How are the margin reflex distances (MRD) measured in unilateral third cranial nerve palsy (oculomotor cranial nerve palsy)?

Updated: Oct 08, 2018
  • Author: James Goodwin, MD; Chief Editor: Andrew G Lee, MD  more...
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Answer

Milder degrees of ptosis are examined best using measurements of the palpebral fissure width; more information can be gained from measuring the levator function and margin reflex distances (MRD), as follows:

  • The examiner sits in front of the patient and directs a point source of light (eg, handlight, Finnoff head, penlight) into both the eyes of the patient.
  • The examiner can observe the pinpoint light reflex in the center of the cornea. The position of this reflex is not affected by deviation of the eye, which makes it a useful reference point for measuring the distance to the upper and lower lids, respectively.
  • The MRD1 usually refers to the distance from the light reflex to the upper lid margin, and the MRD2 is the distance to the lower lid margin.
  • Levator palpebrae weakness reduces the MRD1 but will not affect the MRD2. On the other hand, ptosis from ocular sympathetic palsy (Horner or Claude Bernard syndrome) causes weakness of Mueller muscle in both the upper and lower lids, with consequent reduction of both the MRD1 and the MRD2.
  • Pseudoptosis from widening of the contralateral palpebral fissure may result from mild or old contralateral seventh cranial nerve lesion and orbicularis oculi weakness. In both these cases, MRD1 and MRD2 are increased on the side opposite to the apparent ptosis.
  • Careful examination of facial muscle strength and symmetry usually confirms the cause of the apparent ptosis in these cases.

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