[Pseudotumor cerebri: quantitative in-vivo measurements of markers of intracranial hypertension]. / Pseudotumor cerebri: Quantitative Normalwerte anatomischer Kennstrukturen im kraniellen MRT.

PURPOSE: Intracranial hypertension can change the morphology of anatomical structures that are critical in the evaluation of pseudotumor syndromes. The purpose of our study was to establish the normal range of such markers of intracranial hypertension and to consider a dependency on sex, age and body-mass index (BMI). MATERIALS AND METHODS: 123 persons without signs or symptoms of intracranial hypertension (63 females, 60 males, 18- 86-years-old, mean 49.5 SD17.8 years, mean BMI 25.3 SD42) were prospectively enrolled and MRI was performed at 3T. A STIR sequence in the coronal plane was used to measure the width of the optic nerve, the perioptic fluid rim and the total optic nerve sheath diameter in 4 different locations behind the eyeball. The height and width of the pituitary and Meckel's cave were also measured and the area was calculated. RESULTS: The mean width of the optic nerve sheath narrows significantly from anterior (mean 5.3 SD 0.6 mm) to posterior (mean 4.1 SD 0.4 mm), as does the perioptic fluid rim (mean 1.4 SD 0.3 mm vs. mean 1.0 SD 0.2 mm) and--to a lesser extent--the optic nerve itself (mean 2.4 SD 0.4 mm vs. mean 2 SD 0.3 mm, p=0.000 for all). There was no statistically relevant correlation of the width of the optic nerve sheath with age. The coronal area of Meckel's cave was independent of sex, age or BMI (mean 39 SD 9.3 mm2). The height of the pituitary differed little in women (mean 4.4 SD 0.9 mm) and men (mean 4.2 SD 0.8 mm), but we found a significant negative correlation with age in women only (r = -0.38, p = 0.01). CONCLUSION: The presented typical values and their deviations serve as a basis for the evaluation of pathologies in patients suspected of having pseudotumor syndrome.

Saturation of hemoglobin in intracranial arteries is similar in patients with hemodynamically relevant and irrelevant stenosis of the internal carotid artery.

The aim of this study was to establish if patients with hemodynamically relevant or irrelevant stenoses of the extracranial internal carotid artery have different intracranial arterial oxygen saturation as measured by transcranial pulse oximetry using near infrared spectroscopy. Patients with unilateral stenosis > 70% according to North American Symptomatic Carotid Endarterectomy Trial (NASCET) were included. Hemodynamic relevance was assessed using ultrasound criteria. Transcranial spectroscopy recordings were taken before and after surgical or interventional treatment of the stenosis. Optodes were placed bilaterally on the intact frontoparietal aspect of the skull. Oxygen saturation and diversion angle alpha from the hemoglobin plane were measured. There were no significant differences regarding arterial oxygen saturation between the two groups. Oxygen saturation ranged from 0.910 +/- 0.08 to 0.957 +/- 0.028 in the subgroups (all values as mean +/- S.E.). These values are consistent with previous studies and theoretical values. In smokers we found a significantly shifted diversion angle from the hemoglobin plane to the negative side. This indicates the presence of an absorber other than oxy- and desoxyhemoglobin in the optical field. We conclude that transcranial pulse oximetry cannot distinguish between patients with hemodynamically relevant and irrelevant stenosis of the internal carotid artery. However it seems to be capable of distinguishing smokers from non-smokers.

Reversibility of venous sinus obstruction in idiopathic intracranial hypertension.

Elevated intracranial intravenous pressure seems to be of importance in pseudotumor cerebri syndromes, either as a cause (secondary intracranial hypertension) or as a consequence (idiopathic intracranial hypertension) of increased intracranial pressure. We present 3 case reports in which diagnostic imaging before and after CSF diversion provided evidence that narrowing of the transverse sinuses is a secondary phenomenon. Stent angioplasty of the venous sinuses should not be considered a therapeutic approach in these cases.

Selective affection of hippocampal CA-1 neurons in patients with transient global amnesia without long-term sequelae.

The aetiology, pathomechanisms and anatomical correlates of transient global amnesia (TGA) still remain obscure. Recently, focal MR-signal diffusion-weighted imaging (DWI) changes in the hippocampus have been described in patients with TGA, but the exact localization, long term outcome and pathophysiological nature of these lesions still remain unknown. The topography and time course of hippocampal DWI lesions in 41 TGA patients was studied using serial 3 T high-resolution MR-imaging and correlated to clinical and neuropsychometric results. Of these, 29 patients showed 36 DWI lesions with corresponding T(2) lesions in the hippocampus within a time window of 48 h after onset. Almost all lesions (94%; 34/36) were selectively found in the CA-1 sector (Sommer sector) of the hippocampal cornu ammonis. Most DWI lesions (8/10) were already detectable in the peri-acute phase <6 h after onset of symptoms. A follow-up study 4-6 months after the episode did not show evidence for residual structural sequelae of these lesions (n = 20/20). A venous MR angiography of the intracranial dural sinus showed an asymmetric venous drainage in 21/24 (88%) patients. In 11/16 (69%) patients with unilateral lesions, the asymmetry corresponded to the side of the DWI lesion. Significant episodic verbal memory deficits in the acute phase (n = 14/18) were associated with lesions of the dominant hemisphere while impairment of visuospatial memory was associated with lesions of the non-dominant hemisphere. Persistent neuropsychological sequelae were not detected 4-6 months after the episode (n = 16). This is the first prospective study combining high-resolution imaging and neuropsychometry analysing the detailed functional anatomy and outcome of hippocampal DWI/T(2) lesions in TGA supporting the view the TGA being a benign transient disorder. The TGA can be considered a model for a focal transient perturbation of memory circuits in the temporo-mesial region.

MR spectroscopy in patients with intracranial meningiomas.

With magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS), we tried to get more pre-operative information in patients with suspicious meningioma concerning the histologic diagnosis especially regarding WHO Grades I and II meningiomas. Apart from the known spectra and metabolites such as choline, creatine and N-acetyl-aspartate (NAA), recent publications have shown that lactate is often found in necrotic tumor tissue. Within a 2 year period, 39 patients with an intracranial meningioma were studied with MRS. In 62.5% of histologic atypical meningiomas (WHO Grade II), a lactate peak could be demonstrated in the pre-operative MRS. Interestingly, also patients with multiple meningiomas show different spectra of their tumors.

[Treatment of carotid artery aneurysms with covered stents]. / Aneurysmabehandlung der Arteria carotis interna mit gecoverten Stents.

PURPOSE: Evaluation of the use of covered stents in treating pseudoaneurysms of the cervical and intracranial/extradural carotid artery and determination of the periprocedural and short- to mid-term complication rate. MATERIALS AND METHODS: 8 patients with 9 spontaneous dissecting aneurysms of the cervical carotid artery--5 of which were symptomatic--plus one patient with ofthalmoplegia due to an aneurysm of the cavernous carotid artery were studied. While the latter was treated with a PTFE-covered balloon-mounted stainless steel stent (Jostent/Graftmaster), a self-expanding PTFE-covered Nitonol Stent (Symbiot) was used in all other cases. Intervention was performed with local anesthesia. Aspirin and Clopidogrel were both used as antiplatelet drugs. Clinical signs and symptoms and vascular imaging with DS, MR, CT angiography and ultrasound were recorded during patient follow-up, with a mean follow-up period of 14.6 months (4 - 30). RESULTS: We were able to treat 8 out of 10 aneurysms (80%) using covered stents. The aneurysms were immediately occluded and the associated stenoses of the parent vessel were eliminated. No clinically relevant complications occurred during the procedure or in the follow-up interval. In two cases, elongation of the carotid artery prevented the stent from being positioned over the aneurysm neck. These cases were shown to be stable with the use of antiplatelet drugs. CONCLUSION: Covered stents can be used in the treatment of pseudoaneurysms of the carotid artery as an alternative to long-term antithrombotic medication or surgery. In our study treatment was effective (80%) and free of complications in the short- and mid-term follow-up. Possible indications, technique and the use of imaging modalities for patient follow-up are discussed.

[Endovascular therapy of stroke]. / Endovaskuläre Behandlung des Schlaganfalls.

Standard therapy of acute ischaemic stroke during the first three hours after symptom onset is intravenous thrombolysis. When contraindications exist, an endovascular approach might be possible. Intraarterial administration of thrombolytic drugs is the most common way. Additionally, there is an increasing use of mechanical tools that are expected to be faster and more effective. Three principles of mechanical thrombolysis are known: aspiration, extraction and fracturing of the thrombus. Different tools were tested clinically and some demonstrated its high efficiency. Larger studies are needed to compare those tools. Apart from recanalization of a cerebral vessel occlusion implantation of a stent might be necessary for prophylaxis of recurrent stroke.

Glycoprotein IIb/IIIa Inhibitor Bridging and Subsequent Endovascular Therapy in Vertebrobasilar Occlusion in 120 Patients.

PURPOSE: The treatment mode in acute vertebrobasilar occlusion (VBO) remains uncertain. We analyzed efficacy and safety of intravenous glycoprotein IIb/IIIa inhibitor (IV GPI) plus subsequent intra-arterial thrombolysis with or without additional endovascular mechanical therapy (percutaneous transluminal angioplasty/stenting or thrombus aspiration) and sought treatment factors that predict good clinical outcome. METHODS: We retrospectively analyzed 120 cases of patients with angiographically proven acute VBO. Multivariate logistic regression was used to identify independent predictors for clinical outcome and included level of consciousness, age, sex, time to angiography, GPI agent, admission mode, occlusion type, recanalization success, and endovascular treatment mode. Clinical follow-up was dichotomized in no to moderate disability (modified Rankin scale (mRS) 0-3) vs. severe disability or death (mRS 4-6). RESULTS: Median National Institutes of Health stroke scale (NIHSS) score on admission was 32, and mean NIHSS score was 24. A total of 49 patients (41 %) developed no to moderate disability (mRS 0-3), and 39 patients (33 %) died. Thrombolysis in myocardial infarction 2/3 recanalization success was achieved in 97 patients (80.8 %). Symptomatic intracerebral hemorrhages occurred in 11 patients (9 %). Mild impairment of consciousness (p < 0.001) and embolic occlusion type (p = 0.01) were significant predictors of favorable outcome. Clinical outcome in recanalized patients was better, but not statistically significant (p = 0.055). CONCLUSIONS: Our results indicate that combined therapy with IV GPI and subsequent endovascular therapy may be a valid treatment strategy in acute VBO. With this treatment approach, a preserved vigilance before treatment and an embolic occlusion type are associated with no to moderate disability.

[MRI of carotid stents: influence of stent properties and sequence parameters on visualization of the carotid artery lumen]. / MRT bei Karotisstents: Einfluss von Stenteigenschaften und Geräteparametern auf die Darstellbarkeit des Karotislumens.

PURPOSE: To evaluate MR artifacts of carotid artery stents and to optimize stent properties and sequence parameters. MATERIAL AND METHODS: Four carotid artery stents - Wallstent (mediloy), Precise (nitinol), ACCULINK (nitinol) and a stent prototype (nitinol) - were investigated in a flow model of the cervical vessels. The model was made of silicon tubing and a flow pump that produces realistic flow curves of the carotid artery. To investigate the effects of magnetic susceptibility and radiofrequency induced shielding artifacts, turbo spin echo and gradient echo sequences as well as CE-MRAs were measured. To improve the visualization of the stent lumen in a CE-MRA, flip angle as well as geometry and covering of the stent prototype were altered. RESULTS: Susceptibility artifacts in stents of the carotid artery only influence the lumen visualization at the proximal and distal end of the braided mediloy stent. A change of stent coverings has no significant influence on radiofrequency artifacts, whereas a reduction in linking elements between stent segments and a change in diameter of stent struts improves visualization of the stent lumen. By increasing the flip angle in a CE-MRA, visualization of the stent lumen is possible in both mediloy and nitinol stents. CONCLUSION: The choice of stent material and changes in stent geometry as well as the optimization of the flip angle of the CE-MRA may reduce susceptibility and radiofrequency artifacts, rendering feasible the CE-MRA of a stented carotid artery.

Contrast-enhanced magnetic resonance angiography in stroke diagnostics: additional information compared with time-of-flight magnetic resonance angiography?

PURPOSE: The aim of this study was a comparison of the diagnostic value of time-of-flight magnetic resonance angiography (TOF-MRA) and contrast-enhanced (CE) MRA in the setting of acute stroke MRI. The hypothesis was that CE-MRA has at least the same diagnostic value as the commonly used TOF-MRA. MATERIALS AND METHODS: A total of 66 stroke patients underwent MRI up to 24 h after symptom onset and again after 3­6 days. Primary slices and maximum intensity projections (MIP) of both techniques were evaluated separately and in combination by two readers in consensus. The quality of imaging and degree of vascular pathologies were evaluated. RESULTS: Out of 109 examinations 105 could be evaluated. There were no significant differences in imaging quality in normal vascular segments. For arterial segments distal to an occlusion CE-MRA allowed better visualization of vessels than TOF-MRA. A combined evaluation of both techniques allowed a significantly better assessment than evaluation of images by one technique alone. In contrast to TOF-MRA, CE-MRA included extracranial segments. CONCLUSION: CE-MRA and TOF-MRA do not differ regarding the evaluation of normal intracranial vessels. CE-MRA provides the advantage of good visualization of vessels distal to occluded segments. Furthermore CE-MRA allows visualization of extracranial vessels and faster image acquisition. TOF-MRA can be equivalently used if the administration of contrast agents is not possible.

MR imaging findings in patients with secondary intracranial hypertension.

BACKGROUND AND PURPOSE: IH can alter the configuration of anatomic structures of the central nervous system. We determined the sensitivity and specificity of MR imaging to detect these changes in patients with secondary IH. MATERIALS AND METHODS: Patients (n = 36) with IH were prospectively investigated with MR imaging and were matched to 36 controls. MR images were evaluated for elongation and edema of the optic nerves, protrusion of the optic disc, flattening of the posterior sclera, height of the pituitary gland, and width of the optic nerve sheath. On MRV, we recorded venous sinus abnormalities and measured the luminal width of the superior ophthalmic veins. A grading score was introduced to define cranial venous outflow obstruction. RESULTS: Cranial venous outflow obstruction and ONS hydrops were the most valid signs indicating IH with a sensitivity of 94% and 92% and a specificity of 100% and 89%, respectively. Sensitivities and specificities were 56% and 97% for reduced pituitary height, 64% and 78% for flattening of the posterior sclera, 31% and 97% for widening of the superior ophthalmic veins, 33% and 100% for optic disc protrusion, 14% and 100% for optic nerve edema, and 6% and 100% for elongation of the optic nerve. At least 2 MR imaging findings could be demonstrated in each patient but in none of the controls. The number of positive MR imaging findings correlated with CSF pressure (r = 0.62, P = .01). CONCLUSIONS: The combination of cranial and orbital MR imaging and MRV can be highly sensitive and specific in the diagnosis of patients with IH.

Focal lesions of human hippocampal CA1 neurons in transient global amnesia impair place memory.

A critical role in place learning has been attributed to place cells within the cornu ammonis 1 (CA1) sector of the hippocampus in rodents. The role of CA1 cells in the human hippocampus with regard to place learning remains elusive. Using a virtual Morris water maze, we investigated patients with acute transient global amnesia (TGA), a rare self-limiting dysfunction of the hippocampal system. Fourteen individuals with selective and focal lesions in the CA1 sector of the hippocampus showed a profound impairment in place learning. The size of the lesions and the duration of the TGA correlated with the deficit in the performance.

A subarachnoid haemorrhage primarily presenting as Terson's syndrome.

We present a case of a subarachnoid haemorrhage due to a ruptured basilar artery aneurysm in a 61-year-old woman. She presented with visual deterioration as the only clinical sign, no previous episodes of headache were reported. Fundoscopy showed a bilateral vitreous haemorrhage. A consecutively performed cranial computer tomography demonstrated subarachnoid bleeding. Further diagnostics revealed a basilar artery aneurysm that was successfully treated with coil embolisation. As a scarring of the vitreous bodies developed, they had to be removed. The patient's eyesight improved gradually and no further complications occurred.

[Caudal regression syndrome - caudal agenesis]. / Kaudales Regressionssyndrom - kaudale Agenesie.

BACKGROUND: Neural tube defects are caused by complex genetic and environmental factors. The congenital anomaly most specific to pregnant women with diabetes mellitus is caudal regression syndrome. PATIENT: A 4-year-old boy with a history of mild delay in motor development presented with primary enuresis and encopresis. On physical examination, he had no sensory and motor deficits, but a short anal cleft. On questioning, the mother reported insulin-dependent diabetes mellitus during pregnancy. MRI of the spinal cord demonstrated a thoracic syringomyelia, a dysplastic conus medullaris, and an absence of coccyx and distal sacrum, called caudal regression syndrome or caudal agenesis. CONCLUSION: The caudal regression syndrome refers to sacral agenesis associated with spinal cord anomalies, e.g. syringomyelia. Sacral agenesis is marked by total absence of the coccyx and total or distal absence of the sacrum. An abnormal backside combined with a history of maternal diabetes mellitus in pregnancy is highly suggestive for the presence of caudal regression syndrome.

The Penumbra System: a mechanical device for the treatment of acute stroke due to thromboembolism.

BACKGROUND AND PURPOSE: Data from recent reports have indicated that mechanical thrombectomy may have potential as a treatment for acute ischemic stroke. The purpose of this study was to assess the safety and performance of the Penumbra System (PS): a novel mechanical device designed to reduce clot burden in acute stroke due to large-vessel occlusive disease. MATERIALS AND METHODS: A prospective, single arm, independently monitored and core laboratory adjudicated trial enrolled subjects with an acute neurologic deficit consistent with acute stroke, presenting within 8 hours of symptom onset and an angiographically verified occlusion (Thrombolysis in Myocardial Infarction [TIMI] grade 0 or 1) of a treatable intracranial vessel. The primary end point was revascularization of the target vessel to TIMI grade 2 or 3. Secondary end points were the proportion of subjects who achieved a modified Rankin Scale (mRS) score of 2 or less or a 4-point improvement on the National Institutes of Health Stroke Scale (NIHSS) score at 30-day follow-up, as well as all-cause mortality. RESULTS: Twenty-three subjects were enrolled, and 21 target vessels were treated in 20 subjects by the PS. At baseline, mean age was 60 years, mean mRS score was 4.6, and mean NIHSS score was 21. Postprocedure, all 21 of the treated vessels (100%) were successfully revascularized by the PS to TIMI 2 or 3. At 30-day follow-up, 9 subjects (45%) had a 4-point or more NIHSS improvement or an mRS of 2 or less. The all-cause mortality rate was 45% (9 of 20), which is lower than expected in this severe stroke cohort, where 70% of the subjects at baseline had either an NIHSS score of more than 20 or a basilar occlusion. CONCLUSION: Thus, early clinical experience suggests that the PS allows revascularization in certain subjects experiencing acute ischemic stroke.

Focal MR spectroscopy of hippocampal CA-1 lesions in transient global amnesia.

OBJECTIVE: The pathomechanisms of transient global amnesia (TGA) remain enigmatic. Focal MR signal diffusion changes in the CA-1 sector of the hippocampus have been described in transient global amnesia, but the pathophysiologic correlate of these lesions is unknown. METHODS: We studied the metabolic spectra of diffusion lesions in the CA-1 sector hippocampus of seven patients with TGA using MR spectroscopy (MRS) between 24 and 72 hours after onset and 2 to 5 months later. The amnestic deficit was studied using a neuropsychometric test battery. RESULTS: Four out of seven patients with an acute TGA showed a diffusion lesion with a corresponding T2 lesion in the CA-1 sector of the hippocampus. Selective hippocampal MRS of diffusion lesions showed a lactate peak in three of four patients, but not in patients without a diffusion lesion. The NAA/creatine ratio was normal. CONCLUSION: Lactate as a marker of anaerobic glycolysis indicates acute metabolic stress of CA-1 neurons in TGA whereas long-term neuronal metabolic changes are not found. This implies that the acute effect on hippocampal CA-1 neurons is the functional correlate of a transient global amnesia reflecting a transient perturbation of memory relevant circuits in the hippocampus.

Electrode implantation for deep brain stimulation in dystonia: a fast spin-echo inversion-recovery sequence technique for direct stereotactic targeting of the GPI.

OBJECTIVE: Deep brain stimulation (DBS) of the globus pallidus internus (GPi) is an effective treatment for medically refractory primary dystonia. We present our technique for direct preoperative visualization of the target using a fast spin-echo inversion-recovery (FSE-IR) sequence. METHODS: Twenty-three consecutive patients (mean age 41 years, range 9-68 years, male to female ratio 11:12) with severe dystonia were operated using a combination of FSE-IR imaging for direct visualization of the globus pallidus internus with stereotactic, gadolinium-enhanced T1-MPRage images. The complete procedure, including stereotactic MRI, was performed under general anesthesia with propofol and remifentanyl. We used multichannel microdrive systems (Medtronic; Alpha-Omega) to introduce up to five parallel microelectrodes for microelectrode recordings (MER) and test stimulation with the central trajectory directed at the anatomically predefined target. The initial standard coordinates in relation to the mid-commissural point (mid-AC-PC) were as follows: lateral 21 mm, anterior 3 mm, and inferior 2 mm, which were then adapted to the individual case based on direct visualization of the target area and further refined by the intraoperative neurophysiology. RESULTS: In ten patients (43%) atlas-based standard coordinates were modified based on the direct visualization of the GPi in the FSE-IR images (bilaterally in seven patients, unilaterally in three). The modified targets ranged from 18.5 to 23.5 mm (mean 20.76 mm) laterally, 1-7 mm (mean 2.75 mm) anteriorly and 1-2 mm (mean 1.95 mm) inferiorly to the mid-AC-PC. We implanted the permanent electrode based on the results of MER and intraoperative stimulation performed to determine the threshold for pyramidal tract responses on the central trajectory in 67%, medially in 16%, anteriorly in 11%, laterally in 4%, dorsally in 2%. The procedure resulted in excellent clinical benefits (average reduction of the Burke-Fahn-Marsden Dystonia Rating Score (BFMDRS) or the Toronto Western Spasmodic Torticollis Rating Scale (TWSTRS) were respectively 65.9%, range 20.9-91.4%) within the first year after surgery. Safety was demonstrated by the absence of intracranial bleeding or other surgical complications causing neurological morbidity. CONCLUSION: Inversion recovery sequences are an excellent tool for direct visualization of the GPi. These images can be fused to stereotactic MRI or CCT and may help to improve anatomical targeting of the GPi for the implantation of DBS electrodes.

Voxel-based morphometry shows no decreases in cerebellar gray matter volume in essential tremor.

OBJECTIVE: To investigate cerebellar gray matter volume in patients with essential tremor (ET). METHODS: We used voxel-based morphometry (VBM) based on high-resolution T1-weighted MRI to compare gray and white matter density between 27 patients with ET and 27 age- and sex-matched healthy control subjects. Fourteen patients had only postural tremor, whereas 13 patients showed additional intention tremor. RESULTS: VBM failed to demonstrate regional decreases in gray and white matter volume in patients with ET. There was, however, an expansion in gray matter depending on the type of tremor. Compared with age-matched control groups, patients with intention tremor showed a relative expansion of gray matter bilaterally in the region of the temporoparietal junction and the right middle occipital cortex. CONCLUSIONS: The lack of a consistent decrease in gray and white matter density argues against a progressive neurodegenerative process in essential tremor that leads to a substantial decrease in cerebellar gray matter volume. Patients with predominant intention tremor show a relative expansion of gray matter areas involved in higher order visuospatial processing, which might represent a long-term result of adaptive reorganization compensating the higher demands on the visuospatial control of skilled movements in case of trembling.

[Meningeal irritation--a complication of herpes zoster]. / Meningeale Reizung--eine Komplikation des Herpes zoster.

HISTORY AND CLINICAL FINDINGS: A previously healthy 26-year-old man complained of gradually increasing headache after an attack of flu. After 4 days an erythema with papules but no blisters was noted in the area of distribution of the left 10th thoracic nerve. As a child he had varicella (chickenpox) without complications. INVESTIGATIONS: Lymphocytic pleocytosis and evidence of an abnormal blood-brain barrier were noted in cerebrospinal fluid (CSF). Serology for varicella zoster virus revealed an IgG titre of > 7400 IU/l in serum and 21 IU/l in CSF. The corresponding IgM titres were negative. TREATMENT AND COURSE: The headaches and cutaneous changes regressed under i.v. treatment with acyclovir, 10 mg/kg body weight, 3 x daily for 10 days. Repeat CSF examination after 10 days showed merely minimal residual changes of inflammation. CONCLUSION: This case illustrates the risk of severe neurological complications of herpes zoster infection. A seemingly minor rash with headache must be correctly diagnosed and immediate high-dosage acyclovir treatment instituted to prevent life-threatening and severe complications of herpes zoster meningitis or encephalitis.