Subacute Sclerosing Panencephalitis: Recent Advances in Pathogenesis, Diagnosis, and Treatment

Abstract

Subacute sclerosing panencephalitis (SSPE) is a relentless progressive brain disorder caused by the persistent presence of mutated measles virus in the central nervous system. The disease typically develops years after primary measles infection, with the highest risk observed in children infected before the age of 2 years. The global incidence of SSPE is notably higher in low- and middle-income countries and in regions with low measles vaccination coverage. The pathogenesis of SSPE involves viral persistence through mutations in viral proteins, enabling immune evasion and cell-to-cell propagation within the brain. Neuroinflammation, immune-mediated damage, and neuronal loss further contribute to disease progression. Clinical manifestations range from progressive cognitive decline and behavioral changes, along with myoclonus, seizures, movement disorders, visual impairment, and, finally, a vegetative state. Diagnosis is confirmed through cerebrospinal fluid analysis showing elevated antimeasles antibodies, characteristic periodic electroencephalography discharges, and neuroimaging findings like white matter hyperintensities and cerebral atrophy. Treatment remains challenging, with isoprinosine, interferon-α, ribavirin, and newer agents like favipiravir and aprepitant offering new hope. Symptomatic management and palliative care are needed in all patients. SSPE is invariably fatal. Notably, reports of prolonged survival and disease stabilization have been documented, particularly with early and combined therapy. The coronavirus disease 2019 pandemic’s adverse impact on measles vaccination rates highlights the urgent need for robust measles immunization campaigns. Future directions involve exploring antiviral fusion peptide inhibitors and artificial intelligence-driven diagnostic tools to improve early detection, treatment efficacy, and outcome prediction in SSPE.

Introduction

Subacute sclerosing panencephalitis (SSPE) is a rare but severe neurodegenerative disease caused by the persistent presence of the measles virus within the central nervous system (CNS). It typically emerges years after an initial measles infection, often affecting children and young adults. Despite significant advances in measles vaccination programs, SSPE continues to be a major health challenge, particularly in areas with inconsistent immunization coverage. The condition arises from a complex interplay of factors, including viral persistence, immune response dysregulation, and potential genetic susceptibility. Mutations within the measles virus enable it to evade the host’s immune system, leading to a chronic infection that progressively damages CNS. Clinically, SSPE often begins with subtle behavioral changes and cognitive decline, followed by more pronounced neurological deficits such as motor dysfunction and seizures. Over time, it leads to severe disability and, in most cases, death.[1]

The diagnosis of SSPE is based on a combination of clinical presentation, characteristic findings on electroencephalography (EEG), and detection of measles-specific antibodies in the cerebrospinal fluid (CSF). While these tools are effective in confirming the diagnosis, early identification remains a challenge due to the insidious onset of symptoms. Treatment options for SSPE are limited, and no definitive cure exists. Antiviral therapies and immunomodulatory interventions have shown promise in some cases, but outcomes remain highly variable. As such, the focus often shifts toward supportive care and efforts to improve quality of life for the affected individuals.[1]

This review explores the current understanding of SSPE, including its epidemiology, pathogenesis, clinical features, diagnostic approaches, and management strategies. It also highlights recent advances and identifies critical gaps in knowledge, offering insights into potential areas for future research and therapeutic development.

Epidemiology

The World Health Organization estimates that the incidence of SSPE ranges from four to 11 cases per 100,000 measles cases worldwide. However, this risk significantly increases when measles infection occurs at a very young age. There are an estimated 18 cases of SSPE per 100,000 measles cases among children who contract measles early in life. Furthermore, lower- and middle-income countries report an even higher incidence of SSPE, with rates as high as 27.9 cases per 100,000 measles cases. Early measles virus infection is the primary risk factor for developing SSPE, with a higher risk found in individuals who contract measles below 2 years of age.[2]

In resource-rich countries with universal measles vaccination coverage, an increasing number of SSPE cases are reported following repeated measles outbreaks. Georgia experienced measles outbreaks in 2004–2005 and 2013–2015, with reported cases numbering 8377 and 11,495, respectively. After these outbreaks, 16 cases of SSPE were diagnosed between 2008 and 2017. Notably, 11 out of 12 SSPE patients with a documented history of measles contracted the infection at or below the age of 2 years, while one patient contracted it at 3 years of age. Based on these figures, the estimated risk of developing SSPE following the 2004–2005 outbreak ranged from 1 in 1396 to 1 in 13,960.[3] There has been a recent spike in severe and fulminant cases of SSPE in the UK. Between 2022 and 2024, six children in the UK were diagnosed with SSPE. The children were young, with ages ranging from 2 to 7 years and a typical age of 5 years. Symptoms began appearing 2–6 years after the initial infection, with 3 years being the average latency period from infection to symptom onset. Most of the children had been exposed to measles during infancy.[4] In Japan, a developed country, SSPE has frequently been reported. A nationwide survey was conducted in Japan in 2022; this survey investigated the 37 surviving patients of SSPE and the factors associated with its progression. No new cases have been reported in the survey since 2017. The survey revealed a median age of 32 years, with 83.8% having a history of measles (median age of infection: 12.5 months). Most (83.3%) contracted measles before 24 months of age. The median incubation period from measles to SSPE onset was 9 years. At the time of survey, 89.2% of patients lacked speech or comprehension, and 59.5% were bedridden, with 88.9% requiring persistent nursing care. Despite reduced SSPE cases due to lower measles incidence in Japan, surviving patients in 2022 had advanced disease.[5]

India, Pakistan, Turkey, and Papua New Guinea are four countries that report the highest number of SSPE cases every year. Unfortunately, precise incidences of SSPE in these countries are not readily available.[6] A retrospective study conducted in Papua New Guinea identified a significant incidence of SSPE between 1988 and 1991. During this period, 87 cases were diagnosed, with confirmation based on the detection of high levels of antimeasles antibodies in CSF. In 1990, the annual incidence of SSPE in Papua New Guinea was estimated at 56 cases per million population below 20 years of age. The average age at diagnosis was 4.9 years, and a marked male predominance was observed.[7] Analysis from 1994 to 2009 in Germany, using capture–recapture methods, estimated the SSPE incidence at 1 per 1700–3300 measles cases. SSPE predominantly affected males (2.6:1 ratio), with a median latency of 8 years from infection to diagnosis.[8]

A notable shift in the clinical profile of SSPE shows increasing cases in children under 3 years. A recent review reported a 3-year-old boy with SSPE and reviewed 70 cases in children aged 3 years or younger, revealing a mean onset age of 26.34 months and a gender ratio of 3.2:1. Most cases originated from Turkey, India, and Japan. The absence of measles vaccination was frequently identified as a contributing factor.[9]

Pathogenesis

The measles virus belongs to the Morbillivirus genus within the Paramyxoviridae family. The measles virus is composed of a single-stranded RNA genome enclosed within an envelope. Its lipid bilayer envelope contains viral glycoproteins, specifically hemagglutinin (H) and fusion (F) proteins, which are essential for viral attachment, entry, and replication. The viral genome, comprising approximately 16,000 nucleotides, encodes eight proteins: nucleocapsid (N), phosphoprotein (P), matrix (M), hemagglutinin (H), fusion (F), virulence (V) protein, cytoplasmic (C) protein, and large (L) protein, which functions as an RNA-dependent RNA polymerase. The V, C, and L proteins play key roles in viral replication and pathogenesis. The N protein encapsulates the RNA genome, forming a helical nucleocapsid. The viral envelope, derived from the host cell’s lipid bilayer, incorporates the H and F glycoproteins, which play pivotal roles in the virus’s infectivity.[10]

The pathogenesis of SSPE is a complex process that involves intricate molecular mechanisms, including viral entry, replication, and spread within the brain. Most of the work on the pathogenesis of SSPE has primarily been conducted using experimentally neuronal cell cultures, infected animals, or biopsied/autopsied brain tissues from infected individuals.

Host’s susceptibility

Genetic predisposition plays a crucial role in determining susceptibility to SSPE. Research on familial cases, where multiple siblings contract the disease, suggests that host susceptibility factors and specific viral genotype features play crucial roles in the development of SSPE.[11] Various studies have identified abnormalities in immune-related genes that enhance the individual’s susceptibility to develop SSPE. These genes include those involved in innate immunity, such as nucleotide-binding oligomerization domain-containing proteins and Toll-like receptors, as well as cytokine genes, such as interleukin-12 and interferon (IFN)-γ. In addition, abnormalities in genes involved in apoptosis and immune regulation have also been implicated in the pathogenesis of SSPE. Specific haplotypes and genotypes have been linked to increased susceptibility, while others may offer protection[12,13] [Supplementary File 1].

Supplementary File 1.

Summary of Genetic Studies on SSPE

Study	Patients	Controls	Genetic Abnormalities	Clinical Significance
Kocaaga et al., 2023 doi: 10.1177/08830738221144081	64	70	NOD1 (rs2075820, rs2075818) and NOD2 (R334Q, R334W) polymorphisms	NOD1 rs2075820 A allele is protective; rs2075818 C allele increases susceptibility to SSPE
Genc et al., 2018 doi: 10.1177/08830738221144081	64	68	IL28B, IL29, and miR-548 polymorphisms	Increased IL-29 and miR-548 expression in SSPE; rs8099917 G allele increases SSPE risk
Dundar et al., 2016 doi: 10.1007/s13365-016-0442-7	78	90	IL-12 (-1188 A/C) and IFN-γ (+874 A/T) polymorphisms	IL-12 (-1188) A/C polymorphism correlates with increased SSPE risk; IFN-γ (+874) not significant
Yentur et al., 2014 doi: 10.1055/s-0034-1378129	118	221	Granzyme B (rs8192917, G>A) polymorphism	GG genotype of Granzyme B polymorphism is protective for SSPE
Karakas-Celik et al., 2014 doi: 10.1016/j.gene. 2014.03.056	54	81	TLR4 (Asp299Gly) and IL-17 (His161Arg) polymorphisms	Asp299Gly TLR4 polymorphism increases SSPE risk; Arg161 variant of IL-17 is protective
Piskin et al., 2013 doi: 10.1089/dna. 2013.1997	54	72	IL-18 (-137, -607) and IL-2 (-330) polymorphisms	IL-18 (-607) AA genotype increases SSPE risk; IL-18 (-137) C allele increases SSPE risk
Piskin et al., 2013 doi: 10.1055/s-0033-1338134	109	55	PD-1 (rs2227982) gene polymorphism	PD-1 rs2227982 T allele is protective for SSPE
Ishizaki et al., 2010 doi: 10.1055/s-0033-1338134	40	84	PD1 haplotype (-606G) polymorphism	PD1-606G allele linked to higher promoter activity and increased PD1 expression in SSPE
Ishizaki et al., 2008 doi: 10.1080/13550280802298120	40	84	TLR3 (rs3775291) gene polymorphism	TLR3 (rs3775291) 412Phe allele and haplotype increase susceptibility to SSPE
Yilmaz et al., 2007 doi: 10.1080/13550280701455383	87	106	IL-2, IL-12, and IFN-γ polymorphisms	IL-12B C allele and CC genotype increase SSPE risk; IL-2 (-330) GG genotype is protective
Deveza et al., 2006 doi: 10.1055/s-2006-924724	60	120	MxA, IL-4, and IRF-1 gene polymorphisms	No significant association of MxA, IL-4, or IRF-1 genes with SSPE in Filipino population
Tasdemir et al., 2006 doi: 10.1002/ajmg.b. 30343	43	100	ACE I/D and AT1R (A1166C) polymorphisms	ACE D allele and DD genotype increase SSPE risk; AT1R polymorphism not associated with SSPE
Torisu et al., 2004 doi: 10.1212/01.wnl.0000106940.95749.8e	87	106	MxA promoter (-88 G/T) polymorphism	MxA -88T allele associated with higher promoter activity and SSPE susceptibility
Inoue et al., 2002 doi: 10.1001/archneur.59.5.822	54	55	IL-4 (-589 T) and IRF-1 allele 1 polymorphisms	IL-4 (-589 T) and IRF-1 allele 1 combination increases SSPE risk
Kusuhara et al., 2000 doi: 10.1086/315386	40	32	CD46 gene (G/A176) polymorphism	No clear role of CD46 polymorphism in SSPE susceptibility
Rittner et al., 1984 doi: 10.1007/BF00345615	23	None	C4 deficiency (C4A QO) polymorphism	Partial C4 deficiency (C4A QO) may predispose individuals to SSPE

Viral entry into the brain

The entry of the measles virus into the brain is a critical event in the pathogenesis of SSPE. Although the precise pathogenetic mechanism is not fully understood, research suggests that the measles virus binds to specific receptors on the surface of neurons to gain entry. Infected immune cells, particularly lymphocytes and monocytes, are thought to act as “Trojan horses,” spreading the virus across the blood–brain barrier (BBB), while the measles virus may also exploit areas of BBB compromise during systemic infection. The measles virus utilizes its F and H proteins to infect CNS cells, with a leucine-to-tryptophan substitution at position 454 (L454W) in the F protein enhancing its ability to spread in brain tissue. Measles virus targets neurons and glial cells by binding to receptors such as signaling lymphocytic activation molecule/CD150 and nectin-4.[14,15,16]

Viral persistence in the brain

A hallmark feature of SSPE pathogenesis is persistence of the measles virus in the neuronal tissue. This is achieved through specific genomic mutations acquired during the latent period between the acute infection and disease onset. These mutations primarily occur in the M, F, and H genes, which encode the structural and functional components of the viral envelope. The M protein, crucial for virion assembly and release, is often defective in SSPE-associated strains, leading to the suppression of infectious particle production. Mutations in the M protein, such as the F50S mutation, have recently been shown to enhance neurotropism of the measles virus and contribute to the development of SSPE. This defect forces the virus to rely on cell-to-cell fusion for propagation, thus evading immune recognition.[15]

In addition, mutations in the F protein confer a hyperfusogenic phenotype, enhancing the virus’s ability to form multinucleated syncytia in neuronal and glial cells. This fusion activity is critical for viral spread within CNS.[15] Similarly, mutations in the cytoplasmic tail of the H protein enable receptor-independent interaction with host proteins, such as cell adhesion molecule (CADM)1 and CADM2, which are predominantly expressed in neurons. Mutations in the F protein and/or M protein enable the measles virus to acquire neurotropism.[14]

One key factor contributing to the persistence of the measles virus is the formation of defective interfering particles, which are incomplete viral particles that can still infect cells and interfere with the replication of standard viral particles. The measles virus can spread within the brain through “collective infectious units,” which facilitate the persistence and spread of the virus. These units allow the virus to undergo rapid mutation and adaptation, enabling it to evade the immune system.[16,17]

Spread of the virus within the brain

In SSPE, viral propagation occurs almost exclusively via cell-to-cell transmission, facilitated by the hyperfusogenic F protein and the receptor-independent interactions of the H protein. This mode of spread is highly efficient in the brain, where cells are densely interconnected. The formation of multinucleated syncytia, a hallmark feature of measles virus infection, allows the virus to exploit neuronal networks for rapid dissemination.[15]

Immune changes in the brain

The immune changes are crucial in the pathogenesis of SSPE. During measles infection, the immune system clears the virus from most tissues, but it persists in the brain due to an impaired immune response. Activated immune cells produce proinflammatory cytokines, but fail to clear the virus. Elevated cytokine levels, including interleukin-12, interleukin-17, and IFN-γ, contribute to the progression of SSPE.[18] Continued inflammation in the brain results in progressive neuronal loss.

Pathology

Histopathologic analysis of brain tissue in SSPE reveals a distinctive pattern of diffuse encephalitis, characterized by perivascular lymphocytic infiltration, prominent gliosis, neuronal degeneration, and the presence of intranuclear inclusion bodies containing measles virus RNA and proteins. These pathological changes suggest significant inflammation and immune cell infiltration, predominantly localized around cerebral blood vessels, contributing to disruption of BBB. This compromise facilitates extensive neuronal degeneration and reactive gliosis, culminating in progressive brain atrophy. Detection of intranuclear inclusion bodies serves as a diagnostic hallmark of SSPE, indicative of chronic measles virus persistence within neuronal tissues.[19]

Immunohistochemistry and electron microscopy

Immunohistochemical analysis demonstrates the presence of measles virus antigens within glial cells and neurons. The staining patterns reveal a widespread distribution of viral antigens, including N and M proteins. Immunoreactivity is predominantly localized within the brain, highlighting the extensive involvement of neural tissue in measles virus-associated pathology. Electron microscopy also reveals characteristic ultrastructural changes, including intranuclear inclusions within oligodendrocytes, which are composed of interwoven tubular strands typical of measles virus nucleocapsid along with granular material representing the viral nucleoprotein. In addition, viral particles, although rarely seen, mitochondrial abnormalities, such as swelling and degeneration, and axonal damage, including degeneration and formation of axonal spheroids, may be observed.[19]

Evidence of tauopathy

Trans-Activation Response (TAR) DNA-binding protein 43 (TDP-43) regulates gene expression, maintains genomic stability, and influences RNA metabolism. A recent study examined TDP-43 pathology in 16 autopsied SSPE brains, identifying TDP-43 inclusions in 31% of cases, primarily in the atrophied temporal and parietal cortices as tangle- and thread-like neuronal cytoplasmic inclusions. TDP-43 pathology was linked to longer disease duration (>4 years) and tau-positive neurofibrillary tangles, highlighting an association between TDP-43 and tau aggregates. These findings suggest that TDP-43 and tau proteins contribute to the neurodegenerative process triggered by viral inflammation.[20] Another study examined brain tissue from five SSPE cases; pathological examination noted marked brain atrophy, demyelination, and fibrillary gliosis. Abnormal tau protein accumulation and neurofibrillary tangles were found, primarily in the oculomotor nuclei, locus coeruleus, and limbic cortex. Although antiviral treatments were given to two cases, which controlled the measles virus spread, they did not stop the progression of tauopathy.[21]

Clinical Features

The classical clinical course of SSPE progresses through several stages. Initially, subtle cognitive decline and poor academic performance are observed, followed by forgetfulness, behavioral changes, and gait abnormalities. As the disease advances, verbal output decreases and patients experience difficulty walking. Eventually, they become akinetic and mute. In the terminal stages, patients enter in a vegetative state [Table 1].

Table 1.

Staging of SSPE

Stage	Clinical features
Stage 1	Subtle behavioral changes, personality alterations, irritability, and mild intellectual decline Episodes of inappropriate laughter Poor school performance Subtle myoclonic jerks
Stage 2	Marked myoclonic jerks, often periodic and synchronous Progressive cognitive decline, speech deterioration Gait disturbances Pyramidal signs
Stage 3	Severe cognitive decline with loss of speech and voluntary movements Akinetic mutism Autonomic dysfunction (e.g., hyperthermia, tachycardia, diaphoresis)
Stage 4	Coma or persistent vegetative state Loss of cortical function Autonomic instability, leading to death due to infections or respiratory failure

SSPE: subacute sclerosing panencephalitis

One of the hallmark clinical features of SSPE is periodic myoclonus. These sudden, involuntary muscle contractions predominantly involve axial muscles. A slow relaxation phase is a characteristic feature of periodic myoclonus in SSPE. Periodic myoclonus results in difficulties with walking and frequent falls. While myoclonic jerks do not impair consciousness, they can be worsened by excitement. In some cases, myoclonus appears as a subtle, periodic slow blinking or upward rolling of eyes. Seizures are another common symptom of SSPE, which may present as generalized tonic–clonic, focal, or atonic episodes and are often challenging to manage. Many patients display repetitive motor behaviors and vocalizations, such as clapping, finger-clicking, hand rubbing, and palilalia.[1,22]

As SSPE advances, patients may develop pyramidal and extrapyramidal signs, including ataxia, dystonia, and dyskinesia. The intensity of myoclonus may diminish in the advanced stage. In the terminal stages, hypothalamic failure may occur, resulting in intermittent autonomic abnormalities. In the final stages of SSPE, patients may experience irregular breathing, decerebrate and decorticate rigidity, and become vegetative.[23]

Movement disorders

In addition to periodic myoclonus, SSPE can also exhibit several kinds of hyperkinetic and hypokinetic movement disorders. Hyperkinetic movement disorders, such as dystonia, chorea, and tremors, are more common and often manifest in the early stages of SSPE. On the contrary, hypokinetic movement disorders, including parkinsonism, tend to occur in later stages. Development of movement disorders in SSPE is thought to be related to the inflammation and degeneration of specific brain regions, including the basal ganglia and thalamus.[24]

Ocular manifestations

Vision loss is a common presenting complaint in SSPE. Vision loss in SSPE can occur due to various reasons, including retinal lesions, optic nerve involvement, and cortical vision loss. A systematic review of 96 cases with vision loss revealed that the majority of patients experienced visual manifestations before the onset of encephalopathy. The retina was the most common site of vision loss. Retinal lesions, including viral chorioretinitis with macular involvement or necrotizing retinitis with or without pigmentary retinal changes, were typical findings. The inner plexiform and outer plexiform layers of the retina were dominantly affected. The optic nerve is commonly affected, with optic atrophy and papillitis being characteristic findings. One-third of patients had cortical vision loss. The measles virus possibly spreads to the retina and brain via the hematogenous route during early childhood infection. Retinal histopathologic changes are similar to those described in the affected brain. Histopathology of the retina in SSPE shows retinal necrosis, inclusion bodies, perivascular cuffing, gliosis, photoreceptor loss, and retinal pigment epithelium disruption.[25]

Psychiatric manifestations

Many patients initially present with psychiatric symptoms, including schizophrenia-like psychosis featuring hallucinations, delusions, and disorganized thinking. They may also develop catatonia, marked by motor immobility, rigidity, altered consciousness, and affective disturbances such as mania and depression.[26]

Diagnostic Workup

CSF changes

Typically, the CSF analysis reveals a normal or mildly elevated cell count, normal glucose levels, and a normal or slightly elevated total protein count. However, the key to diagnosing SSPE lies in the detection of elevated antimeasles antibody titers in CSF, which indicates the host’s defense against the measles virus. This is characterized by high levels of measles-specific immunoglobulin G (IgG) and immunoglobulin M antibodies, an elevated IgG index indicating intrathecal synthesis of IgG antibodies, and the presence of oligoclonal bands signifying an abnormal immune response. CSF shows a markedly elevated IgG titer against measles virus.[27]

Measles-specific IgG titers in CSF are key for SSPE diagnosis. Among 59 paired SSPE samples, 94.9% had CSF IgG ≥0.5 IU/mL and a CSF/serum ratio ≥0.05, while non-SSPE cases had CSF IgG <0.1 IU/mL and a CSF/serum ratio <0.03. These thresholds effectively differentiate SSPE from non-SSPE cases.[28]

Electroencephalography

The characteristic EEG pattern in SSPE features periodic discharges, also known as Radermecker complexes, which consist of distinctive high-amplitude, slow-wave complexes. These complexes typically are stereotyped and recur at regular intervals, often every 3–10 sec, and may be accompanied by muscle contractions or myoclonic jerks. As the disease progresses, the EEG pattern may evolve, with the periodic discharges becoming more frequent and the background activity becoming increasingly disorganized. In some cases, EEG may also show focal or lateralized abnormalities, reflecting the asymmetric involvement of the brain[29] [Figure 1].

Figure 1.

EEG recorded at a speed of 20 mm/sec shows periodic complexes (Radermecker complexes) every 4 sec amid a slow background of theta activity. EEG: electroencephalogram

Neuroimaging

Early neuroimaging in SSPE can appear normal. In the initial stages, it may reveal subtle changes, such as mild abnormalities in the white matter. As the disease progresses, more significant findings emerge, including periventricular demyelination, progressive cerebral atrophy, and ventricular enlargement.

Magnetic resonance imaging (MRI) characteristically shows hyperintensities in the subcortical white matter, basal ganglia, and brainstem on T2-weighted images. Periventricular white matter signals are dominantly seen in parieto-occipital regions. Basal ganglia hyperintensity and brainstem involvement have been described in patients with other movement disorders.[30,31] In the advanced stages of the disease, marked cerebral atrophy along with the presence of multiple small white matter cystic lesions are significant findings[32] [Figures 2-4].

Figure 2.

MRI of the brain shows prominent periventricular hyperintensities. There are signal changes in both basal ganglionic regions. MRI: magnetic resonance imaging

Figure 4.

MRI of the brain shows brain atrophy, white matter hyperintensities, and cystic changes involving the right putamen, left caudate, and right occipital regions in a 28-year-old male diagnosed with fulminant subacute sclerosing panencephalitis. MRI: magnetic resonance imaging

Figure 3.

MRI of the brain shows prominent pontine hyperintensities. MRI: magnetic resonance imaging

Diagnostic Criteria

Dyken’s criteria are used for SSPE diagnosis by assessing the clinical symptoms, EEG patterns, and CSF/serum measles antibody titers to confirm the presence of measles virus [Table 2].

Table 2.

Dyken’s diagnostic criteria for SSPE

Dyken’s diagnostic criteria	Description
Clinical features	The patient exhibits a slow, progressive deterioration in cognitive abilities, including personality changes, memory loss, and learning difficulties. This is often accompanied by myoclonic jerks (sudden, involuntary muscle spasms), progressive motor dysfunction (spasticity, dystonia, or coordination problems), and sometimes visual disturbances
EEG findings	EEG shows characteristic periodic, high-voltage, synchronous slow-wave discharges. These discharges coincide with myoclonic jerks and are typically bilaterally symmetrical
Elevated gamma globulin or oligoclonal bands in CSF	Analysis of CSF reveals elevated IgG or the presence of oligoclonal bands, which indicates an abnormal immune response within the central nervous system
Presence of measles antibodies in serum	Elevated levels of measles-specific IgG antibodies are detected in serum, which signifies prior exposure to the measles virus or vaccination. However, this alone is not definitive for SSPE.
Presence of measles antibodies in CSF	Measles-specific IgG antibodies in CSF confirm intrathecal antibody production. The CSF/serum antibody index supports the diagnosis of SSPE
Histopathologic findings from brain biopsy or autopsy	In cases where brain tissue is available (e.g., biopsy or autopsy), inclusion bodies are seen inside neurons and glial cells. Other findings include neuronal loss, gliosis, and evidence of chronic viral infection

CSF: cerebrospinal fluid, EEG: electroencephalogram, IgG: immunoglobulin G, SSPE: subacute sclerosing panencephalitis. Key diagnostic rule: To establish a diagnosis of SSPE, at least three or more of the six criteria must be fulfilled

Differential Diagnosis

SSPE diagnosis requires careful differential diagnosis to rule out other conditions that may present with similar symptoms. Patients with early subtle cognitive decline are often misinterpreted as having functional symptoms or malingering and may initially seek psychiatric evaluation.

Differential diagnoses for SSPE also include epileptic encephalopathies, like Dravet syndrome, Lennox-Gastaut syndrome, and West syndrome. These conditions share similarities with SSPE, including frequent seizures, cognitive decline, and characteristic EEG patterns. Progressive myoclonic epilepsies also resemble SSPE, featuring seizures, myoclonus, and cognitive decline. Accurate diagnosis requires careful consideration of clinical presentation, EEG findings, and laboratory results to distinguish these conditions from SSPE.[33]

Neurodegenerative disorders such as juvenile Huntington’s disease, Wilson’s disease, neuroacanthocytosis, and mitochondrial disorders (e.g., Leigh syndrome, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome, Kearns-Sayre syndrome) should also be considered. Other conditions like brain tumors, vascular malformations, and multiple sclerosis can also present with similar symptoms and neuroimaging abnormalities, making a comprehensive diagnostic workup essential to establish an accurate diagnosis.

Infectious diseases such as other viral encephalitis should be considered. In addition, autoimmune and inflammatory conditions like autoimmune encephalitis, Hashimoto encephalopathy, and acute disseminated encephalomyelitis (ADEM) can present with similar clinical and neuroimaging features.

Acute Fulminant Variant

Acute fulminant SSPE is a rapidly progressive and severe form of SSPE, often leading to moderate disability within months of onset, followed by death or an akinetic mute state. Unlike the classic form, which evolves over years, fulminant SSPE progresses aggressively, often within 6 months. A recent UK study identified six cases over 2 years, primarily in younger children with shorter latency periods after measles infection. These cases exhibited atypical symptoms such as visual impairment, seizures, headache, vomiting, and movement disorders. EEG abnormalities were present in most cases but were not always apparent initially, while MRI findings were normal in some patients. Antiviral and immunomodulatory therapies showed minimal benefit, with rapid neurological deterioration.[4] In another case series, four children presented with unusual SSPE manifestations. One had congenital measles infection, another developed gait disturbances, the third one exhibited features resembling ADEM with refractory seizures, and the fourth one had unilateral myoclonus with hemiparesis. Diagnosis relied on characteristic EEG findings, CSF analysis, and serum antimeasles antibodies. Given the aggressive course of fulminant SSPE, early recognition of atypical features and timely diagnostic evaluation are crucial for further management, though treatment remains largely ineffective.[34]

Pregnancy

SSPE in pregnancy is associated with severe maternal outcomes and potential risks to newborns. A recent systematic review analyzed maternal and fetal outcomes in pregnant women with SSPE, encompassing 19 reports describing 21 cases. Most women (14 out of 21) exhibited clinical symptoms of SSPE during the antepartum period, with vision loss being a frequent initial complaint. Nine women experienced unilateral or bilateral vision loss, while two had chorioretinitis. All cases eventually progressed to the akinetic mute stage and subsequently became vegetative. Neuroimaging in 13 of 15 women revealed T2/fluid-attenuated inversion recovery hyperintensities in the parieto-occipital regions. Brain biopsy findings confirmed florid panencephalitis in all cases, with measles virus components detected in four. Maternal outcomes were severe, with 13 women succumbing after delivery. Despite this, 15 of the 19 newborns survived, though most were preterm. Five fetuses either died shortly after birth or were stillborn.[35]

Treatment

Currently, the management of SSPE is primarily symptomatic. Despite the use of antiviral medications and immunomodulatory therapies, the disease’s progressive nature often remains ineffective. As such, a comprehensive care plan emphasizing supportive measures, symptom management, and palliative care is essential to optimize patient outcomes.[36,37]

Immunomodulatory drugs

Treatment for SSPE is primarily focused on immunomodulation. The first-line treatment involves the use of isoprinosine, an immunomodulatory and antiviral agent that enhances T-cell proliferation and natural killer cell activity, leading to increased proinflammatory cytokine levels.

Isoprinosine

Isoprinosine is the most frequently used drug in SSPE. It is believed to exert immunomodulatory effects by enhancing the body’s natural defense mechanisms against viral infections. It promotes immune cell activation by increasing the expression of natural killer group 2D ligands on target cells, resulting in enhanced lymphocyte proliferation, cytokine production, and natural killer cell cytotoxicity. Some studies have indicated that isoprinosine may slow the progression of SSPE, although its efficacy remains unconfirmed. For SSPE treatment, the recommended dosage is 50–100 mg/kg/day, with treatment durations ranging from several months to 2–3 years or, in some cases, lifelong.[38,39]

Interferon-α

IFN-α works by modulating the immune response, inhibiting viral replication, and promoting antiviral activity. In the treatment of SSPE, IFN-α has been shown to slow the disease progression and improve outcomes, although it is often used in combination with other drugs. The optimal route for IFN-α administration remains uncertain due to a lack of head-to-head trials. Intrathecal administration involves injecting 1–3 million international units (IU) of IFN-α intrathecally two or three times a week, allowing direct delivery to CNS and enhancing its antiviral effect. Intravenous administration involves injecting 3–10 million IU/m² of IFN-α three times a week, ensuring broader distribution but potentially causing more pronounced systemic side effects. Subcutaneous administration, although less common, involves injecting 3 million IU of IFN-α three times a week, sometimes in combination with intrathecal administration for enhanced efficacy. Theoretically, the intraventricular route with an Ommaya reservoir is the most effective, but it is invasive and expensive. Treatment duration can vary, but it is often continued for 6–12 months or even longer. For example, a patient with SSPE was treated with intrathecal IFN-α, and the patient achieved remission and improved quality of life for 7–8 years. However, severe deterioration followed. This case suggests that IFN-α–induced remission is temporary, highlighting the need for more effective long-term therapies for SSPE patients.[40,41]

A randomized study compared the efficacy of oral inosiplex (isoprinosine) alone versus combined treatment with intraventricular IFN-α in patients with SSPE. No significant differences were noted between the two cohorts in survival rates and neurological disability. However, the study suggested that treatment was superior to no treatment, with satisfactory outcomes (stabilization or improvement) observed in 34%–35% of patients.[42] The most common side effects of IFN-α include flu-like symptoms, psychiatric disturbances, and hematological toxicity including leukopenia and thrombocytopenia.

Amantadine

Amantadine’s mechanism of action against the measles virus is not well understood, but it is believed to entail interference with viral replication, inhibition of viral transcription, and blocking of viral budding, while also possibly having immunomodulatory effects. Studies suggest that amantadine may slow down the progression of SSPE.[43] Most of the information on the usage of amantadine is based on anecdotal evidence from case reports. For example, in an adult case of SSPE, amantadine (200 mg/day) was administered in conjunction with isoprinosine (1000 mg/day). Over 16 months, the patient exhibited cortical blindness and occasional myoclonus, yet maintained ambulation, highlighting the potential role of amantadine in SSPE treatment.[44]

Antiviral drugs

Ribavirin, a nucleic acid analog, can effectively inhibit the replication of SSPE virus strains in laboratory and animal experiments. Clinical observations have revealed partial symptom improvement or slower disease progression in some cases. Notably, administering ribavirin directly into the ventricles of the brain appears to be the preferred method, as it ensures optimal concentrations of the medication in brain tissue. A prospective study conducted in the Philippines assessed the safety and efficacy of combining intraventricular ribavirin with oral isoprinosine in 16 patients diagnosed with SSPE. The results indicated that nearly half of the patients, primarily those in Stage III, exhibited noticeable clinical improvement. Conversely, approximately 38% of patients experienced deterioration. While minor adverse side effects were observed, a significant concern was the high incidence of Ommaya reservoir infections, affecting over 31% of patients. These findings suggest that ribavirin may offer symptomatic benefit and potentially prolong survival, although it may not alter or stop disease progression.[45]

Favipiravir and remdesivir

Recent studies have investigated the potential role of other nucleic acid analogs, favipiravir and remdesivir, in SSPE. These compounds work by disrupting viral RNA polymerase activity. Originally developed to target the influenza virus, favipiravir has demonstrated efficacy against various RNA viruses, including the SSPE strain. However, achieving sufficient CSF concentrations via oral administration remains a challenge.

Remdesivir has also shown therapeutic activity against many RNA viruses, including the measles virus, highlighting its potential as a treatment option. A 5-year-old patient with SSPE was treated with remdesivir. Initially, the patient experienced a temporary improvement in quality of life during the first two treatment courses. However, further administration of the drug provided no additional benefit, and the patient eventually succumbed to the disease. Autopsied brain examination revealed significant histopathologic changes, like neuronal loss and demyelination, along with plenty of measles virus RNA-positive cells. Next-generation sequencing identified a complete measles virus genome containing characteristic SSPE mutations. Functional analysis of these mutations demonstrated that alterations in the F protein gene led to an unusually high fusion capacity, resulting in a hyperfusogenic phenotype.[46]

Aprepitant

Aprepitant is a neurokinin-1 receptor antagonist primarily used to prevent chemotherapy-induced nausea and vomiting. Recent studies suggest its potential antiviral properties, particularly in targeting measles virus spread, making it a candidate for investigation in SSPE. A randomized trial involving 62 SSPE patients evaluated the efficacy of aprepitant in modifying disease progression. The drug demonstrated a favorable safety profile, with adverse effects comparable to its standard use in nausea management. However, despite being well tolerated, no significant clinical improvement was observed in treated patients. Both the treatment and placebo groups exhibited progressive cerebral atrophy, suggesting that aprepitant did not alter the overall disease trajectory.[47]

Symptomatic treatment

Carbamazepine is commonly used for SSPE, showing efficacy in controlling myoclonus. Alternative medications, such as clobazam, levetiracetam, and valproate, are also considered viable options for controlling both myoclonus and seizures.

Ketogenic diet

The ketogenic diet, known for managing drug-resistant epilepsy, also exhibits antioxidant and anti-inflammatory effects. It has shown promise in SSPE, temporarily improving myoclonic jerks, clinical symptoms, cognitive function, and EEG findings.[48]

Prognosis

SSPE is considered an invariably fatal condition with a highly variable natural course. The disease typically follows a non-linear trajectory. A follow-up study of 118 SSPE patients revealed that only 20% of patients progress predictably through defined stages. Most patients experience a fluctuating course marked by episodic deterioration, periods of stability, and occasional temporary improvement. A sudden transition from stability to rapid neurological decline is commonly reported, occurring in 25% of cases, and may result in death. Patients who appear stable may abruptly deteriorate to a vegetative state or die within a few days to weeks. Spontaneous improvement or stabilization has been observed in 53% of cases, with remission lasting several months to years. While rare, substantial improvement occurs in 5% of patients, even in advanced disease. However, the overall prognosis remains poor. Survival times vary significantly; around 40% die within the first year, 41% survive beyond 2 years, and 20% live for 4 years or more. Longer survival is associated with slower progression and, in some cases, better long-term outcomes.[23]

Prolonged survival or substantial improvement

Prolonged survival in SSPE has been observed in many cases, with durations ranging from 3 years to over 13 years. Some patients experienced temporary stabilization, delaying disease progression for several years. Functional independence was maintained in certain cases for up to 8 years. While some patients showed initial improvements in quality of life, most eventually experienced disease progression. Treatments such as intraventricular IFN-α, inosiplex, and antiviral therapies have been linked to extended survival, though subsequent neurological decline always happens.[49,50]

Future Directions

SSPE, a severe complication of measles, urgently requires innovative solutions. Gaining insight into how the measles virus triggers SSPE is essential for developing targeted therapies. Promising approaches, such as fusion peptide inhibitors, aim to prevent viral fusion and replication, potentially slowing disease progression. Advances in artificial intelligence provide new opportunities by supporting early diagnosis, predicting disease outcomes, and integrating neuroimaging, electrophysiology, and clinical data for effective monitoring. Universal measles vaccination is critical to prevent measles virus infections and reduce SSPE cases. These breakthroughs hold the potential to revolutionize SSPE prevention and treatment globally.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.