Although often misinterpreted in today's vernacular, the idiom “the exception that proves the rule” should never be disregarded in medicine as it results in a correct, albeit unexpected, diagnosis. From the Latin “exceptio firmat regulam in casibus non exceptis,” the legal interpretation of the idiom emphasizes that the exception to a rule proves the existence of that rule (1). As clinical practice has become increasingly complex, this idiom is ever more important as it compels us to question our decisions and diagnoses when clinical evidence is not always straightforward.
We present 3 guiding principles in neuro-ophthalmology with an illustrative case that violates each principle. These exceptions prove that indeed these rules do exist.
Bitemporal Hemianopia Respecting the Vertical Midline Indicates an Optic Chiasmal Syndrome
Case 1: A 59-year-old hypertensive woman was referred to the neuro-ophthalmology clinic for evaluation of bitemporal visual field defects. On examination, visual acuity with myopic correction was 20/25 in the right eye and 20/20 in the left eye. Color vision using Hardy-Rand-Rittler (HRR) pseudoisochromatic plates was 10/10 bilaterally. Automated perimetry showed mild but definite bitemporal defects that obeyed the vertical midline (Fig. 1). Kinetic perimetry using a Goldmann perimeter confirmed bitemporal scotomatous field defects (Fig. 2). Pupils were normally reactive to light stimulation; there was no relative afferent pupillary defect in either eye. Slit-lamp biomicroscopy revealed minimal cataracts in both eyes, and intraocular pressures were normal. On ophthalmoscopy, there were tilted optic discs with extensive peripapillary nasal and inferior chorioretinal atrophy (Fig. 3). Brain magnetic resonance imaging (MRI) showed a small enhancing lesion involving the left anterior clinoid process, most likely a meningioma, with no chiasmal compression. It also showed bilaterally elongated globes. The patient has been followed for more than 2 years with no changes in her clinical examination, no progression of visual field defects, and no change in the size or shape of the intracranial lesion.
Comment: Visual field defects that respect the vertical meridian usually indicate a neurological process. In the case of bitemporal hemianopia, one must suspect a process compressing or infiltrating the optic chiasm and proceed with neuroimaging (2). Sellar and suprasellar lesions, including pituitary adenoma, meningioma, craniopharyngioma, and aneurysm, are known to produce bitemporal hemianopia. Bitemporal hemianopia is also a finding in tilted disc syndrome, characterized by inferior or inferonasal tilting of the optic disc, segmental hypoplasia and ectasia of the choroid and retinal pigment epithelium, and situs inversus of the retinal vessels (3). The superotemporal visual field defects associated with tilted disc syndrome typically do not respect the vertical midline (4). However, Sowka and Luong (5) have reported a series of 5 patients with tilted disc syndrome and bitemporal field defects that seemingly respected the vertical midline on frequency doubling threshold perimetry. Similar to our patient, neuroimaging in these patients did not show any intrinsic or compressive lesion involving the optic chiasm. These authors suggested repeating the perimetric studies using an alternate device and test strategy that may be more sensitive in detecting neurological field defects. Indeed, manual kinetic perimetry is superior to automated static perimetry in distinguishing bitemporal hemianopia due to an intraocular process from a neurological disorder (6). Even though a left anterior clinoid lesion was present in our patient, it was not responsible for her bilateral visual field defects; rather, the bitemporal hemianopia was a result of her tilted disc syndrome as opposed to a chiasmal process, making this case an exception that proves the rule.
Slowly Progressive Unilateral Visual Loss Associated With a Retrobulbar Optic Neuropathy Is Caused by a Compressive Lesion
Case 2: A 33-year-old woman complained of slowly progressive visual loss in her right eye over the previous 4 months. Her medical history was unremarkable, except that 1 year earlier, she had experienced a 3-week period of numbness of her left arm and leg that spontaneously resolved. On examination, her visual acuity was 20/70 in the right eye and 20/15 in the left eye. Color vision using HRR plates was 2/10 in the right eye and 10/10 in the left eye. Automated perimetry revealed a moderate reduction in sensitivity of the right visual field; the field of the left eye was full. There was a right relative afferent pupillary defect. Ocular motility was full, with smooth pursuit and accurate saccades. The patient was orthophoric at distance, near, and in the cardinal positions of gaze. Anterior segment findings and intraocular pressures were normal. The right optic disc was mildly pale; the left optic disc appeared normal. Optical coherence tomography of the peripapillary retinal nerve fiber layer (PRNFL) showed thinning in several sectors in the right fundus, with an average thickness of 84 µm. The thickness of the PRNFL on the left was normal in all but 1 sector, with an average thickness of 102 µm. Brain MRI revealed multiple, ovoid, periventricular white matter lesions on fluid-attenuated inversion recovery images, some of which enhanced after intravenous administration of gadolinium (Fig. 4). A subsequent evaluation that included lumbar puncture established a diagnosis of multiple sclerosis (MS).
Comment: Slowly progressive unilateral visual loss associated with evidence of a retrobulbar optic neuropathy almost always is due to a lesion that compresses the posterior orbital, canalicular, or intracranial portion of the optic nerve (7). A high-quality MRI of the brain and orbit with gadolinium is recommended in such cases. Chronic demyelinating optic neuritis can also cause slowly progressive, painless vision loss in 1 or both eyes in patients with established MS. Nevertheless, it is a diagnosis made only after exclusion of any compressive lesion to the anterior visual pathway and alternative MS mimics (8). In rare cases, chronic demyelinating optic neuritis can be the presenting finding in a patient with an underlying neurological disorder (9,10), as was the case in our patient. This patient is an exception to the rule that slowly progressive monocular retrobulbar optic neuropathy is caused by a compressive lesion.
A Complete Third Nerve Palsy With Sparing of the Pupil Is Never Caused by an Aneurysm
Case 3: A 65-year-old hypertensive woman reported a 1-month history of complete ptosis of the right upper eyelid (this case has been previously reported ). She denied any headache or right orbital pain. On initial examination, the patient had a complete right third nerve palsy (TNP) (Fig. 5). The right eye had full abduction, and there was definite intorsion of the eye when the patient attempted to look down and to the right, indicating intact functions of the sixth and fourth cranial nerves, respectively. Both pupils measured 3 mm in light and 5 mm in darkness, and both constricted briskly to light stimulation. Results of the remainder of the ocular examination were normal. An edrophonium chloride (Tensilon) test resulted in no change in ocular motility, alignment, or ptosis. Topical 10% cocaine hydrochloride drops were placed in both eyes and produced equal dilation of both pupils. The erythrocyte sedimentation rate was normal, and a glucose tolerance test also gave normal results. A diagnosis of presumed ischemic TNP was made, and it was elected to observe the patient at regular intervals.
The patient returned 1 month later (2 months after the onset of ptosis), and repeat examination showed no change. The patient continued to deny headache, pain, or discomfort. Nevertheless, computed tomography was performed, which disclosed a large bilobed mass in the region of the junction of the basilar and posterior cerebral arteries (Fig. 6). A cerebral angiogram revealed an aneurysm measuring 25 mm in diameter originating from the tip of the basilar artery and projecting toward the right side (Fig. 6). The patient subsequently underwent clipping of the aneurysm, at which time, the oculomotor nerve was found to be compressed by the aneurysm. Unfortunately, the patient experienced a postoperative brainstem infarct that resulted in her ultimate demise.
Comment: According to the “rule of the pupil,” isolated complete TNP with pupil sparing in an elderly patient with vasculopathic risk factors is most likely from microvascular infarction of the peripheral nerve. Neuroimaging including angiography usually is not indicated unless there is no improvement in 2–3 months (12,13). However, TNP caused by intradural aneurysms is commonly associated with ipsilateral pupillary abnormality, that is, pupillary dilation and decreased velocity and extent of constriction (14). Although pupil-sparing third nerve palsies caused by aneurysmal compression occasionally occur, they are typically accompanied by incomplete ocular motility deficits. As ocular motility becomes maximally affected, the pupil invariably becomes involved (15,16). This patient violates the rule that intracranial aneurysms never cause a pupil-sparing complete TNP and, thus, is an exception that proves the rule.
In neuro-ophthalmology, as with most fields of medicine, one determines a diagnosis mostly by history and examination. Once evidence is gathered and is shuttled through the various clinical decision algorithms, appropriate ancillary tests are carried out and a final diagnosis is made. As there is no perfect process, clinical judgment remains of critical importance and much of it relies on certain tenets of neuro-ophthalmology.
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