Does Dolichoectasia of the Carotid Artery Cause Optic Neuropathy?

Point Counter-Point Section Editors: Andrew G. Lee, MD Gregory Van Stavern, MD Does Dolichoectasia of the Carotid Artery Cause Optic Neuropathy? Valerie I. Elmalem, MD, Valerie A. Purvin, MD There is ongoing debate regarding whether dolichoectasia can cause optic neuropathy, or whether this represents an incidental neuroradiologic finding. Two experts discuss this issue. Pro: Valerie I. Elmalem, MD Dolichoectatic intracranial vessels have abnormal configuration with tortuosity and distension. Symptomatic vascular compression by dolichoectatic arterial vessels is a well-known phenomenon that may cause variable symptoms and signs such as hemifacial spasm, downbeat nystagmus, lateral medullary syndrome with the ocular tilt reaction, and sixth nerve palsy (1). Auditory and vestibular findings have been reported with involvement of the vertebrobasilar (V-B) vessels. The main focus of this discussion will be dolichoectasia of the internal carotid artery (ICA) and other surrounding vessels causing compression of the anterior visual pathways. In 1997, Jacobson et al (2) reported that among 100 patients, anatomic contact between the optic nerve and ICA was common (70%). Anatomic compression, however, was rare (12% bilateral and 5% unilateral). Freeman and Newman (3) described a case of presumed vascular compressive optic neuropathy that deteriorated after surgical decompression and slowly improved with subsequent steroid treatment. Jacobson (4) argued that this might have been due to ischemia of the compressed optic nerve with slow recovery. He proposed that the mechanism of optic neuropathy with vascular compression included direct compressive injury and chronic ischemia due to compromised regional perfusion. The characteristics of a dolichoectatic ICA are not well defined. Jacobson et al (2,4) found that increased diameter of the ICA correlated with degree of anatomic compression of the optic nerve. The mean diameter of the artery in his symptomatic patients was significantly greater at 5.1 vs 3.8 mm in asymptomatic patients. On neuroimaging, the dolichoectatic ICA often was in contact with the anterior visual pathway with asymmetric displacement and compression. Department of Ophthalmology (VIE), SUNY Downstate Medical Center. Brooklyn, New York; and Midwest Eye Institute (VAP), Indianapolis, Indiana. The authors report no conflicts of interest. Address correspondence to Valerie I. Elmalem, MD, 451 Clarkson Avenue, Box 58, Brooklyn, NY 11203; E-mail: [e-mail] 368 Golnik et al (5) reported further magnetic resonance imaging (MRI) characteristics in patients with unexplained optic neuropathy. Patients were excluded if they had symptoms or findings of other identifiable etiologies, age was less than 46 years at onset of symptoms, or if there was a history of optic disc cupping. Extensive workup to evaluate for other causes of optic neuropathy was performed. Among the 20 patients evaluated, the distance between the optic nerve and ICA flow void on digitized coronal magnetic resonance images was significantly less on the atrophic side (mean 0.42 mm) compared with the normal side (mean 1.04 mm) and control group (mean 0.96 mm). Men and women were equally affected and average age was 63 years. Visual loss was slowly progressive in most of the patients and median visual acuity was 20/30 (range: 20/20-4/200). Contrary to Jacobson's study, Golnik et al found that the affected ICA flow void diameter was not significantly different from the normal side or control group (mean 3.75 mm on the atrophic side, 4.03 on the normal side, and 4.20 in controls). One limitation of the study, however, was that flow void measurement did not include arterial wall thickness, which could also theoretically contribute to compression of the optic nerve. The study by Golnik et al (5) highlighted that a normalappearing ICA can potentially cause symptomatic compression of the optic nerve if the anatomic distance is significantly less than 1 mm. This anatomic relationship is best seen on T1-weighted coronal images with a slice thickness of 3 mm and gap of 0.3 mm (4). I find the thin-section coronal T2 steady-state free procession sequence to be especially helpful in visualizing the vascular-nerve anatomic contact (Fig. 1). Unless alerted to the possibility of compressive optic neuropathy, neuroradiologists may not mention this asymmetric anatomic variation in their report. In most of the reported cases of dolichoectatic arterial compression of the anterior visual pathways, visual loss progressed slowly over several years and rarely required surgical intervention (5-9). Jacobson (4) reported the largest series to date with 18 patients and 24 affected eyes, and Elmalem and Purvin: J Neuro-Ophthalmol 2018; 38: 368-374 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Point Counter-Point FIG. 1. Thin-section coronal T2 steady-state free procession sequence shows a dolichoectatic carotid artery (arrows) compressing the left optic nerve (arrowhead). median age of 72 years without sex predilection. Vascular risk factors were common including hypertension (56%), hypercholesterolemia (28%), cigarette smoking (28%), previous unrelated stroke (22%), diabetes mellitus (11%), and cardiac disease (11%). Dolichoectasia of the ICA was present in 4/18 patients, the V-B system in 2/18, and both in 5/18 cases. Consistent with other reports, visual loss was slowly progressive over several years in most cases. Only 1 patient of the 18 underwent neurosurgical decompression, which resulted in stabilization of visual acuity. Contrary to previous reports of fusiform aneurysms (10,11), only half of Jacobson's patients had abnormally wide and elongated (dolichoectatic) arteries and he proposed that progressive increase in the diameter of the artery as occurs with aging or hypertension leads to compression in anatomically susceptible patients. In another series, Purvin et al (6) reported 10 patients with 16 eyes experiencing compression of the anterior visual pathway by a dolichoectatic artery. The most common cause of compression was the ICA (7/10 patients) followed by the basilar artery (3/10 patients). There have been other reports of the V-B system compressing the anterior visual pathways displaying similar patient demographics and characteristics (12-15), as well as a previous report of compression of the optic chiasm by the anterior cerebral arteries (9). In the study by Purvin et al (6), there was significant progression of visual loss in 2/10 patients and one had successful surgical decompression resulting in subsequent improvement. Most of the patients were women aged older than 70 years. The 2 patients who progressed were younger and male. Colapinto et al (15) reported a 34-year-old woman with progressive visual loss over 2 weeks with 20/40 visual acuity in her right eye and markedly constricted visual field in the affected eye. Investigation revealed elevation and Elmalem and Purvin: J Neuro-Ophthalmol 2018; 38: 368-374 compression of the right optic nerve and chiasm by the unfolded siphon of the ICA. The patient had surgical decompression with unroofing of the bony canal and incision of the dural sheath. Visual acuity recovered to 20/30 with marked improvement of the visual field. In another case reported by Uchino et al (16), a 71-year-old woman with progressive visual field loss from ICA compression of the right optic nerve was treated with surgical decompression. She had an uncomplicated postoperative course and visual field improvement persisted during several months after surgery. An older case report and review by Nonaka et al (17) described decompressive surgery resulting in 41% of 34 operated cases compared with 27% of 26 nonoperated cases improving. Often, patients who are later found to have vascular compression are initially thought to have "low-tension glaucoma" (4,8). Gutman et al (8) studied 62 patients with classic signs of low-tension glaucoma. Computed tomography (CT) neuroimaging demonstrated that 56/62 patients, or 90.3%, had abnormal intracavernous carotid arteries contacting and displacing the optic nerve adjacent to the intracranial segment of the optic canal compared with 21% of age-matched controls who displayed this finding. There was asymmetric cupping in 28 patients (45.2%), which correlated with the severity of ICA abnormality (calcification, dilatation, and ectasia). In Jacobson's series (4), there were 21/24 affected eyes with glaucomatous features of the optic disc. Normaltension glaucoma was initially suspected in 4/18 patients who later developed atypical optic disc appearance or progression of visual field defects with visual acuity loss despite adequate control of intraocular pressure. In all the series of dolichoectatic compression of the anterior visual pathways, patients with known diagnosis of glaucoma and history of elevated intraocular pressure were excluded. These patients are likely at risk for more rapid progression of visual loss if they also have dolichoectatic ICA compression of the optic nerve. In my practice, infrequently I have seen cases of markedly asymmetric glaucoma or significant progression of visual field loss with central scotoma, despite longstanding control of intraocular pressure. Asymmetric vascular contact/compression of the affected optic nerve was identified on MRI with otherwise negative workup. The progression on visual fields may be misinterpreted as glaucoma progression and these patients may undergo very aggressive yet unnecessary IOP-lowering surgical interventions. Although the literature on this topic is limited to case reports and small case series, I believe that dolichoectatic arterial compression of the anterior visual pathways is more common clinically than is reported in the literature. Most patients can be managed conservatively, but some with more rapid progression may benefit from surgical intervention to stabilize or improve vision, if the benefits of surgery outweigh the risks for the individual patient. 369 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Point Counter-Point Con: Valerie Purvin, MD Definition Visual loss The term dolichoectasia is derived from "dolichos" meaning elongation and "ectatic" meaning distention, reflecting the presence of both these anatomic distortions in most cases. These vascular abnormalities are alternatively referred to as fusiform aneurysms (distinguishing them from the saccular type in which a neck separates the distention from the parent vessel) and sometimes described simply as vascular tortuosity or ectasia. Dolichoectatic aneurysms occur on a spectrum, ranging from small fusiform dilations of a single vessel to giant aneurysms filled largely with thrombus. The latter are sometimes referred to as giant serpentine aneurysms. Visual loss attributed to dolichoectatic compression was first described in 1932 by Caramazza (21). Four additional cases and a review of 10 cases from the literature were reported in 1956 by Mitts and McQueen (22). In all 14 patients, the diagnosis was established by exploratory craniotomy. With improvements in MRI, allowing for simultaneous visualization of arteries and optic nerves with high spatial resolution, such cases could be diagnosed without surgical intervention, leading to a renewed interest in this area. Gutman et al (8) studied the CT scans of 62 patients diagnosed with low-tension glaucoma and found that 90% had calcification or dilation of the adjacent ICA compared with only 21% of controls (8), concluding that chronic compression was a cause of optic disc cupping and visual loss. By contrast, Stroman et al (23), using MRI rather than CT, found no difference in the distance between the ICA and the optic nerve (ON) in 20 such patients vs controls (23). Furthering this controversy, Golnik et al (5) studied 23 optic nerves of 20 patients with unexplained optic neuropathy and found significantly less distance between the ON and ICA than in controls (5), again suggesting a causal relationship of dolichoectasia to optic neuropathy. Although advances in neuroimaging allowed for a more accurate picture of the relationship between the ICA and the optic nerve in patients with visual loss, the significance of these findings remained difficult to interpret without more information regarding this relationship in normal individuals. In an effort to fill this lacune, Jacobson (4) studied this anatomic feature in a series of asymptomatic patients. He examined the T1 coronal images of 100 patients, grading them as 0 (no contact), Grade 1 (contact without distortion), or Grade 2 (contact with distortion- i.e., compression). He found no contact in 13%, contact on at least one side in 70%, and compression of one optic nerve in 5 patients with bilateral compression in 12. Thus, there was a surprisingly high frequency of contact but not of compression. The main limitation of the study was its retrospective design and, more specifically, its assumption that because patients scanned for visual loss were excluded, these asymptomatic individuals had normal optic nerve structure and function. Nevertheless, the study was very helpful for interpreting the finding of simple ON contact when evaluating a patient for unexplained optic neuropathy; the clinicians should continue searching for the cause of the visual loss. Prevalence Dolichoectasia of the intracerebral vessels is a rare disorder, accounting for ,0.1% of aneurysms in autopsy (18) and angiographic (19) studies. The prevailing wisdom is that the V-B system is more often affected than the internal carotid. However, this has not been borne out by more recent studies (11). It has been suggested that carotid ectasia may be more likely than V-B lesions to become symptomatic, thereby accounting for their higher representation in clinical series. In a review of autopsies at Mayo Clinic, only 7 (30%) of 23 patients with V-B ectasia were symptomatic (20). Etiology Dolichoectasia is typically caused by weakening of the arterial wall due to atherosclerosis. However, the condition is sometimes found in children and in conditions with mesodermal dysgenesis (e.g., neurofibromatosis type 1, fibromuscular dysplasia, Ehlers-Danlos syndrome). The arterial wall also can be weakened by infectious, inflammatory, or neoplastic disease and by dissection. Pathophysiology Fusiform aneurysms produce symptoms in a variety of ways, including compression of adjacent structures, impairment of circulation to neural tissue by pressure, thrombosis, by obliteration of nutrient vessels derived from the dolichoectatic arterial segment, and by giving rise to emboli. In contrast to saccular aneurysms, rupture (subarachnoid hemorrhage) is uncommon. Specific signs and symptoms are determined by location, size, and extent. Close approximation of the ICAs to the prechiasmal optic nerves creates the possibility of visual loss due to dolichoectatic compression. How often this actually occurs is still unclear. Rare cases in which dramatic dilation of the tip of the basilar artery compresses the chiasm also have been described (6). 370 Elmalem and Purvin: J Neuro-Ophthalmol 2018; 38: 368-374 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Point Counter-Point Does dolichoectasia cause visual loss? The difficulty in establishing a causal role for dolichoectatic ON compression is, in part, due to the inexact correlation between the radiographic and the clinical findings. This issue was discussed by Jacobson and Corbett (24) in relation to a case of downbeat nystagmus associated with dolichoectasia of the vertebral artery They noted that some patients have a more severe degree of compression by other (nonvascular) abnormalities without similar deficits, and other individuals exhibit more severe dolichoectatic compression without clinical manifestation. There must be other factors at play. These authors suggested that perhaps the exact site of the compression is important or possibly a combination of compression plus ischemia is required to produce neurologic deficits. Another confounding aspect is that the definition of a dolichoectatic ICA remains "arbitrary and subjective," as pointed out by Jacobson. In his series of patients with presumed ON compression by the ICA, half of the involved eyes did not actually demonstrate a dolichoectatic artery, despite clear radiographic evidence of compression by the adjacent ICA. Similarly, in a case with bitemporal hemianopia reported by Bergaust in 1963 (25), surgical exploration revealed compression of one optic nerve and the chiasm by a nondolichoectatic ICA. Neural compression by a nondolichoectatic vessel is well established as a cause of trigeminal neuralgia and other forms of cranial nerve irritability but the validity of these syndromes was only established after multiple cases of successful surgical treatment (26). An additional source of difficulty in establishing a causal connection concerns the limitations of clinical diagnosis for optic neuropathies in general. For example, a patient with visual loss and a pale optic disc may have suffered a previous episode of anterior ischemic optic neuropathy. Unless examined during the acute stage, when disc edema is present, this diagnosis may be difficult to establish. Similarly, the diagnosis of glaucoma is essentially a clinical one, particularly for those cases in which intraocular pressures are normal. Although there are clinical features that help to differentiate glaucomatous from nonglaucomatous cupping, they are not definitive. Perhaps the clinical features of dolichoectatic compressive optic neuropathy are sufficiently distinctive as to establish the diagnosis. In addition to individual case reports, 2 studies have looked at these features in a series of patients in whom the compression was so marked and the evidence for another cause of visual loss was so scant that a causal assumption seemed justified. Jacobson (4) reported the findings in 24 eyes of 18 such "highly selected" cases. In this series, age at diagnosis ranged from 28 to 86 years with a median of 72 years; men and women were equally affected. Patterns of visual field loss included arcuate, altitudinal, and paracentral Elmalem and Purvin: J Neuro-Ophthalmol 2018; 38: 368-374 defects, central or cecocentral scotomas, and, rarely, bitemporal or homonymous loss. Most eyes demonstrated optic disc excavation. In a series involving 10 such patients, the average age at presentation was similarly 71 years, there was a female predominance (80%), and patterns of visual loss were similarly varied (6). In both series, progression of visual loss was impressively slow in almost all patients. Reviewing these features, it does not seem that a clear clinical picture emerges by which we might confirm a diagnosis of arterial compressive optic neuropathy. Another approach to the question of causality is to look at the effects of decompression. Significant improvement of optic nerve function after surgery would be convincing evidence for a causal connection. Is surgical treatment beneficial? A variety of surgical techniques have been used in the neurosurgical management of these vascular lesions (3). Treatment options include clip reconstruction of the parent vessel, isolating the segment of artery by ligation, balloon or coil, resection with reanastomosis, trapping with bypass, and muslin wrapping. Anticoagulation has a role in occasional cases in which emboli are a threat. To evaluate treatment options, comparing one with another and with conservative management would ideally require a randomized, prospective, controlled study. Given the rarity of this disorder, such studies are not feasible. For example, the 1996 report by Anson et al (11) described the surgical results in 40 patients with symptomatic fusiform aneurysms, of which 21 involved the anterior circulation only one of whom presented with progressive optic neuropathy. Furthermore, case series reporting surgical outcomes are biased toward patients who were referred for surgery. This will tend to select cases with more severe and progressive symptoms. In our series, we found that most patients with dolichoectatic compression of the anterior visual pathways experienced tolerable symptoms and demonstrated only mild, very slow progression (6). In comparing different forms of treatment, ideally each case would be treated with the same, single surgical technique. In reality, different techniques often are combined. For example, in our single patient who underwent surgery, the optic canal was unroofed and the offending carotid artery was moved away from the optic nerve. There was no question that optic nerve function improved postoperatively but no way to tell which aspect of the surgical treatment was responsible. Similarly, in the series of 4 cases reported by Mitts and McQueen (22), the one patient in whom vision improved postoperatively underwent both internal carotid ligation and unroofing of the optic canal. 371 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Point Counter-Point Rebuttal: Dr. Elmalem Although it is true that this entity needs to be studied further and remains a diagnosis of exclusion, the current literature does suggest a causal relationship of ICA compression with optic neuropathy, even if dolichoectasia is not present. This is likely unique to the regional anatomic vulnerability of the optic nerve. As mentioned by Dr. Purvin, presence of additional factors that make the arterial wall less compliant, in addition to ischemia, may be what is necessary to produce symptomatic compression. In Jacobson's study that found a surprisingly high rate of contact but not compression (2), he proposed that there must be risk factors that alter the rigidity of the arterial wall and the dynamics of contact between the artery and optic nerve. Symptomatic patients in his study had vascular risk factors including hypertension, hypercholesterolemia, cigarette smoking, previous unrelated stroke, diabetes mellitus, and cardiac disease. Gutierrez et al (27) found that the Circle of Willis can adapt its configuration in response to certain vascular risk factors (diabetes mellitus and hypertension), and that the presence of hypoplastic vessels in the posterior circulation increases the risk for dolichoectatic vessels in the anterior circulation. The mean age of affected patients in the literature (63-72 years) also is suggestive of aging changes in the arterial wall playing a role. There may not be much distortion of the ICA necessary to produce a visual deficit if dilative arteriopathy is present. We have learned from numerous reports of compressive irritation of other cranial nerves (trigeminal and facial) that neurosurgical decompression of nondolichoectatic vessels causes improvement of symptoms (26,28). This infers that the ICA need not be dolichoectatic to result in optic neuropathy, and potentially improve with surgical decompression. Dr. Purvin suggested that neurosurgical decompression has limited efficacy in treating ICA compression of the optic nerve. On the contrary, there have been several case reports of neurosurgical optic nerve decompression that resulted in stabilization and even improvement in optic nerve function (15,29-32). Earlier reports of neurosurgical interventions focused on the vessel itself rather than the dynamic contact between the vessel and the optic nerve (11,22). Decompression of the bony canal and falciform ligament to allow for more compliance/movement of the optic nerve in response to the adjacent ICA pulsations likely contributes to improved outcomes, as shown in Figure 2 (30). Success is most likely affected by timing and technique of intervention, as well as patient selection. Younger patients with rapid progression seem to be more ideal candidates for surgical management. Clearly, the efficacy of neurosurgical decompression needs to be studied further, but is limited by the infrequent detection of this condition as well as the possible morbidity associated with the neurosurgical procedure. In summary, compressive optic neuropathy from an adjacent ICA with abnormal configuration should be considered once other possible etiologies are excluded. Elderly patients with vascular comorbidities are more commonly affected and the clinical course is usually slowly progressive over several years. Younger patients tend to have a more rapidly progressive course and this subset of patients may benefit from surgical decompression. FIG. 2. Optic nerve decompression. A. A semicircular incision is made in the dura of the anterior clinoid and planum sphenoidale. B. The bony roof of the optic canal is removed and the falciform ligament is opened. Reproduced with permission from (30). 372 Elmalem and Purvin: J Neuro-Ophthalmol 2018; 38: 368-374 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Point Counter-Point Rebuttal: Dr. Purvin In presenting the evidence for a causal role of intracranial dolichoectasia in the production of optic neuropathy, it is interesting how much overlap there is in the references cited. We may look at the same data and yet reach different conclusions. Some of the limitations of these various studies are presented above; others may be less apparent. For example, in the study of Nonaka et al (17) cited by Dr. Elmalem, 41% of operated cases showed improvement of optic neuropathy vs only 27% of nonoperated cases. This apparent response to surgery certainly seems to support a causal role for dolichoectatic compression. However, we have to wonder what kind of compressive neuropathy shows such significant spontaneous improvement. Although such cases have been reported, they are rare. How certain can we be as to the correct diagnosis and the mechanism of visual loss in this group of patients? Dr. Elmalem points out that individuals with glaucoma and/or elevated intraocular pressure are typically excluded from studies of this subject, noting that these patients "are likely at risk for more rapid progression of visual loss if they also have dolichoectatic ICA compression of the optic nerve." This statement is provocative but not supported by a reference, presumably because we do not really know if this is true. The idea that there is an additive quality for different risk factors for optic neuropathy, for example, compression and intraocular pressure, is commonly assumed but not really proven and may be not be correct. Further study of this question would be invaluable. Summary (Andrew G. Lee, MD and Greg Van Stavern, MD) Although dolichoectasia of the ICA is a known and relatively commonly seen radiographic finding, contact with the optic nerve alone seems an unlikely mechanism for compressive optic neuropathy in otherwise unexplained optic atrophy. Dolichoectasia in other neuroophthalmic conditions (e.g., hemifacial spasm) can result in nerve dysfunction but generally requires mass effect and visible radiographic compression to be credibly hypothesized as the source of the neuropathy. We believe that dolichoectasia of the ICA with secondary compressive optic neuropathy should be considered in the differential diagnosis of unexplained optic atrophy but should be a diagnosis of exclusion. 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