Complete Bilateral Ophthalmoplegia Due to a Strategic Midbrain Infarct.

Sir,

The rostral midbrain and thalamo-mesencephalic junction are concerned with vertical and torsional eye movements, whereas the primary horizontal gaze centers are located in the pons. Impaired abduction in mesencephalic lesions can be due to pseudo-abducens palsy (PAP) in conjunction with other vertical gaze disorders. We present a case of complete bilateral ophthalmoplegia due to a discrete midbrain infarct, where abduction was demonstrable only by caloric testing.

A 59-year-old woman with hypertension developed acute onset of drooping of the left eyelid and swaying to the right side while walking. As she presented to the hospital outside the window period, intravenous thrombolysis was deferred. On examination, she was conscious and oriented and had a left complete 3rd nerve palsy with right ataxic hemiparesis. The next morning, she developed altered mental status, fixed and dilated pupils (left >right) with bilateral ptosis, exotropia, and dense right hemiplegia [0/5 power]. Voluntary, roving eye movements and vestibulo-ocular reflexes (VOR) (both horizontal and vertical) were absent [Figure 1 and Video 1]. An urgent magnetic resonance imaging (MRI) brain showed multiple infarcts involving the left thalamus, left posterior temporal lobe, left midbrain extending to the midline, left cerebral peduncle, and right cerebellum. Bilateral cold caloric testing (30°C) revealed preserved bilateral abduction. Bilateral complete ophthalmoplegia due to a strategic midbrain lesion was diagnosed. One week later, abduction movements normalized, and she was left with bilateral ptosis, upgaze impairment, sluggish pupillary reactions to light, left oculomotor palsy, and right hemiplegia.

Figure 1

Shows bilateral exotropia. The panel on the right shows the strategic midbrain infarct that extends from the crus cerebri to the midline oculomotor nucleus

Bilateral complete ophthalmoplegia is an extremely rare finding in thalamo-mesencephalic lesions as only vertical gaze abnormalities or an internuclear ophthalmoplegia are usually encountered.[123]

Horizontal ophthalmoplegia can be seen in bilateral midbrain lesions if the descending fronto-pontine saccadic (FPS) and the occipito-temporal pursuit (OTP) horizontal gaze pathways that lie adjacent to the riMLF/internuclear fibers (INC) complex are involved.[4]

Pseudo-abducens palsy (PAP) (”a failure of ocular abduction not caused by dysfunction of the sixth nerve”) can rarely result from mesencephalic lesions. Components of the mesencephalic descending convergence pathway (DCP) can be disrupted.[5] The DCP from the temporal-parietal-occipital junction traverses the paramedian thalamus and inhibits the contralateral premotor vergence neurons (near-response neurons) in the supra-oculomotor area (SOA) in the midbrain. Interneurons from the SOA are excitatory to the ipsilateral medial rectus (MR) oculomotor subnucleus and inhibitory to the ipsilateral abducens nucleus, facilitating convergence. DCP lesions disinhibit the SOA and lead to convergence excess and abducens overinhibition. PAP can also occur with lesions of the decussating descending excitatory internuclear fibers (De-INF) from the MR subnucleus to the contralateral abducens nucleus, that facilitates horizontal gaze, with resultant contra-lateral abducens overinhibition [Figure 2].[6]

Figure 2

Representative diagram of the horizontal gaze pathways coursing through the mesencephalon. For simplicity, only one side is illustrated. (-); refers to an inhibitory pathway. (+); refers to an excitatory pathway. DCP: descending convergence pathway. SOA: supra-oculomotor area, EWN; Edinger Westphal nucleus. MR; medial rectus subnucleus. LPS: levator palpebrae superioris subnucleus. SR: superior rectus subnucleus. MLF: medial longitudinal fasciculus. De-INF: Decussating descending excitatory internuclear fibers

Thus, either an ipsilesional or contra-lesional pseudo-abduction deficit can occur depending on the rostrocaudal location of the mesencephalic lesion. The lesion location also determines if PAP and esotropia coexist as in thalamic esotropia.[78] As the lesion in our probably extended to the MR subnucleus, bilateral exotropia occured.

PAP is overcome by VOR in the majority of cases.[9] However, the VOR may rarely be absent in PAP, and caloric testing (a much stronger stimulus) is necessary to overcome PAP.

Exceptionally, complete midbrain ophthalmoplegia unresponsive to both VOR and caloric testing is described.[310]

Our patient had completed bilateral ophthalmoplegia with exotropia, which could not be overcome by VOR. In our case, the complete bilateral ophthalmoplegia with exotropia was possibly due to an oculomotor nuclear palsy along with bilateral involvement of the FPS/OTP or the DCP or the De-INF pathways.[11] Bilateral ptosis and impaired pupillary reactions with a left oculomotor palsy are suggestive of a nuclear 3rd palsy. The subsequent partial improvement of right eye movements was probably due to the fact that the lesion extended only to the midline oculomotor nucleus, sparing the oculomotor fascicles on the right side.

Although VOR is usually preserved in supranuclear lesions, only caloric testing could elicit ocular abduction in our case. Low-frequency sustained vestibular stimulus (caloric testing) is known to be more robust than high-frequency VOR in eliciting horizontal or vertical reflex eye movements.[12] In conclusion, a single strategic paramedian midbrain infarct can produce bilateral complete ophthalmoplegia with a lesion involving the 3rd nerve nucleus and the descending mesencephalic pathways that influence the horizontal gaze center or the abducens nucleus in the pons.

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