96Chapter 6: Failure of pattern recognition

proximity of the cavernous sinus, superior orbital fissure and orbital apex, any pathologic process can begin in one of these regions and then spread to involve neighboring structures, as in the above case. The patient’s initial diplopia was due to tumor in the superior orbital fissure which subsequently extended into the orbital apex producing optic nerve compression.

Disease processes that cause the orbital apex syndrome include primary and metastatic orbital neoplasms, tumors invading the orbit from adjacent structures (sinuses, nasopharynx and sphenoid wing), ischemia and a variety of inflammatory disorders such as mucormycosis, aspergillosis and herpes zoster. In most cases of orbital disease due to any of these mechanisms, the presence of orbital signs, particularly proptosis, helps to localize the disease process. At the orbital apex, however, there is so little extra room that even a small mass can produce significant visual loss in the absence of proptosis. In addition, this area is notoriously difficult to visualize on radiographic studies. MRI is superior to CT in the evaluation of soft tissue lesions in the posterior orbit, but even with a high quality scan the responsible abnormality may difficult to appreciate, particularly at low field or if fat suppression is not used. Familiarity with the pattern of clinical findings in this syndrome should aid in its recognition.

Diagnosis: Orbital apex syndrome

Tip: The combination of ipsilateral ophthalmoplegia and optic neuropathy is a distinctive pattern indicating a lesion at the orbital apex.

Painless diplopia

Case: This 14-year-old girl had a six-month history of double vision on left gaze, unassociated with headache or eye pain. She had been generally healthy and reported no prior history of head trauma, diplopia or ptosis. On examination, eye movements were full except for moderate limitation of adduction in the right eye. The right pupil was 2 mm larger than the left pupil and its direct and consensual response to light stimulation was a bit sluggish. The eyelids were aligned in primary position but changed on gaze to either side (Figure 6.6). There were no other focal neurologic deficits. CT and MRI were read as normal.

What is this motility pattern, and

what does it tell you about the mechanism of the patient’s diplopia?

This patient exhibits lid retraction on adduction, which represents a synkinesis between third nerve neurons. This is an example of third nerve misdirection, and it indicates mechanical damage to the peripheral oculomotor nerve. Her radiographic studies were critically re-evaluated in light of this rather subtle finding, and a very small mass was identified within the right cavernous sinus (Figure 6.7). This lesion was compatible with either a meningioma or schwannoma and she was managed expectantly. Over the next few years her third nerve palsy showed slow additional progression.

Discussion: Third nerve misdirection, also termed aberrant regeneration or oculomotor synkinesis,

occurs when stimulation of one branch of the third nerve results in co-activation of another third nerve branch. The various patterns of misdirection can be predicted by “mixing and matching” any two third nerve muscles. The most commonly recognized form of oculomotor synkinesis is lid retraction on adduction (as seen in this patient) or on downgaze (Figure 6.8). When third nerve misdirection involves the pupil, there is a poor response to light stimulation but constriction on attempted adduction or vertical gaze. While this occurs more commonly than aberrant lid innervation, it is technically more difficult to detect, often requiring the use of a slit-lamp. In any of its forms, third nerve misdirection is easy to overlook because its demonstration requires doing one thing while watching another, for instance having the patient look to the side but observing the lids. In a patient with a third nerve palsy, particularly one in which the diagnosis is in question, it is important to specifically look for

misdirection after assessing range of motion, pupillary responses and lid position and function.

The mechanism underlying oculomotor synkinesis is misdirection of regenerating axons with subsequent faulty reinnervation of target muscles. The significance of this phenomenon is that it indicates mechanical damage to third nerve axons. This kind of injury is common after aneurysmal rupture or head trauma, and in this setting is sometimes referred to as “secondary” aberrant regeneration. In contrast, “primary” aberrant regeneration indicates the presence of oculomotor synkinesis without a previous history of third nerve palsy. Patients who present with primary misdirection almost always harbor a compressive lesion, typically an intracavernous carotid aneurysm or a meningioma, as in the case under discussion. Thus, the finding of lid retraction on adduction in this patient strongly indicated the presence of an occult structural lesion in contact with the third nerve, which was ultimately

Figure 6.8 Aberrant regeneration of the third nerve following trauma. Four months after transtentorial herniation due to a subdural hematoma, this patient exhibits lid retraction on downgaze. This distinctive finding was the only residual sign of his previous third nerve palsy.

demonstrated on her MR scan. Recognition of this particular pattern of ocular motor dysfunction can be the key to the detection of small or subtle lesions.

Diagnosis: Oculomotor nerve palsy with aberrant regeneration

Tip: For practical purposes, oculomotor synkinesis always indicates mechanical damage to the third nerve. The presence of oculomotor synkinesis without a previous history of acute third nerve palsy indicates a compressive lesion.

Right-sided visual field loss

Case: A 67-year-old, hypertensive, diabetic homemaker experienced sudden onset of difficulty seeing to the right. A brain MRI showed a number of scattered hyperintensities on T2-weighted images, which were interpreted as non-specific small vessel disease (Figure 6.9A). She was told that her scan had

ruled out a stroke. Subsequent neuro-ophthalmic examination included Goldmann visual fields that showed a right homonymous sectoranopia (Figure 6.9B).

What is the significance of this visual field pattern? Does it help to illuminate the findings on her MRI?

This unusual wedge-shaped homonymous defect with its apex pointing to fixation is termed a homonymous horizontal sectoranopia. It constitutes a distinctive pattern of field loss produced by a vascular injury to the lateral geniculate body. Although the patient’s MRI shows a number of white matter lesions consistent with small vessel disease, the one in the left lateral geniculate body represents an acute infarct, which was the cause of her visual loss.

Discussion: Axons from the retinal ganglion cells synapse in the lateral geniculate body (LGB) to form the geniculocalcarine radiations. This nucleus is a cap-like structure that is located in the posterior aspect of the thalamus, below and lateral to the pulvinar and above the lateral recess of the ambient cistern (Figure 6.10A). The dorsal region of the nucleus subserves macular function, the lateral and medial aspects subserve the superior and the inferior fields respectively (Figure 6.10B). The LGB has a dual blood supply: the lateral choroidal artery (branch of the posterior cerebral artery) supplies the macular zone, whereas the anterior choroidal artery (a branch of the internal carotid artery) supplies the lateral and medial horns. As a consequence of this anatomic arrangement, occlusion of either vessel can produce a distinctive visual field pattern. Infarction in the territory of the lateral choroidal artery results in a congruous, wedge-shaped defect termed a “horizontal sectoranopia”, as in the above case. Occlusion of the anterior choroidal artery produces a complementary pattern, termed a “quadruple sectoranopia” (Figure 6.11).

Despite rare case reports of horizontal sectoranopia due to a lesion of the geniculocalcarine