Neurological disorders – epilepsy, Parkinson’s disease and multiple sclerosis

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Chapter 21 Neurological disorders – epilepsy, Parkinson’s disease and multiple sclerosis

Epilepsy

Definitions

A seizure is a clinical symptom or sign caused by abnormal electrical discharges within the cerebral cortex.1 For example, a tonic–clonic seizure refers to a pattern by which a patient loses consciousness, becomes generally stiff (tonic), and subsequently jerks all limbs (clonus); whereas a complex partial seizure refers to a constellation of impaired consciousness, déjà vu sensations, epigastric rising sensation, olfactory hallucinations and motor automatisms, e.g. lip smacking.2 By contrast, epilepsy refers to the clinical syndrome of recurrent seizures, and implies a pathological state that predisposes to further future seizures. Hence having one, or even a single cluster of seizures (i.e. over a few days) does not in itself qualify as epilepsy, since these seizures may have been due to a febrile illness or drug intoxication that themselves later resolve. By contrast, having at least two seizures, separated by at least a few weeks, is usually sufficient to signify epilepsy. Only one-third of people having seizures develop chronic epilepsy.

Pathology and seizure types

Epilepsy affects 0.5–1% of the general population, while the lifetime risk of having a seizure is 3–5 %. There are both multiple causes and multiple seizure types.3 Approximately half of adult epilepsy is believed to be due to genetic or early developmental causes, although the exact nature of these – e.g. sodium channel mutations or cerebral palsy – are determined in only a small minority. The other half of adult epilepsy is due to acquired causes, such as alcohol, stroke, traumatic head injury or brain tumours.

The cause of epilepsy determines the seizure type. Genetic causes (i.e. ‘primary’) predispose to generalised seizures4 characterised by tonic–clonic or absence seizures (lapses of consciousness lasting seconds), myoclonus (random limb jerk at other times), photosensitivity (seizures triggered by flashing lights), EEG showing a 3-Hz spike-and-wave pattern, and a normal MRI brain. Conversely, where focal brain injury has occurred, e.g. brain tumour or stroke, and the brain scan is abnormal, focal epileptic discharges occur within the brain leading to a partial seizure – i.e. when only a narrow set of brain functions are disturbed, e.g. causing single limb jerking (implying motor cortex involvement). Importantly, partial seizures can propagate very quickly to become a ‘secondary generalised seizure’. Another common cause of adult-onset partial epilepsy is maldevelopment of the medial temporal lobes (‘mesial temporal sclerosis’) believed to be due to injury, e.g. hypoxia or infection, during fetal or early childhood life, and sometimes apparent as atrophic hippocampi and amygdala on high-resolution MRI.

Principles of management

Practical guide to antiepilepsy drugs

1. When to initiate. Following a single seizure the chance of a further seizure is approximately 25% over the following 3 years. Furthermore, only 33% of single-seizure patients develop chronic epilepsy. Hence the majority of first seizures are provoked by a reversible, and often recognisable, factor, e.g. infection, drug toxicity, surgery. For these reasons, following a single seizure6 anticonvulsants are not generally prescribed, whereas after two or more distinct seizure episodes (i.e. with more than a few weeks apart between episodes), they generally are prescribed. Immediate treatment of single or infrequent seizures does not affect long-term remission but introduces the potential for adverse effects. Patients need to be made aware that anticonvulsant therapy reduces harm caused by generalised seizures, and may also reduce the risk of sudden death in epilepsy (SUDEP), that usually occurs during sleep.

2. Monotherapy. Although the choice of anticonvulsants is large (approximately 20), firstline therapy is generally restricted to one of only a few drugs that have a good track record and are relatively safe and well-tolerated. Initial therapy is confined to a single drug (i.e. monotherapy) that is usually effective in stopping seizures or at least significantly decreasing their frequency. The majority of epilepsy patients (70%) can remain on monotherapy for adequate control, although sometimes the choice of monotherapy may need to be switched to allow for tolerance or optimisation of seizure control. As the number of single anticonvulsants tried increases, the incremental likelihood that any new one will offer a significant reduction in seizures decreases: from 50% response to a first drug, to an additional 30% to a second drug, to an extra 10% to a third drug, and less than 5% for any subsequent drug tried.

3. What drug to initiate. For older types of anticonvulsants, knowing the seizure type – i.e. whether partial or primary generalised – mattered, because in certain cases the spectrum of seizure efficacy is limited, and, moreover, certain seizure types can be worsened by ill-chosen drugs. For example, carbamazepine is an effective first-line therapy for partial seizures but may worsen primary generalised, absence or myoclonic seizures; similarly phenytoin can worsen absence and myoclonic seizures. Ethosuximide, by contrast, is only effective in primary generalised, and not partial, seizures.

More modern anticonvulsants, by contrast, are in general effective over a much broader range of seizure types allowing for more confidence of use even when seizure type is uncertain. Thus sodium valproate, lamotrigine and levetiracetam are active against both primary and secondary generalised epilepsy, and being relatively well tolerated, account for most first-line prescriptions. In one head-to-head study comparing popular first-line therapies for generalised and partial seizures, lamotrigine was generally tolerated better than other drugs, while valproate was the most efficacious; carbamazepine and topiramate were more likely to cause unwanted effects.7

4. Women of reproductive age and children. These categories of patients prompt selection of particular drugs and avoidance of others (see below for more detail).

5. Polytherapy. If a trial of three or so successive anticonvulsants (i.e. taken as monotherapy at adequate dosage for at least several months) does not control a patient’s epilepsy, it may be worthwhile trying dual therapy. Polytherapy offers the theoretical advantage of controlling neuronal hyperexcitability by more than one mechanism, that can be synergistic. In reality, increasing polytherapy often adheres to the law of diminishing returns, viz. the proportion of uncontrolled patients who show a positive response decreases at each addition of drug number And at the same time, adverse effects become more likely.

6. Abrupt withdrawal. Effective therapy must never be stopped suddenly, as this is a well-recognised trigger for status epilepticus, which may be fatal. But if rapid withdrawal is required by the occurrence of toxicity, e.g. due to a severe rash or significant liver dysfunction, a new drug ought to be started simultaneously. The speed by which the dose of a new drug can be raised varies according to drug type and urgency.

7. Circumstantial seizures. In cases where fits are liable to occur at a particular time, e.g. the menstrual period, adjust the dose to achieve maximal drug effect at this time or confine drug treatment to this time. For example, in catamenial epilepsy, clobazam can be useful given only at period time.

Once treatment is stable, patients should keep to a particular proprietary brand as different brands of the same generic agent (e.g. carbamazepine) may exhibit varying pharmacokinetics.

Pregnancy and epilepsy

Pregnancy worsens epilepsy in about a third of patients, but also improves epilepsy in another third. One of the main concerns in this patient group is that all anticonvulsants increase the chance of teratogenicity slightly, with valproate, phenytoin and phenobarbital carrying most risk. The toxicological hazard must be weighed against the risk of seizures which themselves can be harmful to mother and unborn baby, and are likely to worsen if anticonvulsants are discontinued. For instance, the risk of major congenital anomalies in the fetus is 1% for healthy mothers, 2% in untreated epileptic mothers (in observational studies, so generally not severe epileptics), and 2–3% in mothers on epilepsy monotherapy. Valproate, by contrast, has been associated with a malformation rate of approximately 10%,10 while 20–30% of children are subsequently found to have mild learning disabilities or require ‘special needs’ education. The UK maintains a national drug monitoring register of all pregnant women taking antiepileptic drugs.

Doctor–patient discussions about what antiepileptic drug, if any, and at what dose, are required pre-conception. Advance planning is preferred because:

During pregnancy, liver enzymes become induced, which has implications in epilepsy. Firstly, patients on lamotrigine before conception require a gradually increased dose during the pregnancy, to cope with enhanced catabolism (lowering lamotrigine plasma concentration). Secondly, enzyme-inducing drugs often aggravate a relative deficiency of vitamin K that occurs in final trimester women, predisposing to postpartum haemorrhage; vitamin K is therefore given by mouth during the last 2 weeks of pregnancy.

Epilepsy in children

Seizures in children tend to arise from different sets of causes (usually genetic or cerebral palsy) from those arising in adults, and can carry either very good long-term outcomes, e.g. spontaneous resolution, or, less commonly, bad outcomes, e.g. gradual deterioration. Treatments are similar to those used in adults, but certain seizure types necessitate drugs that are rarely used in adults, e.g. ethosuximide for absence seizures, or vigabatrin for refractory partial seizures (partly because children may become irritable or more cognitively impaired with drugs such as valproate and phenobarbital).

Febrile convulsions. Seizures triggered by fever due to any cause (typically viral infection) are common in young children (3 months – 5 years old). Two-thirds of such children will have only one attack, and in total only 2% will progress to adult epilepsy. For this reason, continuous prophylaxis is seldom given, except for those cases where atypical febrile seizures occur, e.g. lasting for more than 15 min, have focal features or recur within the same febrile illness. Long-term antiepileptic therapy is avoided where possible in children due to recognised adverse effects of most such drugs on learning and social development. Febrile convulsions may be treated on an ad hoc basis by issuing parents with a specially formulated solution of diazepam for rectal administration (absorption from a suppository is too slow) that allow for easy and early administration. Febrile convulsions may be prevented by treating febrile children with paracetamol and cooling with sponge soaks.

Status epilepticus

Status epilepticus refers to continuous or repeated epileptic seizures for more than 30 min. It often arises in patients already known to have epilepsy, in whom antiepileptic drug therapy has been inappropriately withdrawn or not taken. It can be the first presentation of epilepsy, due to an acquired brain insult, e.g. viral encephalitis.

Status epilepticus is a medical emergency. In the first instance, general resuscitation (airway control, oxygen, intravenous saline, etc.) is required. Treatment of seizures is initially with the intravenous benzodiazepine lorazepam (0.5–4 mg). Lorazepam is preferred to diazepam because it has a longer effective t½ and is less lipophilic and so accumulates less in fat, causing less delayed toxicity (hypotension and respiratory depression). The speed of action of lorazepam and diazepam are both rapid. Phenytoin i.v. may be started simultaneously to suppress further seizures, given as a loading dose (15–20 mg/kg body-weight) over 1 h, while monitoring ECG and blood pressure for arrhythmias and hypotension. Subsequently a maintenance dose of approximately 300 mg/day is given and adjusted according to plasma levels (corrected for albumin). Phenobarbital may be given i.v. as a third-line drug when seizures continue. At this point, the level of sedation (due both to seizures and drugs) is usually sufficiently great to warrant general anaesthesia, e.g. with propofol or thiopental, combined with intubation, mechanical ventilation and intensive care management. Pharmacologically induced sedation is removed periodically to allow for assessment of seizure activity (both from clinical observations and using EEG).

If resuscitation facilities are not immediately available, diazepam by rectal solution is a useful option. In some cases, midazolam (nasally) may be preferred, e.g. in children or those with severe learning disability. Intravenous benzodiazepines should not be used if resuscitation facilities are unavailable as there is risk of respiratory arrest.

Always investigate and treat the cause of a generalised seizure. Give aciclovir i.v. if viral encephalitis is suspected or, if status is triggered by removing an antiepileptic drug, it must be re-instituted. Magnesium sulphate is the treatment of choice for seizures related to eclampsia (see also p. 125).11

Details of further management appear in Table 21.1.

Table 21.1 Treatment of status epilepticus in adults

Status Treatment
Early Lorazepam 4 mg i.v., repeat once after 10 min if necessary, or clonazepam 1 mg i.v. over 30 s, repeat if necessary, or diazepam 10–20 mg over 2–4 min, repeat once after 30 min if necessary
Established Phenytoin 15–18 mg/kg i.v. at a rate of 50 mg/min, and/or phenobarbital 10–20 mg/kg i.v. at a rate of 100 mg/min
Refractory Thiopental or propofol or midazolam with full intensive care support

Pharmacology of individual drugs

Modes of action

Antiepilepsy (anticonvulsant) drugs aim to inhibit epileptogenic neuronal discharges and their propagation, while not interfering significantly with physiological neural activity. They act by one of five different mechanisms given below. It is generally recommended that when more than one drug is needed to control seizures, then drugs chosen should be selected from different classes of action, both to target epileptogenesis at more than one control point (resulting in synergistic effects) and to reduce unwanted effects.

Enhancement of gamma-aminobutyric acid (GABA) transmission

Examples: benzodiazepines, phenobarbital, valproate, vigabatrin, tiagabine.12 By enhancing GABA, the principal inhibitory transmitter of the brain, neuronal membrane permeability to chloride ions is increased, which secondarily reduces cell excitability. Benzodiazepines and barbiturates activate the GABA receptor via specific benzodiazepine and barbiturate binding sites.

Sodium channel blockers

Carbamazepine

Carbamazepine (Tegretol) acts predominantly as a voltage-dependent sodium channel blocker, thereby reducing membrane excitability.

Phenytoin

Phenytoin (diphenylhydantoin, Epanutin, Dilantin) acts principally by blocking neuronal voltage-dependent sodium ion channels; this action is described as membrane stabilising, and discourages the spread (rather than the initiation) of seizure discharges.

Lacosamide

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