises from the vascular arcade formed by the Basilar communicating arteries. Three, both the arteries arise from a single Basilar communicating artery.
   
   If the third variant of the posterior thalamic paramedian arteries is there, an occlusion of a single vessel can cause ischemic lesions of riMLF on both sides leading to downgaze palsy. For a more detailed account see reference no. 4.
5. Other sites reported as being affected by lesions having caused DDP are the interstitial nucleus of Cajal, the periaqueductal grey matter^{6, 7 and 8} and the posterior commissure.

To sum up, the etiology is as follows:
(1) Congenital palsy of inferior rectus alone or accompanied by superior oblique

Figure 48-a and b, preoperative and postoperative (respectively) photographs of a case of cong. DDP (differential diagnosis: Cong. Aplasia of inferior rectus muscle). Surgery performed: trans-position of both horizontal recti to insertion-site of inferior rectus. For details see case report 48-1

paralysis. In the latter case the inferior rectus palsy may be due to a partial III CN paralysis that is associated with IV CN palsy. Congenital absence of inferior rectus muscle has been reported¹³. The lone case of DDP operated in my clinic many years ago (figure 4..-a & b above) also had a congenital absence of the inferior rectus muscle (case report 4..-1 below).

(2) Acquired DDP

Although congenital DDP has been described as being more common in ophthalmic literature^{1 and 2} many more acquired cases of DDP have been reported in Neurological literature than in ophthalmic journals^{3, 4, 5, 6, 7, 8, 9, 10, 11 and 14}. A case of DDP after hemorrhagic conjunctivitis has also been reported⁵.

1. Most of the acquired cases, however, are due to cerebrovascular disease involving the pons and the cerebellum^{3, 4, 8, 9, 10, 11 and 15.} Patients in this group are usually unconscious when they are first examined and therefore under the care of a neurologist or a physician.
2. Trauma to the orbit causing blowout fractures entrapping or injuring the inferior rectus muscle can also cause monocular downgaze deficiency (MDD).
3. Monocular downgaze deficiency (MDD) may be secondary to superior rectus contracture after strabismus surgery.

The main points are as follows:

• Thorough general systemic, neurological and ophthalmologic examinations should be carried out as a matter of routine and also to rule out associated cerebrovascular disease in cases of acquired DDP.

• A careful history to rule out previous strabismus surgery involving SR or IR.

• Congenital cases usually do not complain of diplopia as they have already developed suppression due to a constant strabismus. However, diplopia is a common complaint in acquired cases.

• Traumatic cases present with varying symptoms after orbital injury involving the inferior rectus muscle.

• Doll’s eye movements or vestibulo-ocular reflexes usually are intact on downgaze (on moving the head from side to side the eyes move simultaneously to the opposite side). This supports the view that DDP is a supranuclear disorder and the nuclear reflexes are present.

• Sometimes pupillary anomalies may be there¹².

• On orthoptic examination the findings are fairly typical:

1. In primary position there is marked hypertropia.
2. Gross limitation of downgaze or depression: The infraduction of the affected eye is limited in all three directions (direct depression, dextrodepression and levodepression).
3. Upshoot of the affected eye in adduction and abduction
4. Overaction of the affected eye in elevation in all three directions (dextroelevation, direct elevation and levoelevation).
5. Convergence may be found to be defective. This may be secondary to a longstanding constant hypertropia leading to disuse of convergence.
6. Amblyopia, suppression and loss of binocular vision in long-standing cases
7. Upper lid retraction may be present in primary position with the sound eye fixing. This is because of the attachments between the superior rectus and the levator palpebrae superioris¹².
8. Pseudoptosis in downgaze when the affected eye does not depress but the upper lid goes down (droops or depresses)¹².

Differential diagnosis (Table 48-1 and 48-2)

Apart from the tests mentioned above under symptomatology the following tests should be done if possible:

Table 2:

Apart from the tests mentioned above under symptomatology the following tests should be done if possible:

1. Forced duction test is a must, particularly to rule out the presence of fibrosis/scarring/contracture of superior rectus muscle and also of the inferior oblique.
2. If facilities are available CT scan (computed tomography) of the brain and orbits: The latter is especially useful as it can give a good idea of the condition of the extraocular muscles, e.g., aplasia, hypoplasia or fibrosis/contracture.
3. Other more specialized tests like recording of saccadic velocity,
4. Electromyograms and
5. Magnetic resonance imaging can be of help and are usually required for research purposes.

Management of monocular depression deficiency or double depressor palsy

• Nonsurgical treatment consists of routine correction of refractive errors and treatment of amblyopia if indicated.

• The treatment of DDP is basically surgical.

• The procedure of choice, in the absence of superior rectus contracture, is a transposition of the horizontal recti to the IR insertion site. The horizontal rectus muscles (medial rectus and lateral rectus) are detached from their insertions and transposed to the inferior rectus insertion.

• If, after the transposition procedure there is residual hypertropia of the affected eye, recession of superior rectus is carried out in second stage.

• However, if there is a contracture of superior rectus muscle as diagnosed by positive forced duction test, superior rectus recession is the first procedure to be carried out ^{13 and 17}.

• Postoperative complications are the usual ones encountered after strabismus surgery, i.e., diplopia, undercorrection, overcorrection and anterior segment ischemia.

1. Noorden, G.K. von: Binocular vision and Ocular Motility: Theory and Management of Strabismus, 5^th edition, 1996, St. Louis, Mosby-Year Book, p. 417.
2. Noorden, G.K. von and Hansel, R.: Clinical characteristics and treatment of isolated inferior rectus paralysis, 98:253, 1991.
3. Trojanowsky, J.Q. and Lafontaine, M.H.: Neuroanatomical correlates of selective downgaze paralysis, J. Neurol. Sci. 52:91, 1981.
4. Trojanowsky, J.Q. and Wray, S.H.: Vertical gaze ophthalmoplegia: Selective paralysis of downgaze, Neurology 30:605, 1980.
5. Prakash, P., Menon, V.M., Gupta, A.K. et al: Acquired double depressor palsy following acute hemorrhagic conjunctivitis, Indian J. Ophthalmol. 36:35, 1988.
6. Ozdemir, N. et al: Downgaze palsy due to periaqueductal lesion diagnosed by Magnetic Resonance imaging, Ophthalmologica 209:225, 1995.
7. Jacob, L. et al: Selective paralysis of downward gaze caused by bilateral lesions of the mesencephalic periaqueductal grey matter, Neurology 35:516, 1985.
8. Jacobs, L. et al: Selective paralysis of downward gaze caused by bilateral lesions of the mesencephalic periaqueductal grey matter and commissure of the superior colliculi, Neurology 34:95, 1984.
9. Jacob, L. et al: The lesions producing paralysis of downward but not upward gaze, Arch. Neurol. 28:319, 1973.
10. Bogousslavsky, J. and Regli, F.: Upgaze palsy and monocular paresis of downward gaze from ipsilateral thalamo-mesencephalic infarction: A vertical "one-and-a half" syndrome, J. Neurol. 231:43, 1984.
11. Green, J.P. et al: Paralysis of downgaze in two patients with clinical radiological correlation, Arch. Ophthalmol. 111:219, 1993.
12. Rosenbaum, A.L. and Santiago, A.P.: Clinical Strabismus Management: Principles and Surgical Techniques, 1998, Philadelphia, PA, W.B. Saunders Company, p. 280.
13. Cooper, E.L. and Greenspan, J.A.: Congenital absence of the inferior rectus muscle, Arch. Ophthalmol. 86:451, 1971.
14. Buttner-Ennever, J.A. et al: Ptosis and supranuclear downgaze paralysis, Neurology 39:385, 1989.
15. Halmagyi, G.M. et al: Failure of downward gaze: The site and nature of the lesion, Arch. Neurol. 35:22, 1978.
16. Cogan, D.C.: Paralysis of downgaze, Arch. Ophthalmol. 91:192, 1974.
17. Dunlap, E.A.: Vertical displacement of the horizontal recti. In: Symposium on Strabismus, 1971, St. Louis, C.V. Mosby and Company, p. 307.