Early diagnosis of severe myoclonic epilepsy in infancy.

Of 329 epileptic patients referred in a six year period with the first seizure occurring in the first year of life, 20 met the following criteria: generalized seizures excluding infantile spasms, myoclonic, tonic or absence seizures, at least one afebrile seizure, normal development prior to the first seizure, normal CT scan, and no etiology. Seventeen of these 20 patients developed the full pattern of severe myoclonic epilepsy in infancy (SMEI). This syndrome was recognizable from the second or third seizure in the first year of life, although epileptiform EEG abnormalities were lacking until the age of 11 to over 30 months. Therefore, based on the clinical pattern, the diagnosis of SMEI can be made with quite good reliability by the end of the first year of life.     

Clinical spectrum of mutations in SCN1A gene: severe myoclonic epilepsy in infancy and related epilepsies

Severe myoclonic epilepsy in infancy (SMEI) manifests very frequent generalized tonic-clonic seizures (GTC), accompanied by myoclonic seizures, absences and partial seizures [Dravet, C., 1978. Les épilepsie grave de l'enfant. Vie Méd. 8, 543-548; Dravet, C., Roger, J., Bureau, M., Dalla Bernardina, B., 1982. Myoclonic epilepsies in childhood. In: Akimoto, H., Kazamatsuri, H., Seino, M., Ward, A. (Eds.), Advances in Epileptology. Raven Press, New York, pp. 135-140; Dravet, C., Bureau, M., Oguni, H., Fukuyama, Y., Cokar, O., 2002. Severe myoclonic epilepsy of infancy (Dravet syndrome). In: Roger, J., Bureau, M., Dravet, C., Genton, P., Tassinari, C.A., Wolf, P. (Eds.), Epileptic Syndromes in Infancy, Childhood and Adolescence, third ed. John Libbey, London, pp. 81-103]. However, there is a group of severe epilepsy that has many characteristics common to SMEI except for myoclonic seizures. We reported this group of epilepsy as intractable childhood epilepsy with GTC (ICEGTC) [Watanabe, M., Fujiwara, T., Yagi, K., Seino, M., Higashi, T., 1989b. Intractable childhood epilepsy with generalized tonic-clonic seizures. J. Jpn. Epil. Soc. 7, 96-105 (in Japanese); Fujiwara, T., Watanabe, M., Takahashi, Y., Higashi, T., Yagi, K., Seino, M., 1992. Long-term course of childhood epilepsy with intractable grand mal seizures. Jpn. J. Psychiatr. Neurol. 46, 297-302]. Recently, mutations of the neuronal voltage-gated sodium channel alphasubunit type 1 gene (SCN1A) have been found in SMEI [Claes, L., Del-Favero, J., Ceulemans, B., Lagae, L., Van Broeckhoven, C., De Jonghe, P., 2001, De novo mutations in the sodium-channel gene SCN1A cause severe myoclonic epilepsy of infancy. Am. J. Hum. Genet. 68, 327-1332]. Mutations in SCN1A are found in both SMEI and ICEGTC at high rates of 70-81%. The loci of the mutations seen in ICEGTC are quite similar to those found in SMEI, suggesting a genotypic continuity between these entities. The clinical spectrum of epilepsies harboring SCN1A mutations may be consisted of various phenotypes with GEFS+ on the mildest end and SMEI on the severest end of the spectrum.     

Long-Term Course of Childhood Epilepsy with Intractable Grand Mal Seizures

Abstract: Twenty-nine children with childhood epilepsy characterized by frequent grand mal (generalized tonic-clonic) seizures in spite of maximal doses of antiepileptic drugs and by an early onset of seizures (before 1 :year of age) were followed up for more than 5 :years. The children were divided into 3 :groups: severe myoclonic epilepsy in infancy (SME), no SME, and intractable childhood epilepsy with generalized tonic-clonic seizures (GTC). In all the 3 :groups, the grand mal seizures persisted, whereas the other types of seizures tended to disappear as the patients aged, and the prognosis for mental development was poor. In the majority of cases in all the 3 :groups, the waking grand mal seizures altered to sleep grand mal seizures with aging. Two pairs of monozygotic twins with SME suggested that genetic factors play a role in this epileptic syndrome. Intractable childhood epilepsy with GTC is distinguished by the absence of other types of generalized seizures. It cannot be regarded as an epileptic syndrome, but its pathogenesis and treatment require further studies.     

Absence seizures with evolution into generalized tonic-clonic activity: clinical and EEG features

PURPOSE: To report the clinical and electrographic features of absence seizures evolving into generalized tonic-clonic (GTC) activity in six patients with idiopathic generalized epilepsy. METHODS: All patients were referred for evaluation of refractory seizures and underwent video-EEG monitoring after discontinuation of their antiepileptic drugs (AEDs). We analyzed the video-EEG recordings for seizure semiology as well as ictal and interictal activity. We also reviewed the initial clinical data in all patients. RESULTS: All patients were women, with a mean age of 27 years (range, 14-43 years). The mean age at seizure onset was 12 years (range, 5-15 years). Family history was positive for epilepsy in four patients. All patients had recorded seizures with an onset that was characteristic of generalized absence clinically and electrographically, with evolution into GTC activity. The EEG onset was with generalized 2.5-to 5-Hz spike-and-wave discharges, with evolution into faster rhythmic activity. Interictal EEG recordings showed generalized 2-to 5-Hz spike-and-wave discharges. All had normal background activity. All patients were treated with divalproex monotherapy. Five patients have been seizure free, and one had a single breakthrough GTC seizure during a follow-up period of 12-36 months. CONCLUSIONS: GTC activity may evolve from typical absence seizures. This seizure type should be included in the International Classification of Seizures. Its recognition and distinction from complex partial seizures with secondary generalization are important for appropriate therapy.     

Severe Myoclonic Epilepsy of Infancy: Extended Spectrum of GEFS+?

PURPOSE: Severe myoclonic epilepsy of infancy (SMEI) is an intractable epilepsy of early childhood of unknown etiology. It is often associated with a family history of seizure disorders, but epilepsy phenotypes have not been well described. We sought to characterize the seizure phenotypes of relatives to better understand to the genetic basis of SMEI. METHODS: Probands with SMEI were identified, and systematic family studies were performed. Epilepsy syndromes were characterized in affected family members. RESULTS: Twelve probands with SMEI were identified. Eleven of the 12 probands with SMEI had a family history of seizures, and the twelfth was the result of a consanguineous marriage. We found that 16.7% of full siblings and 8.3% of parents had definite seizures. A total of 39 affected family members was identified. The most common phenotype was febrile seizures in 14, febrile seizures plus in seven, partial epilepsy in two, and there were single individuals with SMEI, myoclonic-astatic epilepsy, Lennox-Gastaut syndrome, and 13 cases with unclassified or unconfirmed seizures. CONCLUSIONS: The family history of seizures in SMEI is in keeping with the spectrum of seizure phenotypes seen in generalized epilepsy with febrile seizures plus (GEFS+). Our findings suggest that SMEI is the most severe phenotype in the GEFS+ spectrum.     

Adult-onset autosomal dominant myoclonic epilepsy: report of a family with an overlooked epileptic syndrome.

OBJECTIVE: Myoclonic epilepsy is a common epileptic syndrome with high genetic contribution. We described a pedigree in which 10 individuals presented with a non-progressive, adult-onset myoclonic epilepsy. MATERIALS AND METHODS: The pedigree was constructed and analyzed. Six affected members were studied with clinical grounds, mental status, neurophysiology, video-electroencephalographic (EEG), brain magnetic resonance imaging (MRI) and mutational analysis of GABRA1 (GABRA1A, which endoces the alpha1 subunit of the gamma-aminobutyric acid receptor subtype A). Clinical and EEG data were collected from six unaffected members. RESULTS: Autosomal dominant hereditary was shown. The age of seizure onset was approximately 40. All the individuals had myoclonic seizures and a normal cognitive level. Bilateral symmetric jerks of the shoulders, arms or legs featured the myoclonic seizure. Ictally, the consciousness was not affected. The ictal EEG demonstrated bilateral spikes-and-waves. The occurrence of myoclonic seizures was not associated with sleepiness. Rare generalized tonic-clonic seizures occurred in two individuals. No absence or accompanying involuntary movements were observed. A lower dose of valproic acid (200-500 mg/D) (clonazepam 0.5 mg/D in a patient) was required to stop the myoclonic seizures. CONCLUSIONS: The clinical features of late adult-onset autosomal dominant myoclonic epilepsy are similar to juvenile myoclonic epilepsy (JME), which is a common generalized epileptic syndrome with a significant hereditary component. But the age of onset, rare association of other seizure patterns, and non-relation of seizure onset to sleepiness suggest that this may be a distinct familial epileptic syndrome different from recognized familial myoclonic epilepsies.     

Levetiracetam monotherapy for primary generalised epilepsy.

PURPOSE: To evaluate the efficacy of levetiracetam in cases of refractory primary generalised epilepsy. METHODS: Three patients with refractory primary generalised epilepsy were treated with levetiracetam monotherapy; one with absence seizures, myoclonic jerks and generalised tonic-clonic (GTC) seizures one with myoclonic jerks and GTC seizures, and one with only GTC seizures. All three patients had generalised spike wave on the EEG and had failed at least three antiepileptic drugs (AEDs) before trying levetiracetam. RESULTS: All three patients tolerated levetiracetam well and became seizure free for at least 6 months. Therapeutic doses of levetiracetam ranged from 1250 to 3000 mg/day. CONCLUSION: Levetiracetam, a new AED with a novel mechanism(s) of action, should be considered for patients with refractory primary generalised epilepsy.     

Malignant migrating partial seizures in infancy: An epilepsy syndrome of unknown etiology

The syndrome of malignant migrating partial seizures in infancy was first reported in 1995, and is now included among the childhood epilepsy syndromes in development in the proposal of the revision of the International League Against Epilepsy (ILAE) classification of the epilepsies and epilepsy syndromes. The main clinical features are seizure onset in the first 6 months of life, occurrence of almost continuous migrating polymorphous focal seizures, combined with multifocal ictal electroencephalography (EEG) discharges, and progressive deterioration of psychomotor development. Etiology is so far unknown. Seizures are markedly drug resistant and outcome is generally severe. Based on age at onset, migrating partial seizures in infancy (MMPEI) may be placed between early epileptic encephalopathies (early myoclonic encephalopathy [EME] and early infantile epileptic encephalopathy [EIEE]) and infantile spasms.     

Topiramate as add-on drug in severe myoclonic epilepsy in infancy: an Italian multicenter open trial

PURPOSE: This study was to evaluate the efficacy and safety of topiramate (TPM) in patients with severe myoclonic epilepsy in infancy (SMEI) and refractory seizures. METHODS: We performed a prospective multicentric open label add-on study in 18 patients (age 2-21 years, mean 9 years) with SMEI and refractory seizures of different types. TPM was added to one or two other baseline drugs and the efficacy was rated according to seizure type and frequency. RESULTS: TPM was initiated at a daily dose of 0.5-1 mg/kg, followed by a 2-week titration at increments of 1-3 mg/kg/24 h up to a maximum daily dose of 12 mg/kg. After a mean period of 11.9 months (range 2-24 months), three patients (16.7%) had 100% fewer seizures and ten patients (55.5%) had a more than 50% seizure decrease. In no patient there was a seizure worsening. Mild to moderate adverse events were present in four patients (22.2%), represented by weight loss, hypermenorrhoea, renal microlithiasis, nervousness and dysarthric speech. CONCLUSION: TPM may be a useful drug in patients with SMEI, being particularly effective against generalized tonic-clonic seizures. Further studies are needed to evaluate the early use of this drug in such a severe syndrome.     

Epidemiologic Study of Epilepsy in Young Singaporean Men

PURPOSE: This survey of 20,542 Singaporean men born in 1974 studied the clinical features of young men diagnosed with epilepsy on preenlistment screening. METHODS: All male citizens in Singapore are medically screened at age 18 years before enlistment for compulsory military service. Patients suspected to have epilepsy are then referred to government hospitals for further management. We interviewed the patients and their parents and reviewed their hospital records. RESULTS: Eighty-nine patients with epilepsy were identified, indicating a lifetime prevalence of 4.9/1,000 males by age 18 years. The lifetime prevalence of epilepsy among Chinese, Malays, and Indians were 5.2, 2.8, and 6.4/1,000, respectively; these differences were not statistically significant. The mean age of seizure onset was 11.1 years. Generalized seizures (65.2%) were commoner than partial seizures (34.8%); common seizure types included generalized tonic-clonic seizures (52.8%), complex partial seizures with secondary generalization (24.7%), and myoclonic seizures (5.6%). Common epileptic syndromes included temporal lobe epilepsy (16.9%), juvenile myoclonic epilepsy (5.6%), and frontal lobe epilepsy (2.2%). Eighty-four (94.4%) patients sought medical treatment, and seven (7.9%) patients sought additional traditional treatment. Although 70 (78.7%) patients responded to medication, 14 (15.7%) patients remained refractory to treatment. CONCLUSIONS: The lifetime prevalence of epilepsy in young Singaporean men was 4.9/1,000. The majority (65.2%) had generalized seizures. Temporal lobe epilepsy was the commonest (16.9%) defined epilepsy syndrome. More patients with epilepsy (94.4%) sought medical treatment, although 15.7% remained refractory to medication.     

A patient with myoclonic epilepsy in infancy followed by myoclonic astatic epilepsy

Myoclonic epilepsy in infancy (MEI) is a primary generalized epilepsy. According to the literature, the outcome of MEI is usually benign. Here we report a patient who developed myoclonic astatic epilepsy at age four, having been seizure free without antiepileptic drug treatment for 2 years after his recovery from MEI. At age four, a video-EEG-recording showed frequent head nodding (atonic seizures) and myoclonic astatic seizures associated with diffuse spikes or polyspikes and waves. The interictal EEG revealed frequent bursts of generalized 100-200 μV, 2-4 Hz spike-and-slow-wave complexes. Despite a general favorable outcome, more severe epilepsy syndromes may develop after MEI, and mental retardation is sometimes observed. Our case and the previous literature suggest that epilepsies following on from MEI often involve myoclonic seizures.     

Diagnostic issues and treatment of cryptogenic or symptomatic generalized epilepsies

To clarify the diagnostic issues and treatment of patients with cryptogenic or symptomatic generalized epilepsies, not including West syndrome (WS), we investigated electroclinical change during the clinical course, and treatment effects in these patients. The selection criteria were minor generalized seizures as their main seizure type and diffuse epileptic discharges as their main EEG findings. Regarding EEG, we included EEGs that predominantly displayed multifocal spike-waves because of the inclusion of severe epilepsy with multiple independent spike foci (SE-MISF). We divided the subjects into two groups according to their main seizure types: Group A (54 patients) with brief tonic seizures and Group B (24 patients) with myoclonic seizures and/or atypical absences. The main epileptic syndromes were considered to be Lennox-Gastaut syndrome and SE-MISF in Group A, and epilepsy with myoclonic-astatic seizures in Group B. A history of WS was often seen in Group A, but it was exceptional in Group B. During the clinical course, seizure types did not basically change in Group A. EEG patterns were changeable in both groups. Although there was some overlap in electroclinical manifestations among epileptic syndromes, a transition between the two groups was not seen. High-dose valproate and ethosuximide were the most effective in Groups A and B, respectively. Long-term prognosis was significantly more favorable in Group B than in Group A.     

Two-year remission and subsequent relapse in children with newly diagnosed epilepsy

PURPOSE: Although remission is the ultimate measure of seizure control in epilepsy, and epilepsy syndrome should largely determine this outcome, little is known about the relative importance of syndrome versus other factors traditionally examined as predictors of remission or of relapse after remission. The purpose of this study was to examine remission and relapse with respect to the epilepsy syndrome and other factors traditionally considered with respect to seizure outcome. METHODS: A prospectively identified cohort of 613 children with newly diagnosed epilepsy was assembled and is actively being followed to determine seizure outcomes. Epilepsy syndrome and etiology were classified at diagnosis and again 2 years later. Remission was defined as 2 years completely seizure-free, and relapse as the recurrence of seizures after remission. Multivariable analysis was performed with the Cox proportional hazards model. RESULTS: Five hundred ninety-four of the original 613 children were followed > or = 2 years (median follow-up, 5 years). Remission occurred in 442 (74%), of whom 107 (24%) relapsed. On multivariable analysis, idiopathic generalized syndromes and age at onset between 5 and 9 years were associated with a substantially increased remission rate, whereas remote symptomatic etiology, family history of epilepsy, seizure frequency, and slowing on the initial EEG were associated with a decreased likelihood of attaining remission. Young onset age (<1 year) and seizure type were not important after adjustment for these predictors. Relapses occurred more often in association with focal slowing on the initial EEG and with juvenile myoclonic epilepsy. Benign rolandic epilepsy and age at onset <1 year were associated with markedly lower risks of relapse. About one fourth of relapses were apparently spontaneous while the child was taking medication with good compliance, and more than half occurred in children who were tapering or had fully stopped medication. CONCLUSIONS: A large proportion of children with epilepsy remit. Symptomatic etiology, family history, EEG slowing, and initial seizure frequency negatively influence, and age 5-9 years and idiopathic generalized epilepsy positively influence the probability of entering remission. Factors that most influence relapse tend to be different from those that influence remission.     

Corpus callosotomy in refractory idiopathic generalized epilepsy

RATIONALE: A small percentage of patients with idiopathic generalized epilepsy (IGE) do not respond to medical therapy. Generalized tonic-clonic (GTC) seizures are especially debilitating and can be associated with severe injuries. The benefit, safety and effect of corpus callosotomy (CC) in patients with IGE have not been studied. METHODS: We reviewed patients with presumed IGE who underwent CC between 1991 and 2000. Criteria for selection included history, examination, brain imagining, interictal and ictal EEG. All patients had refractory and debilitating tonic-clonic seizures (GTCS) and had failed four or more antiepileptic drugs. Seizure frequency was calculated per month over the last year and pre-operative baseline was compared to last follow-up using paired t-tests. IQ, executive function, language and verbal, non-verbal memory and quality of life (QOL) was compared before and after surgery. Serial EEGs after surgery were reviewed. RESULTS: There were nine patients (seven men), mean age 37.9 (range: 22-49), mean IQ 87.3 (range: 75-107). All had anterior CC. Mean follow-up time was 5.4 years (range: 0.6-10.3 years). One patient died from sudden death in epilepsy 9 months after surgery. There was a significant reduction of GTC seizures from 6.3 to 1.1 (p<0.005). Four patients had more than 80% and eight more than 50% reduction. Of five patients with absence seizures, two became seizure free and one had more than 80% reduction and two worsened slightly, and of three with myoclonic seizures one had more than 90% reduction. One patient had completion of the CC with improvement of myoclonus and absence seizures, but not of GTC seizures and suffered a disconnection syndrome. Another had right frontal focal resection without improvement after new seizures of focal onset. Cognitive testing showed a good outcome (improved or no change) in all cognitive domains. Post-surgical EEG showed new focal slowing and sharp waves. There was no change in QOL. CONCLUSION: CC can be effective in reducing GTC, absence and myoclonic seizures in patients with refractory IGE. These findings suggest that interhemispheric communication of the cerebral cortices plays an important role in the generation of seizures in IGE. Anterior CC appears safe while complete callosotomy has a risk of disconnection syndrome.     

How to diagnose and classify idiopathic (genetic) generalized epilepsies

Idiopathic or genetic generalized epilepsies (IGE) constitute an electroclinically well‐defined group that accounts for almost one third of all people with epilepsy. They consist of four well‐established syndromes and some other rarer phenotypes. The main four IGEs are juvenile myoclonic epilepsy, childhood absence epilepsy, juvenile absence epilepsy and IGE with generalized tonic‐clonic seizures alone. There are three main seizure types in IGE, namely generalized tonic‐clonic seizures, typical absences and myoclonic seizures, occurring either alone or in any combination. Diagnosing IGEs requires a multidimensional approach. The diagnostic process begins with a thorough medical history with a specific focus on seizure types, age at onset, timing and triggers. Comorbidities and family history should be questioned comprehensively. The EEG can provide valuable information for the diagnosis, including specific IGE syndromes, and therefore contribute to their optimal pharmacological treatment and management.     

Use of topiramate in childhood generalized seizure disorders

Topiramate is a sulfamate derivative of the naturally occurring monosaccharide D-fructose. It was initially approved in the United States as adjunctive therapy for partial seizures in 1997. However, there is increasing evidence that it is effective in the treatment of generalized seizures and epilepsy syndromes. Initially, open-label studies using topiramate as add-on therapy in children with refractory generalized seizure types were performed. These showed improvement in patients with the following generalized seizure types: typical and atypical absence, atonic, myoclonic, generalized tonic-clonic, and juvenile myoclonic epilepsy. Double-blind, placebo-controlled multicentered studies in patients with refractory primary generalized tonic-clonic seizures and epilepsy syndromes were performed. The median reduction in seizure frequency for primary generalized tonic-clonic seizures was 56.7% for topiramate and 9% for placebo. Additionally, 13.6% of topiramate-treated patients were primary generalized tonic-clonic seizure free for the study period. In the topiramate-treated juvenile myoclonic epilepsy patients, primary generalized tonic-clonic seizures were reduced > 50% in 73% of patients. Open-label extension showed that primary generalized tonic-clonic seizures were reduced >50% in 63% of topiramate-treated patients for > or = 6 months, and 16% were primary generalized tonic-clonic seizure free > or = 6 months. Accumulating evidence suggests that topiramate has a broad spectrum of antiepileptic effect. Moreover, life-threatening organ toxicity has not been attributed to topiramate. Topiramate is an effective treatment for refractory generalized seizure types and epilepsy syndromes encountered in children.     

The core Dravet syndrome phenotype

Dravet syndrome was described in 1978 by Dravet (1978) under the name of severe myoclonic epilepsy in infancy (SMEI). The characteristics of the syndrome were confirmed and further delineated by other authors over the years. According to the semiologic features, two forms have been individualized: (1) the typical, core, SMEI; and (2) the borderline form, SMEIB, in which the myoclonic component is absent or subtle. Clinical manifestations at the onset, at the steady state, and during the course of the disease are analyzed in detail for the typical Dravet syndrome, and the differential diagnosis is discussed. Onset in the first year of life by febrile or afebrile clonic and tonic-clonic, generalized, and unilateral seizures, often prolonged, in an apparently normal infant is the first symptom, suggesting the diagnosis. Later on, multiple seizure types, mainly myoclonic, atypical absences, and focal seizures appear, as well as a slowing of developmental and cognitive skills, and the appearance of behavioral disorders. Mutation screening for the SCN1A gene confirms the diagnosis in 70-80% of patients. All seizure types are pharmacoresistent, but a trend toward less severe epilepsy and cognitive impairment is usually observed after the age of 5 years.     

Idiopathic Generalized Epilepsies Imitating Focal Epilepsies

Summary:  Classification of epileptic seizures and epilepsy syndromes as either focal or generalized is a fundamental and early part in the diagnostic process and is generally fairly easily accomplished. However, in patients with idiopathic generalized epilepsies, seizure and EEG features may suggest, particularly to the unwary, the occurrence of focal rather than generalized seizures. Misinterpretation of typical absence seizures as focal seizures, especially as temporal lobe seizures and of myoclonic seizures as focal clonic seizures, is a relatively common error and focal features during generalized tonic–clonic seizures may also be quite common. Sequences of seizures in idiopathic generalized epilepsies (such as absences or jerks followed by generalized tonic–clonic seizures) may also cause confusion. Versive and circling seizures are seizure types whose ictal semiology is clearly focal; nevertheless such seizures are described in idiopathic generalized epilepsies accompanied by generalized EEG discharges. The occurrence of focal EEG abnormalities in certain idiopathic generalized epilepsy syndromes is common. This is best known in juvenile myoclonic epilepsy.     

Relation of photosensitivity to epileptic syndromes.

Photosensitivity is the most common mode of seizure precipitation. It is age-related, more frequent in females, and most often found in generalised epilepsies. Little is known about its relation to individual epileptic syndromes. This study on 1062 epileptic patients who had 4007 split screen video EEG investigations revealed that the relation to generalised epilepsy is even more close than generally believed. Versive seizures with visual hallucinations was the only focal seizure type related to photosensitivity. Of the syndromes of generalised epilepsy, only childhood absence epilepsy, juvenile myoclonic epilepsy, and epilepsy with grand mal on awakening were related to photosensitivity. The closest correlation was with juvenile myoclonic epilepsy. This is confirmed by a relation to the poly-spike wave pattern, and by an increase of myoclonic seizures by intermittent light stimuli. No relation was found with early childhood syndromes of generalised epilepsy, or generalised tonic-clonic seizures in the evening, or, most remarkably, with juvenile absence epilepsy. In generalised epilepsies with onset around puberty, photosensitivity could thus act as a pathoplastic factor. The female preponderance in both childhood absences and photosensitivity could be due to the same unknown factor.