ABSTRACT
Background
The “insular knife‐cut” sign is a sharp demarcation between hyperintense insular lesions on fluid‐attenuated inversion recovery axial images and the basal ganglia, detected on brain MRI. This sign has been associated with herpes simplex virus encephalitis (HSVE); however, its specificity remains unknown. We assessed the frequency and specificity of the insular knife‐cut sign in a real‐life cohort of patients with suspected HSVE.
Methods
We retrospectively identified patients admitted for suspected HSVE over the past 10 years at three Neurology Units in Italy. Inclusion criteria were cerebrospinal fluid (CSF) tested for HSV‐1/2 PCR and acute brain MRI available.
Results
A total of 188 patients were included: HSVE, 44; alternative diagnoses, 144 (autoimmune encephalitis, 51; infectious encephalitis, 22; other acute encephalopathies, 71). The insular knife‐cut sign was present on the initial brain MRI in 23/44 (52.3%) HSVE patients and 1/144 (0.7%) patients with alternative diagnoses (p < 0.001). The specificity and sensitivity of the sign were 99.3% (95% CI, 96–100) and 52% (95% CI, 38–66), respectively. In eight HSVE patients, the insular knife‐cut sign appeared on subsequent MRIs obtained acutely, raising the sensitivity to 70.5% (95% CI, 56–82). On multivariate regression, the insular knife‐cut sign was the strongest independent predictor (odds ratio [95% CI]) of HSVE (68.9 [11.42–415.54]), followed by temporal pole involvement (8.44 [2.06–34.6]), and CSF pleocytosis (6 [1.7–21.18]).
Conclusions
In patients with suspected encephalitis, the insular knife‐cut sign on MRI strongly predicts a diagnosis of HSVE. Its detection should prompt consideration of HSVE, even when other diagnostic tests are equivocal/unavailable.
Keywords: diagnostic value, encephalitis, herpes simplex virus, MRI, nervous system
1. Introduction
Herpes simplex virus encephalitis (HSVE) is the most incident form of sporadic viral encephalitis worldwide [1, 2], with 2–4 new cases per million inhabitants annually [3]. Due to its severity and unforgiving nature, HSVE is considered a neurological emergency and its outcome strictly depends on the timing of treatment initiation [4, 5]. Patients typically present with fever, focal neurological deficits, seizures, and/or altered mental status, with rapid worsening within days [6, 7]. Brain MRI generally shows asymmetric involvement of the temporal poles and insular cortex on T2‐weighted/fluid‐attenuated inversion recovery (FLAIR) images [8, 9, 10], often associated with restricted diffusion [11, 12]. Other prevalent features include an inflammatory cerebrospinal fluid (CSF) and epileptiform electroencephalogram abnormalities. However, these findings are nonspecific and may be found in other neurologic conditions [13, 14], such as autoimmune encephalitis, seizures, tumors, and other causes of infectious encephalitis [15, 16, 17]. Molecular detection of HSV‐DNA in CSF by PCR confirms the diagnosis [18], although lumbar puncture might be contraindicated in some patients and false‐negative results occur in approximately 5% of cases [19]. In this context, identification of additional early and specific signs of HSVE is key to increase the diagnostic probability and favor early treatment.
A sharp demarcation between insular FLAIR abnormalities and the basal ganglia on axial MRI images, referred to from hereafter as “insular knife‐cut” sign, may be observed in patients with HSVE and is often reported as a characteristic feature of the disease [20, 21]. However, the specificity of this sign is unclear. The aim of the present study was to evaluate the frequency and specificity of the insular knife‐cut sign in a real‐life cohort of patients with suspected HSVE.
2. Methods
2.1. Ethical Standards
The study was approved by the Institutional Review Boards of the University of Cagliari, the Fondazione Policlinico Universitario Agostino Gemelli IRCCS, and the Mondino Neurological Institute (protocol ID 4499). The study was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments. Written informed consent for anonymized use of medical data for research was routinely obtained at admission from patients or from their next of kin.
2.2. Study Design and Study Population
A multicenter, retrospective, cohort study was conducted in three Italian adult Neurology Units between January 2015 and July 2024. We initially identified patients with a diagnosis of HSVE by CSF‐PCR consecutively hospitalized at the three units during the study period. As controls, we selected different cohorts of consecutive patients for whom CSF‐PCR for HSV‐1/2 was requested as part of the diagnostic work‐up but resulted negative: (1) Patients hospitalized for suspected encephalitis at the University Hospital of Sassari with various final diagnoses; (2) patients with antibody‐associated autoimmune/paraneoplastic encephalitis, Creutzfeldt‐Jakob disease (CJD), and neurosyphilis seen at Policlinico A. Gemelli in Rome; and (3) patients with autoimmune limbic encephalitis (both antibody‐associated and antibody‐negative) seen at the Mondino Neurological Institute.
2.3. Inclusion and Exclusion Criteria
Only cases with available brain MRI obtained acutely, and sufficient clinical information, were recruited for the analysis. All patients were adults (age, ≥ 18 years). Patients with clinical‐MRI features not consistent with encephalitis, with suspected encephalitis for whom CSF‐PCR for HSV‐1/2 was not performed, and patients with insufficient clinical information or unavailable/poor quality MRI (e.g., motion or metal artifacts) were excluded.
2.4. MRI Examination
In each participating center, brain MRIs carried out within 60 days from symptoms' onset were independently reviewed by two investigators (either a neurologist and a neuroradiologist, or two neurologists) blinded to the final diagnosis; disagreements were resolved by discussion and consensus. MRIs were variably performed with 1,5‐T or 3‐T scans.
The primary objective was to assess the presence of the insular knife‐cut sign, defined as a sharp, linear demarcation along the external capsule, between cortical–subcortical abnormalities involving the insula extensively on axial FLAIR images and the basal ganglia (Figure 1). The concomitant presence of basal ganglia abnormalities not contiguous with the insular abnormalities was allowed.
FIGURE 1.
Definition of “Insular knife‐cut” sign on MRI. The “insular knife‐cut” sign was defined as a sharp, linear demarcation (A, red line) along the external capsule between insular abnormalities on FLAIR sequences and the basal ganglia (B).
We excluded the following FLAIR abnormalities from the insular knife‐cut sign definition: (1) Those extending from the insula to the basal ganglia through the external capsule; (2) those involving only part of the insula; (3) those restricted to the basal ganglia and not involving the insula. We also excluded insular abnormalities detectable only on other sequences (e.g., diffusion weighted images [DWI], post‐Gadolinium T1‐weighted images) but not on FLAIR.
The following MRI variables were additionally collected: (1) Presence of diffusion restriction (defined as DWI hyperintensity and corresponding apparent diffusion coefficient [ADC]‐map hypointensity) or abnormal Gadolinium enhancement (on post‐Gadolinium T1‐weighted images); (2) presence of FLAIR abnormalities involving brain areas other than the insular–basal ganglia region (e.g., the temporal poles); (3) timing of appearance of MRI abnormalities in case of multiple acute brain MRIs available.
2.5. Statistical Analysis
Categorical variables were presented as absolute frequencies and percentages, whereas continuous variables were summarized with means with SD or medians and ranges, according to their normal or non‐normal distribution. Pearson Chi‐ squared and Fisher's exact test were utilized to compare categorical variables, and the Mann–Whitney U test was adopted to evaluate differences in non‐parametric continuous variables. Specificity and sensitivity of the insular knife‐cut sign were calculated and reported with associated 95% confidence intervals (CI). Firth Logistic regression was used to investigate the association between the insular knife‐cut sign, other clinical‐MRI variables, and HSVE. Odds ratio (OR) and 95% CI were reported for each variable investigated. A p‐value < 0.05 was considered statistically significant. Statistical analysis was performed using R (version 4.3.2) programming language.
3. Results
A total of 188 patients were enrolled: 44 with HSVE (HSV‐1, 42; HSV‐2, 2), and 144 with alternative diagnoses (autoimmune/paraneoplastic encephalitis, 51; CJD, 19; neurosyphilis, 3; other etiologies, 71). A flow chart summarizing the different cohorts included from each center is shown in Figure 2.
FIGURE 2.
Different cohorts of patients included from each center. The flow‐chart shows the different cohorts of patients with HSVE and alternative diagnoses included from each participating center. aThe alternative diagnoses in the 71 patients included as controls were: seizures/status epilepticus, n = 21; other infectious encephalitis, n = 12; idiopathic encephalopathy, n = 12; autoimmune encephalitis, n = 12 (with neural autoantibodies, n = 4 [anti‐LGI1 and anti‐CASPR2, n = 1; anti‐CASPR2, n = 1; anti‐GABAB R, n = 1; anti‐Purkinje cell, n = 1]); neurodegenerative disorders, n = 4; brain tumors, n = 3; metabolic encephalopathies, n = 2; and others, n = 5 (Cerebral Amyloid Angiopathy‐Related Inflammation, n = 2; Posterior Reversible Encephalopathy Syndrome, n = 1; Bipolar disorder type II, n = 1; Syncope with head injury, n = 1). bThe type of antibodies identified in the 42 patients were: anti‐CASPR2, n = 3; anti‐CASPR2 and anti‐LGI1, n = 2; anti‐GABAAR, n = 4; anti‐GABABR, n = 3; anti‐GAD65, n = 2; anti‐GFAP, n = 7; anti‐Hu, n = 2; anti‐IgLON5, n = 2; anti‐KLHL11, n = 1; anti‐LGI1, n = 6; anti‐Ma2, n = 2; anti‐NMDAR, n = 5; anti‐Ri, n = 1; anti‐SOX1, n = 1; anti‐Yo, n = 1. cAntibody‐associated limbic encephalitis, n = 6 (anti‐LGI1, n = 2; anti‐GAD65, n = 1; anti‐Hu, n = 2; anti‐Ma2, n = 1.
3.1. Clinical and Laboratory Characteristics
The demographic, clinical, and MRI characteristics of the included patients stratified by HSVE vs. other etiologies are summarized and compared in Table 1.
TABLE 1.
Comparison of demographics, clinical and brain MRI characteristics of patients with HSVE vs. with alternative diagnoses. Continuous and categorical variables are reported as median (range) and number (%).
| HSVE | Other diagnoses | p | |
|---|---|---|---|
| Number of patients | 44 | 144 | |
| Age at symptoms onset, years | 66 (31–84) | 64 (18–92) | 0.46 |
| Female sex | 30/44 (68%) | 73/144 (51%) | 0.042 |
| Clinical presentation | |||
| Fever | 20/44 (46%) | 23/144 (16%) | < 0.001 |
| Seizures | 14/44 (32%) | 60/144 (42%) | 0.25 |
| Focal neurological deficit | 14/44 (32%) | 38/144 (26%) | 0.46 |
| Altered mental status | 30/44 (68%) | 108/144 (75%) | 0.36 |
| Days from symptoms onset to 1st brain MRI | 5 (1–30) | 5 (0–45) | 0.2 |
| Days from symptoms onset to lumbar puncture | 3 (0–30) | 6 (0–75) | 0.002 |
| CSF pleocytosis | 29/44 (66%) | 36/144 (25%) | < 0.001 |
| CSF white blood cell count, n/mm3 | 26 (0–300) | 0 (0–960) | 0.22 |
| CSF protein levels, g/dL | 0.7 (0.2–2.3) | 0.5 (0.1–1.5) | 0.59 |
| Abnormal brain MRI | 43/44 a (98%) | 95/144 (66%) | < 0.001 |
| Temporal pole involvement | 40/43 (93%) | 41/95 (43%) | < 0.001 |
| Restricted diffusion on DWI | 24/43 (56%) | 40/95 (42%) | 0.13 |
| Gadolinium enhancement | 17/43 (40%) | 30/95 (32%) | 0.4 |
| Insular knife‐cut sign | 31/43 (72%) | 1/95 (1%) | < 0.001 |
| Insular involvement without insular knife‐cut sign | 14/43 (30%) | 21/95 (22%) | 0.2 |
Abbreviations: CSF, cerebrospinal fluid; DWI, diffusion weighted images; HSVE, herpes simplex virus encephalitis.
The single HSVE patient with normal brain MRI had CSF‐PCR positive for HSV‐2.
3.2. Frequency and Specificity of the Insular Knife‐Cut Sign for HSVE
The insular knife‐cut sign was detected on initial brain MRI in 23/44 (52.3%) HSVE patients compared to 1/144 (0.7%) patients with an alternative diagnosis (p < 0.001). The specificity and sensitivity of the sign were 99.3% (95% CI, 96–100) and 52.3% (95% CI, 38–66), respectively. Seventeen HSVE patients underwent brain MRI within 24 h from the lumbar puncture, and 12 (71%) of them already exhibited this sign. A second brain MRI was obtained acutely in 117/188 patients, disclosing the insular knife‐cut sign in eight more HSVE patients (Figure 3). Considering both initial and subsequent MRIs, the overall sensitivity of the sign increased to 70.5% (95% CI, 56–82). One patient with the insular knife‐cut sign and severe HSVE showed concomitant thalamic abnormalities (Figure 4;C4). Representative examples of the insular knife‐cut sign in patients with HSVE are shown in Figure 4; while examples of insular abnormalities not configuring an insular knife‐cut in patients with alternative diagnoses are displayed in Figure 5.
FIGURE 3.
Timing of 1st and 2nd MRI in HSV Encephalitis Patients. In this graph, every horizontal line represents a patient with HSVE. The initial (circles) and follow‐up (triangles) MRIs obtained after symptom onset are colored in red or blue based on the presence or absence of the insular knife‐cut sign, respectively.
FIGURE 4.
Representative examples of “insular knife‐cut” sign in patients with HSVE. Temporal pole (top row) and insular (bottom row) FLAIR abnormalities in patients with HSV encephalitis are shown. In the first patient (A), FLAIR abnormalities extend from the right temporal pole and mesial frontal cortex (A1) to the right insula, with a sharp, clear demarcation from the basal ganglia (A2), delineating the “insular knife‐cut” sign. In the second patient (B), FLAIR abnormalities extend from the mesial temporal poles (B1) to the insula, bilaterally (B2), sparing the basal ganglia and delineating a bilateral “insular knife‐cut” sign. In the third patient (C) the initial brain MRI obtained the day of symptoms onset showed normal temporal poles (C1) and only minimal abnormalities in the left fronto‐temporal cortex on FLAIR sequences (C2) and DWI sequences (C2, lower left square), initially misinterpreted as stroke. On day 4 from symptoms onset, brain MRI was repeated due to clinical worsening, showing extensive involvement of bilateral temporal poles (C3) and insula, with an “insular knife‐cut” sign on the left (C4).
FIGURE 5.
Representative examples of MRI abnormalities in patients with alternative diagnoses. Temporal pole (top row) and insular (bottom row) FLAIR abnormalities in patients with suspected encephalitis and non‐HSV etiologies are shown. In the first patient with a rare form of encephalitis associated with aquaporin‐4 (AQP4)‐IgG (A), brain MRI showed FLAIR abnormalities in the temporal poles (A1) and the right insula, with extension to the bilateral basal ganglia through the external capsule, not configuring an “insular knife‐cut” sign. In the second patient with brain glioma (B), MRI revealed FLAIR abnormalities in the right temporal pole (B1), right fronto‐temporal cortex and only partial insular infiltration not reaching the external capsule (B2). In the third patient with seronegative acute disseminated encephalomyelitis (ADEM) (C), brain MRI revealed multifocal “fluffy” FLAIR abnormalities in the bilateral temporal cortex (C1) and deep gray nuclei, with partial insular involvement through the external capsule (C2). Lastly, in the fourth patient with a diagnosis of probable cerebral amyloid angiopathy‐related inflammation (CAA‐ri) (D), brain MRI revealed FLAIR abnormalities in the right temporal lobe (D1) with faint extension to the right insula and external capsule, without clear demarcation from the basal ganglia (D2).
The single patients without HSVE who showed the insular knife‐cut sign underwent an extensive but inconclusive diagnostic evaluation (including two lumbar punctures with CSF‐PCR for HSV, and both tissue‐based and cell‐based assays for autoantibodies against neural antigens in serum and CSF) and were eventually diagnosed with idiopathic encephalitis.
3.3. Comparison Between HSVE Patients With and Without the Insular Knife‐Cut Sign
The median time from symptom onset to brain MRI was 7 days (range 1–57) for HSVE patients with the insular knife‐cut sign, compared to 5 days (range 1–15) for those without the sign, with no statistically significant difference between the two groups (p = 0.47). In patients not showing the sign, this timing refers to the interval from symptom onset to the first brain MRI, while for positive cases it corresponds to the first MRI showing the insular knife‐cut sign.
Moreover, the frequency of patients who started acyclovir before the first MRI did not differ significantly between those with the insular knife‐cut sign (11/23 [47.8%]) and those without the sign (6/21 [28.6%]; p = 0.23).
HSVE patients with the insular knife‐cut sign were also similar to those without the sign in median days from symptoms onset to lumbar puncture (3 [range, 1–30] vs. 3 [range, 1–15]; p = 0.78); presence of fever (15/31 [48.4%] vs. 5/13 [38.5%]; p = 0.74); and frequency of seizures (11/31 [35.5%] vs. 3/13 [23%]; p = 0.5), altered mental status (21/31 [67.7%] vs. 9/13 [69%]; p = 1) and focal neurological deficits (12/31 [38.7%] vs. 2/13 [15.4%]; p = 0.17).
3.4. Regression Analysis
The results of the univariate and multivariate analysis for the variables explored are shown in Table 2. On multivariate analysis, the insular knife‐cut sign remained the strongest independent predictor of HSVE (OR: 68.9 [95% CI; 11.42–415.54]), followed by temporal pole involvement (OR: 8.44 [95% CI; 2.06–34.6]), and CSF pleocytosis (OR: 6 [95% CI; 1.7–21.18]).
TABLE 2.
Univariate and multivariate Firth logistic regression analysis to exploring clinical/MRI variables associated with a final diagnosis of HSVE (n = 44) among 188 patients included in the study.
| Variable | Univariate analysis | Multivariate analysis | ||
|---|---|---|---|---|
| OR (95% CI) | p | OR (95% CI) | p | |
| Age at symptoms onset, years | 1.01 (0.98–1.03) | 0.46 | — | — |
| Female sex | 2.05 (1.03–4.23) | 0.042 | 1.20 (0.33–4.44) | 0.78 |
| Fever | 4.33 (2.08–9.53) | < 0.001 | 3.44 (0.90–13.11) | 0.07 |
| Seizures | 0.66 (0.35–1.39) | 0.25 | — | — |
| Focal neurological deficit | 1.31 (0.62–2.65) | 0.46 | — | — |
| Altered mental status | 0.71 (0.35–1.62) | 0.36 | — | — |
| CSF pleocytosis | 5.65 (2.79–12.14) | < 0.001 | 6.00 (1.70–21.18) | 0.005 |
| CSF white blood cell count, n/mm3 | 1.00 (0.99–1.01) | 0.22 | — | — |
| CSF protein levels, g/dL | 1.00 (0.99–1.00) | 0.59 | — | — |
| Abnormal brain MRI a | 15.03 (3.85–76.47) | < 0.001 | — | — |
| Temporal pole involvement | 22.50 (7.58–97.48) | < 0.001 | 8.44 (2.06–34.60) | 0.003 |
| Restricted diffusion on DWI | 3.05 (1.54–7.01) | 0.001 | 1.73 (0.49–6.12) | 0.40 |
| Gadolinium enhancement | 1.10 (0.57–2.12) | 0.77 | — | — |
| Insular involvement a | 32.2 (13.29–89.98) | < 0.001 | — | — |
| Insular knife‐cut sign | 122.75 (38.51–195.46) | < 0.001 | 68.90 (11.42–415.54) | < 0.0001 |
| Multivariate analysis — Final model | ||||
|---|---|---|---|---|
| OR | 95% CI | p | ||
| Female sex | 1.203973 | 0.3261856 | 4.443942 | 0.781 |
| Fever | 3.440068 | 0.9027137 | 13.10943 | 0.070 |
| CSF pleocytosis | 5.99885 | 1.698876 | 21.18236 | 0.005 |
| Temporal pole involvement | 8.439807 | 2.058664 | 34.60027 | 0.003 |
| Restrictd diffusion on DWI | 1.730881 | 0.4894952 | 6.120489 | 0.395 |
| Insular knife‐cut sign | 68.89543 | 11.42279 | 415.536 | < 0.001 |
Omitted because of collinearity.
4. Discussion
In this multicenter study of patients admitted for suspected encephalitis of new onset, the detection of the insular knife‐cut sign on brain MRI was the strongest predictor of HSVE, underscoring its potential to enhance diagnostic accuracy in clinical practice.
A clear demarcation between insular MRI abnormalities and the basal ganglia was previously reported as a typical sign of HSVE [20, 21], but its frequency, exact characteristics, and specificity for the disease were unclear. In this study, we established in advance a precise definition of the radiological features defining the insular knife‐cut sign as a sharp demarcation between FLAIR insular abnormalities and the basal ganglia along the external capsule, appreciable on axial images. Insular abnormalities detected on sequences other than FLAIR were not considered, as in our experience they are more represented in mimics of HSVE (e.g., insular DWI abnormalities are common in patients with seizures or status epilepticus). One patient with the insular knife‐cut sign in this study concomitantly showed a focal T2 abnormality in the thalamus, in line with prior studies showing HSVE abnormalities are exceptionally rare outside of the limbic cortex [22]. The term “knife‐cut” sign was originally used to define typical linear erosive skin lesions that may be observed in immunocompromised patients and HSV infection [23], and only subsequently applied to describe this brain MRI feature [24]. To limit overlap, for this article we preferred the term “insular knife‐cut” sign. Notably, 30% of patients with HSVE in this study showed partial insular involvement on FLAIR images (Table 1), which was not considered sufficient to define an insular knife‐cut sign, as in our experience it may be observed also with other etiologies (Figure 5).
Our control group included patients with tumors, seizures/status epilepticus, neurodegenerative disorders, autoimmune encephalitis, and encephalitis due to other infectious agents, although some remained idiopathic, as expected in real‐life practice [25]. We found the insular knife‐cut sign had a specificity of 99.3% for HSVE, with only one patient with idiopathic encephalitis showing the sign among controls. The insular knife‐cut sign was previously reported in a case of neurosyphilis [24], although none of the three neurosyphilis cases in our study exhibited this finding, and rarely in patients with autoimmune limbic encephalitis [26]. Notably, after adjusting the analysis for other variables typically associated with HSVE (e.g., temporal pole involvement on MRI), the insular knife‐cut sign remained the strongest predictor of the disease, confirming its high specificity. Our findings also confirm a negative brain MRI is exceedingly rare in patients with HSVE and may be more common with HSV‐2 rather than HSV‐1 infection [7, 10].
The insular knife‐cut sign was often present at the time of the first brain MRI obtained at hospital admission. This might help support the suspicion of HSVE when other tests are inconclusive or unavailable. In clinical practice, the initiation of antiviral therapy is recommended on the basis of clinical suspicion only, independently of MRI availability/findings. The detection of the insular knife‐cut sign on MRI, however, might become particularly useful for: (1) Orienting the diagnostic suspicion towards HSVE in patients with milder or atypical clinical presentations (e.g., absence of fever, presence of only focal neurological deficits); (2) initiating and/or continuing antiviral therapy in rare patients with an initially negative CSF PCR for HSV (reported to occur in approximately 5% of patients) [27, 28]; and (3) initiating/continuing antiviral therapy when lumbar puncture is contraindicated (e.g., high risk of herniation or major bleeding in patients taking anticoagulants) or CSF PCR for HSV is not available due to limited resources. Lastly, PCR results may require several days, during which MRI findings like the insular knife‐cut sign can provide crucial early diagnostic support [29, 30].
Thirteen patients with HSVE did not exhibit the insular knife‐cut sign on brain MRI, meaning the absence of the sign should not dissuade from suspecting an HSVE and initiating treatment. The clinical characteristics of these patients were comparable to those who were positive for the sign. In particular, the median time from symptom onset to brain MRI did not differ significantly between the two groups, excluding the possibility that the absence of the sign was due to early imaging. Also, early initiation of antiviral treatment could not plausibly explain the negative MRI findings in some HSVE patients, as aciclovir was administered before MRI in a similar proportion of patients in the two groups.
Our study is limited by its retrospective design and the relatively small sample size. We could not assess any gain in sensitivity the insular‐knife‐cut may offer in rare HSVE patients with initially negative CSF PCR for HSV as all patients tested positive at first CSF analysis in this study, but this aspect deserves future investigations in larger cohorts. Rare mimics of HSVE might be underrepresented in our cohort, but we selectively inflated the proportion of disorders that are known to commonly affect temporal poles and more often enter the differential (e.g., autoimmune/limbic encephalitis). To increase the generalizability of our findings and better reflect real‐life practice, we only included patients admitted for suspected HSVE, but the insular knife‐cut sign could be detected in other neurologic conditions that are not in the differential (e.g., stroke in the middle cerebral artery territory; milder or slower forms of infectious/autoimmune encephalitis). Furthermore, our study exclusively included adult patients. Future investigations are needed to determine whether the insular knife‐cut sign is also detectable in pediatric cases.
In conclusion, the insular knife‐cut sign is an early and specific finding frequently detected in patients with HSVE. Its detection in patients with encephalitis of new onset might help support the diagnosis of HSVE and inform treatment decisions.
Author Contributions
Sofia Marini: conceptualization, visualization, writing – original draft, formal analysis, validation, investigation. Maria Luisa Usai: investigation, data curation, validation, visualization, formal analysis. Silvia Falso: investigation, validation, visualization. Davide Turilli: methodology, investigation, validation, visualization. Sabrine Othmani: investigation, validation, visualization. Rossella Meloni: investigation, validation, visualization. Pietro Businaro: investigation, data curation, validation, visualization, writing – review and editing. Giacomo Greco: investigation, validation, visualization. Mariangela Puci: investigation, validation, formal analysis, visualization, methodology. Martina Marini: investigation, validation, visualization. Anna Pichiecchio: methodology, investigation, validation, visualization. Matteo Paoletti: methodology, investigation, validation, visualization. Pietro Zara: investigation, validation, visualization. Salvatore Masala: methodology, investigation, validation, visualization. Giovanni Sotgiu: investigation, validation, writing – review and editing, formal analysis, visualization, methodology. Matteo Gastaldi: investigation, validation, funding acquisition, visualization, resources, writing – review and editing. Paolo Solla: investigation, validation, visualization. Raffaele Iorio: conceptualization, software, data curation, investigation, validation, formal analysis, supervision, funding acquisition, visualization, project administration, resources, writing – review and editing, methodology. Elia Sechi: conceptualization, writing – review and editing, resources, project administration, visualization, funding acquisition, supervision, software, data curation, investigation, validation, formal analysis, methodology.
Conflicts of Interest
The authors declare no conflicts of interest.
Acknowledgments
This study was supported by the Italian Ministry of Health, “Ricerca Corrente 2022‐2024” grant to the IRCCS Mondino Foundation. This study was supported by the Italian Network for the study of Autoimmune Neurology (NINA) of the Italian Association of Neuroimmunology. Open access publishing facilitated by Universita Cattolica del Sacro Cuore, as part of the Wiley ‐ CRUI‐CARE agreement.
Marini S., Usai M. L., Falso S., et al., “Diagnostic Value of the “Insular Knife‐Cut” Sign in Patients With Suspected Herpes Simplex Virus Encephalitis,” European Journal of Neurology 32, no. 8 (2025): e70152, 10.1111/ene.70152.
Funding: This study was supported by Ministero della Salute, Ricerca Corrente 2022‐2024.
Raffaele Iorio and Elia Sechi shared‐senior authorship.
Data Availability Statement
Anonymized data used for this study are available upon reasonable request from the corresponding author.
References
- 1. Granerod J., Ambrose H. E., Davies N. W., et al., “Causes of Encephalitis and Differences in Their Clinical Presentations in England: A Multicentre, Population‐Based Prospective Study,” Lancet Infectious Diseases 10 (2010): 835–844. [DOI] [PubMed] [Google Scholar]
- 2. Boucher A., Herrmann J. L., Morand P., et al., “Epidemiology of Infectious Encephalitis Causes in 2016,” Médecine et Maladies Infectieuses 47 (2017): 221–235. [DOI] [PubMed] [Google Scholar]
- 3. Modi S., Mahajan A., Dharaiya D., Varelas P., and Mitsias P., “Burden of Herpes Simplex Virus Encephalitis in the United States,” Journal of Neurology 264 (2017): 1204–1208. [DOI] [PubMed] [Google Scholar]
- 4. Tunkel A. R., Glaser C. A., Bloch K. C., et al., “The Management of Encephalitis: Clinical Practice Guidelines by the Infectious Diseases Society of America,” Clinical Infectious Diseases 47 (2008): 303–327, 10.1086/589747. [DOI] [PubMed] [Google Scholar]
- 5. Venkatesan A., Tunkel A. R., Bloch K. C., et al., “Case Definitions, Diagnostic Algorithms, and Priorities in Encephalitis: Consensus Statement of the International Encephalitis Consortium,” Clinical Infectious Diseases 57 (2013): 1114–1128. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6. Venkatesan A., “Epidemiology and Outcomes of Acute Encephalitis,” Current Opinion in Neurology 28 (2015): 277–282. [DOI] [PubMed] [Google Scholar]
- 7. Singh T. D., Fugate J. E., Hocker S., Wijdicks E. F. M., Aksamit A. J., and Rabinstein A. A., “Predictors of Outcome in HSV Encephalitis,” Journal of Neurology 263, no. 2 (2016): 277–289, 10.1007/s00415-015-7960-8. [DOI] [PubMed] [Google Scholar]
- 8. Misra U. K., Kalita J., Phadke R. V., et al., “Usefulness of Various MRI Sequences in the Diagnosis of Viral Encephalitis,” Acta Tropica 116 (2010): 206–211. [DOI] [PubMed] [Google Scholar]
- 9. Granerod J., Davies N. W. S., Mukonoweshuro W., et al., “Neuroimaging in Encephalitis: Analysis of Imaging Findings and Interobserver Agreement,” Clinical Radiology 71 (2016): 1050–1058. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10. Kelly M. J., Grant E., Murchison A. G., et al., “Magnetic Resonance Imaging Characteristics of LGI1‐Antibody and CASPR2‐Antibody Encephalitis,” JAMA Neurology 81 (2024): 525–533. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11. McCabe K., Tyler K., and Tanabe J., “Diffusion‐Weighted MRI Abnormalities as a Clue to the Diagnosis of Herpes Simplex Encephalitis,” Neurology 61 (2003): 1015–1016. [DOI] [PubMed] [Google Scholar]
- 12. Sawlani V., “Diffusion‐Weighted Imaging and Apparent Diffusion Coefficient Evaluation of Herpes Simplex Encephalitis and Japanese Encephalitis,” Journal of the Neurological Sciences 287 (2009): 221–226. [DOI] [PubMed] [Google Scholar]
- 13. Iorio R., Damato V., Spagni G., et al., “Clinical Characteristics and Outcome of Patients With Autoimmune Encephalitis: Clues for Paraneoplastic Aetiology,” European Journal of Neurology 27 (2020): 2062–2071. [DOI] [PubMed] [Google Scholar]
- 14. Gastaldi M., Mariotto S., Giannoccaro M. P., et al., “Subgroup Comparison According to Clinical Phenotype and Serostatus in Autoimmune Encephalitis: A Multicenter Retrospective Study,” European Journal of Neurology 27 (2020): 633–643. [DOI] [PubMed] [Google Scholar]
- 15. Granillo A., Le Maréchal M., Diaz‐Arias L., Probasco J., Venkatesan A., and Hasbun R., “Development and Validation of a Risk Score to Differentiate Viral and Autoimmune Encephalitis in Adults,” Clinical Infectious Diseases 76 (2023): e1294–e1301. [DOI] [PubMed] [Google Scholar]
- 16. Dalmau J. and Graus F., “Antibody‐Mediated Encephalitis,” New England Journal of Medicine 378 (2018): 840–851. [DOI] [PubMed] [Google Scholar]
- 17. Hoang H. E., Robinson‐Papp J., Mu L., et al., “Determining an Infectious or Autoimmune Etiology in Encephalitis,” Annals of Clinical Translational Neurology 9 (2022): 1125–1135. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18. Cinque P., Cleator G. M., Weber T., Monteyne P., Sindic C. J., and Loon A. M., “The Role of Laboratory Investigation in the Diagnosis and Management of Patients With Suspected Herpes Simplex Encephalitis: A Consensus Report. The EU Concerted Action on Virus Meningitis and Encephalitis,” Journal of Neurology, Neurosurgery, and Psychiatry 61 (1996): 339–345. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19. Puchhammer‐Stöckl E., Presterl E., Croÿ C., et al., “Screening for Possible Failure of Herpes Simplex Virus PCR in Cerebrospinal Fluid for the Diagnosis of Herpes Simplex Encephalitis,” Journal of Medical Virology 64 (2001): 531–536. [DOI] [PubMed] [Google Scholar]
- 20. Oyanguren B., Sánchez V., González F. J., et al., “Limbic Encephalitis: A Clinical‐Radiological Comparison Between Herpetic and Autoimmune Etiologies,” European Journal of Neurology 20 (2013): 1566–1570. [DOI] [PubMed] [Google Scholar]
- 21. Budhram A., Leung A., Nicolle M. W., and Burneo J. G., “Diagnosing Autoimmune Limbic Encephalitis,” Canadian Medical Association Journal 191 (2019): E529–E534. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22. Damasio A. R. and Van Hoesen G. W., “The Limbic System and the Localisation of Herpes Simplex Encephalitis,” Journal of Neurology, Neurosurgery, and Psychiatry 48, no. 4 (1985): 297–301, 10.1136/jnnp.48.4.297. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23. Cohen P. R., “Clinical Features of Atypical Presentations of Mucocutaneous Herpes Simplex Virus Infection Observed in Immunosuppressed Individuals. Part II: The Knife‐Cut Sign,” Dermatology Online Journal 30 (2024): 30. [DOI] [PubMed] [Google Scholar]
- 24. Liu H. and Dong D., “MRI of Neurosyphilis With Mesiotemporal Lobe Lesions of “Knife‐Cut Sign” on MRI: A Case Report and Literature Review,” Heliyon 9, no. 4 (2023): e14787. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25. Chow F. C., Glaser C. A., Sheriff H., et al., “Use of Clinical and Neuroimaging Characteristics to Distinguish Temporal Lobe Herpes Simplex Encephalitis From Its Mimics,” Clinical Infectious Diseases 60, no. 9 (2015): 1377. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26. Kherbek H., Paramasivan N. K., Dasari S., et al., “Exploring Autoantigens in Autoimmune Limbic Encephalitis Using Phage Immunoprecipitation Sequencing,” Journal of Neurology 272, no. 4 (2025): 292. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27. Weil A. A., Glaser C. A., Amad Z., and Forghani B., “Patients With Suspected Herpes Simplex Encephalitis: Rethinking an Initial Negative Polymerase Chain Reaction Result,” Clinical Infectious Diseases 34 (2002): 1154–1157. [DOI] [PubMed] [Google Scholar]
- 28. Koskiniemi M., Piiparinen H., Mannonen L., Rantalaiho T., and Vaheri A., “Herpes Encephalitis Is a Disease of Middle Aged and Elderly People: Polymerase Chain Reaction for Detection of Herpes Simplex Virus in the CSF of 516 Patients With Encephalitis. The Study Group,” Journal of Neurology, Neurosurgery, and Psychiatry 60 (1996): 174–178. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29. Poissy J., Wolff M., Dewilde A., et al., “Factors Associated With Delay to Acyclovir Administration in 184 Patients With Herpes Simplex Virus Encephalitis,” Clinical Microbiology and Infection 15, no. 6 (2009): 560–564, 10.1111/j.1469-0691.2009.02735.x. [DOI] [PubMed] [Google Scholar]
- 30. Hughes P. S. and Jackson A. C., “Delays in Initiation of Acyclovir Therapy in Herpes Simplex Encephalitis,” Canadian Journal of Neurological Sciences/Journal Canadien des Sciences Neurologiques 39 (2012): 644–648. [DOI] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
Anonymized data used for this study are available upon reasonable request from the corresponding author.