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Synkinesis Following Diabetic Third Nerve Palsy
Arch Ophthalmol. 2000;118:132-134.
A 59-year-old man developed a left pupil-sparing third nerve palsy related to diabetes that recovered gradually over 5 months; the only deficit remaining at 8 months was a trace underaction of adduction. Oculomotor synkinesis was evident at this time, manifest as left upper lid retraction on adduction and on downgaze. There were no signs of oculomotor or pupillary synkinesis. High-resolution magnetic resonance imaging of the third nerve pathway revealed no abnormality. Oculomotor synkinesis following diabetic third nerve palsy is extremely rare, with only one report in the literature to date.
Synkinesis may occur during the recovery phase of a traumatic third nerve palsy or after long-standing compression. The abnormal movements that constitute synkinesis may involve the upper lid, oculomotor system, and the pupil. Synkinesis is rare when the cause is ischemia1 and we have found only one previous report of synkinesis following diabetic third nerve palsy.2 Our report adds a second to the literature.
Report of a Case
A 59-year-old man with type 2 diabetes mellitus was seen with a 2-week history of left ocular and retro-orbital pain, progressive left upper lid ptosis, and diplopia in all directions of gaze. Diabetes was well controlled and there was no history of hypertension or previous neurologic disturbance. Medication consisted of oral hypoglycemic agents only.
The examination revealed corrected visual acuity of 20/30 OU, normal color vision, and full visual fields. The pupils were 4 mm in diameter and reacted briskly to light. There was no proptosis. The upper lid and ductions in the right eye were normal. On the left side, there was complete ptosis with no levator function. The left eye was exotropic, adduction and elevation were absent, and depression was markedly limited. Intorsion on attempted downgaze was present. The remainder of the cranial nerves were normal. A diagnosis of diabetic left pupil-sparing third nerve palsy was made and the patient was observed.
At 2 months, his pain had resolved completely, and left upper lid ptosis was only 3 mm with 5 mm of levator function. The left eye could be adducted just beyond the midline and there was improved but still limited upgaze and inferior rectus function. There was 2 mm of left upper lid retraction on attempted adduction, with the right upper lid remaining stationary on right gaze. There was complete descent of the left upper lid on downgaze at this time. Because of the signs of oculomotor synkinesis, a magnetic resonance scan of the third nerve pathway was obtained, but no intracranial abnormality from the midbrain to the orbit was found.
At 5 months, the patient was asymptomatic and had almost completely recovered. The patient had no ptosis or pupillary abnormality, normal levator function, and full motility except for a trace underaction of adduction on the left. Retraction of the left upper lid on adduction was unchanged and there was a new incomplete descent of the left upper lid by 2 mm on downgaze (Figure 1).
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A, Right gaze demonstrates slight underaction of left medial rectus and elevation of left upper lid. B, Left gaze demonstrates the normal lower position of the left upper lid. C, On downgaze, the left upper lid does not descend fully.
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Although the patient's third nerve palsy had recovered almost completely, definite signs of oculomotor synkinesis were evident. These synkinetic movements were unchanged at the last examination, 8 months after the onset of symptoms.
Comment
Third nerve palsy is the commonest cranial mononeuropathy in patients with diabetes.3 Although the precise etiological role of diabetes-related third nerve palsy is uncertain,4-5 diabetes-related ischemia of the peripheral nerve may be important.6 And, with the use of magnetic resonance imaging, it is now apparent that isolated third nerve palsy (with or without pain and without pupillary involvement) in a diabetic individual may uncommonly be due to midbrain infarction or hemorrhage.7 It should be noted that if the third nerve palsy in a diabetic patient fails to recover, it is likely to be due to a coexistent compressive lesion such as a tumor or an aneurysm.
Recovery of third nerve palsy in diabetes usually occurs within 3 months.8-10 The mechanism of recovery is not known, but almost certainly involves remyelination, present at 6 months11 and complete by 3 years.12 Synkinesis may occur during recovery of any third nerve palsy, particularly following traumatic and compressive lesions,13-15 but only rarely when the nerve palsy appears to be due to microvascular disease1 or ischemia related to giant cell arteritis.16 Oculomotor synkinesis is manifest as abnormal movements of the upper lid, globe, or pupil, occurring in isolation or in a variety of combinations that may be subtle and easily missed by the examiner.
There are currently 3 contesting theories on the pathogenesis of oculomotor synkinesis (see review by Sibony et al17): (1) Aberrant regeneration of injured nerve fibers in which regenerating axons that grow into the wrong nerve sheaths are directed to incorrect targets. (2) "Release" phenomenon or synaptic reorganization of the third nerve prenuclear or nuclear complex.18-19 (3) Abnormal signal conduction (ephaptic transmission) between peripheral axons where there is a secondary partial or complete loss of myelin.
In our patient, it is assumed that the third nerve palsy was due to a lesion of the peripheral nerve since magnetic resonance imaging revealed no evidence of a brainstem, cavernous sinus, or subarachnoid lesion along the third nerve pathway. Magnetic resonance imaging is unhelpful in confirming the peripheral location of the lesion in diabetic third nerve palsy since, as occurred in our patient and in other studies,20-21 the third nerve did not enhance.
Histological studies of diabetic third nerve palsies may provide insight about the pathogenesis of synkinesis in our patient. These studies11-12,22 found that the dominant feature was central focal demyelination of the subarachnoid, intracavernous third nerve with relative axonal sparing. The fascicles and nuclei were normal in two of the examinations. These findings mitigate against the misdirection and central theories but favor the ephaptic theory of electrotonic conduction between bare axons in our case.
In conclusion, this case report is the second of synkinesis following diabetic third nerve palsy. It is emphasized that signs of oculomotor synkinesis may be subtle and thus easily missed unless specifically looked for, but the diagnosis of synkinesis following diabetic third nerve palsy remains a diagnosis of exclusion.
AUTHOR INFORMATION
Dai Barr, FRCOphth;
Mark Kupersmith, MD;
Roger Turbin, MD;
Sherry Yang, MD;
Raymond Iezzi, MD
New York, NY
Corresponding author: Mark J. Kupersmith, MD, INN at Beth Israel North, 170 East End Ave, New York, NY 10128 (e-mail: mkuper{at}bethisraelny.org).
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