Most cases of ischemic optic neuropathy are of sudden onset without any prior indication of visual field loss. In contrast, a gradual loss of the visual field is observed in normal-tension glaucoma (NTG). We here present a case in which we initially diagnosed NTG but upon further examination it was evident that ICA hypoplasia was present.
Horowitz et el. previously reported an uncommon case of anterior ischemic optic neuropathy that was associated with hypoplasia of the interanal carotid aretry (ICA) (Horowitz et al., 2001). Here, we report on a curious case in which clinical expression mimicked NTG, but upon further examination there was considered have been an ischemic event due to ICA hypoplasia that was responsible for the noted visual field change.
2. Case report
In 2006, a 41-year-old female visited an ophthalmologist for the purpose of purchasing contact lenses. During the contact lens examination the patient was found to have excavation of the right optic disc. In the past, she had not been aware of any visual dysfunction, including visual blurring, visual field loss or transient monocular blindness. After being diagnosed as having glaucoma treatment was immediately begun. However, the ophthalmologist in charge of the treatment considered her visual field to be atypical and that her disc morphology was not compatible with the observed field defect as is the case in glaucoma.
Therefore, in February of 2007, 1 year after her original examination, she was referred to our hospital. Upon initial presentation at our clinic, her visual acuity (VA) was 20/20 OU. She had a right relative afferent pupillary defect (RAPD) and her intraocular pressures (IOP) were 12 mmHg OD and 10 mmHg OS. Biomicroscopic examination showed normal findings in both eyes. Fundoscopy revealed large disc excavation in her right eye with an almost normal left eye, and there was chorioretinal atrophy in both eyes (Fig. 1). Goldmann perimetry indicated there was a multidirectional, irregular defect at the temporal and nasal quadrants of her right eye (Fig. 2).
Irregular depression at nasal and inferior quadrant of right eye and normal field of left eye on Goldman perimetry.
Over a 3.5-year period, there was no further significant progress noted for this multidirectional and irregular visual field defect. Fluorescein angiography (FA) showed neither leakage nor hyperfluorescence. The retinal nerve fiber layer thickness, as measured by optic coherence tomography (OCT), showed diffuse thinning in her right eye. Borderline results were found for the Moorfields Classification, which used the Heidelberg retina tomography (HRT).
Since she had a RAPD and her visual field and optic disc findings were not typical of glaucoma, we performed magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) of her head. MRA revealed there was approximately a 50% stenosis of the ophthalmic portion of the right internal carotid artery (ICA) (Fig. 3).
MRA showing approximately 50% stenosis of the right ICA of ophthalmic portion and slight narrowing of the right anterior cerebral artery on brain MRA.
However, there were no abnormality of the right anterior cerebral artery. Three-dimensional computed tomography (3D-CT) indicated that there was a partial stenosis of the right cervical ICA, which was strongly suggestive of hypoplasia (Fig. 4).
A part of hypoplasia and stenosis of the right cervical ICA on 3D-CT.
Color Doppler imaging (CDI) was performed in order to determine the functional characteristics of the internal carotid artery stenosis. As shown in Figure 5, CDI showed a downstream reversed diastolic flow in the right ophthalmic artery. In normal waveforms, there is an initial sharp rise with a small notch noted on the down slope of the peak.
Note downstream of the right ophthalmic artery and normal upstream of the left ophthalmic artery on orbital CDI.
In the current case, there was an atypical pattern with a slow rising wave that lacked the notch found in the normal waveform. There was a normal typical flow in the left ophthalmic artery. These findings indicated that there was regurgitation of the right ophthalmic artery. This can be explained by a significant stenosis of the right ICA leading to a reverse flow in the right ophthalmic artery (Fujioka S et al., 2003).
To assess the optic disc change, we repeatedly conducted HRT during the year that followed. No significant changes were observed.
We also examined the patient using a Humphrey Field Analyzer (program 30-2 SITA standard test). The patient is currently being seen every 3 months. At the present time, she has completed 3.5 years of follow-up and there has been neither progression of the visual field defect nor any change in the disc appearance (Fig. 6).
It has been previously reported that ocular ischemic phenomena can result from disease of the cervical ICA: for example amaurosis fugax (AF), acute retinal artery obstruction, branch retinal artery obstruction (BRAO), ischemic optic neuropathy and the ocular ischemic syndrome where the entire globe shows evidence of poor perfusion. There are also case reports of normal tension glaucoma associated with chronic reduction of retrobulbar blood flow(Costa VP et al.,1998).
In the current case, the initial diagnosis was thought to be NTG, however the atypical disc and visual field defects were against this diagnosis. MRA and 3-D CT examinations suggested that there was hypoplasia of the right cervical ICA and orbital CDI of the right ophthalmic artery indicated reversed diastolic. These results suggested that the presumed ischemic optic neuropathy was associated with stenosis and hypoplasia of the right cervical ICA( Kawaguchi S et al., 1999).
Although the finding in the carotid artery could be incidental and unrelated to the optic cupping in this case, in our opinion the carotid artery abnormalities could have been
associated with ischemia of the nerve head at some time. There may be several other possibilities that could explain the ischemic ocular neuropathy including, the accumulation of subclinical ischemic events on the optic nerve, which is referred to as chronic progressive ischemic optic neuropathy (Knox DL et al.,1971), hypoperfusion of the ophthalmic artery (Burghaus L et al.,2007), ischemic optic neuropathy in childhood(Chutorian AM et al,.2002), and conventional anterior or posterior ischemic optic neuropathy that was not noticed by the patient.
Of these optic neuropathy-related ischemic events, only anterior optic neuropathy has been reported to be associated with ICA hypoplasia. It is common knowledge that there is a sudden onset for ischemic events. In the present case, however, for at least 3.5 years there has been neither significant progression of the disc morphology nor any visual field defect. Within this context, there are two possibilities that might explain our observations. One is that there is a very slow progressive ischemic syndrome that is present. This type of ischemic optic neuropathy has been reported as chronic progressive ischemic optic neuropathy, although at the current time there is still much dispute with regard to this entity (Lessell S et al., 1999).
Another possibility is that this type of ischemic optic neuropathy occurs during infancy. Since the patient did not have any subjective symptoms and the current clinical stage of the optic neuropathy appears to be non-active, it very well could be that the patient may simply have had no previous awareness of the initial ischemic event.
Thus, based upon the ICA congenital anomaly, we speculate that the ischemic optic neuropathy could even have occurred during infancy. An ischemic event of this type in the optic nerve could result in a slight enlargement of cupping (Saito H et al., 2008), as well as a non-progressive change in the visual field.
In the ocular ischemic syndrome there are reports that the flow dynamics of the ophthalmic artery can spontaneously improve (Ward JB et al., 1995). In the current case, CDI documented that the right ophthalmic artery exhibited a reversed flow, indicating that there was the normal flow, indicating that there was the normal collateral pathway from the external carotid artery to the right ophthalmic artery. It is our belief that this collateral pathway provided indispensable circulation.
Hypoplasia of the ICA is rare : less than 100 cases having been reported (Horowitz et al., 2001). Although this condition rarely occurs, the possibility that hypoplasia of the ICA could be the associated with an ischemic ocular syndrome that mimics NTG needs to be taken into consideration when diagnosing such patients.
There is no report of the same or a similar case like our report in the literature, but the neurovascular contact of ICA or the anterior cerebral artery has been described in the literature. Compression of the optic nerve by a fusiform enlagement of the internal carotid artery is originally described in Japanese (Miyake Y et al.,1978).
Subsequently, Uchino et al reported a 71-yaer old woman who had a unilateral visual field defect due to optic nerve compression by sclerotic internal carotid artery. This case underwent decompression surgery and then, the visual field was improved(.Uchino M et al.,1999). According to a retrospective neuroimaging study for reasons unrelated to loss of vision, optic neuropathy, or carotid artery disorders, the frequency of the artery relationship included contact one or both optic nerves in 70 (70%) of 100 patients. The anatomic compression of the optic nerve depended upon the diameter of the carotid artery (Jacobson DM., 1997).
Although uncommon, intracranial compression of the optic nerve by the carotid artery should be always considered when seeing unknown optic neuropathy(Jacobson DM.,1999).
Compression of the optic nerve seems to also occure by megadolichoectatic ophthalmic artery (Gire J.,2010).
The compression may be induced not by anterior cerebral artery (Nishioka T.,1995), but they also suggested the progressive optic neuropathie s due to compression by the plolapsing gyrus rectus (Nishioka T.,2004).
Longstanding compression of the intracranial optic nerve may produce a nerve fiber bundle pattern of visual field loss and excavation of the optic disc, two signs consistent with glaucoma. However most patients with these two signs had other features atypical of glaucoma, including pallor of the neuroretinal rim and impairment of visual acuity and color vision. The presence of such signs should alert one the possibility that a patient with visual field loss and excavation of the optic disc may be harboring a compression lesion, not suffering from glaucoma(Jacobson DM.,1999).
It is necessary to assess a condition of intracranial using thin-slice MRI or angiography, whenever the patients has a optic neuropathy of an unknown etiology or the case having a visual field changes like a glaucoma, but does not agree with clinical findings.