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Nervous System & Special Senses


Temporal Lobe Epilepsy

Worldwide, epilepsy accounts for 1% of the total disease burden, and it affects approximately 2.5 million patients in the United States. There is no gender or age predilection.


Temporal lobe epilepsy (TLE) is the most common cause of intractable epilepsy. It presents as recurrent, unprovoked focal seizures (previously termed "partial seizures") with or without spreading to both sides.


Mesial temporal lobe epilepsy is its most common form, and is thought to be secondary to lesions and functional disturbances from cerebral insults such as febrile seizures, status epilepticus or trauma.


The other major type of TLE is familial temporal lobe seizures, a heterogeneous syndrome of psychic and autonomic auras associated with dysmnesic symptoms that may evolve into focal seizures with impaired awareness (previously termed "complex partial seizures") or focal to bilateral seizures (previously termed "secondary generalized seizures").


Other known causes for this condition are congenital defec

ts and tumors.

Carbamazepine

Carbamazepine, an anticonvulsant, has been widely used in the treatment of newly diagnosed focal or generalized epilepsy.


It is thought to work by inhibiting voltage-gated sodium channel activity.


There is evidence pointing towards its efficacy in focal seizures with complex symptomatology, generalized tonic-clonic seizures, mixed seizure patterns, trigeminal neuralgia and glossopharyngeal neuralgia. It does not, however, appear to control absence seizures.


Contraindications include a history of previous bone marrow depression, significant atrioventricular conduction abnormalities and a known sensitivity to tricyclic compounds such as amitriptyline, desipramine, or imipramine. Use with MAO inhibitors is also not recommended.


Adverse events include Stevens Johnson syndrome/toxic epidermal necrolysis (especially in patients of Han Chinese ancestry where the presence of the HLA-B*1502 allele variant is high), aplastic anemia, leucopenia and hyponatremia.

Focal Seizures

Seizures fall into two broad categories: generalized and focal. A focal seizure originates in a discrete area of the brain, from which it may or may not spread to become bilateral tonic-clonic seizures.


Focal seizures can be further classified with regards to whether there is impaired consciousness or not. Previously, these were respectively termed "complex" and "simple" seizures. This terminology was updated in the International League Against Epilepsy (ILAE) 2017 guidelines.

Phenytoin

Phenytoin is a highly effective anticonvulsant used in the management of epilepsy; as an added advantage, it has a relative absence of sedating side effects, which are otherwise common with these medications. It is also one of the most effective drugs for the treatment of acute seizures and status epilepticus.


Phenytoin is thought to work via membrane-potential-dependent blockade of the sodium channels, decreasing electrical activity. It has also been proposed to reduce glutamate release.


Parenteral phenytoin must be administered in specific solutions to overcome its poor solubility, and should not be given intramuscularly due to the risk of necrosis and abscess formation.


A number of serious adverse reactions, including arrhythmias and hypotension, particularly in patients older than 40 years of age with cardiovascular disease, are associated with this medication. An infusion must be carefully monitored, with measurements of blood pressure and electrocardiogram. Long term side effects include osteoporosis, hirsutism and gingival hyperplasia.

EEG in Temporal Lobe Epilepsy

An interictal electroencephalogram (EEG) is useful for the diagnosis and management of epilepsy. However, a normal EEG does not rule out the disorder.


Long-term EEG monitoring may be useful to identify paroxysmal seizure activity, psychogenic or nonepileptic seizures, to differentiate between nocturnal epilepsy and parasomnias, for the quantification of interictal epileptiform discharges (IED) or seizure frequency and for evaluation for candidacy for epilepsy surgery.


In mesial temporal lobe epilepsy associated with unilateral hippocampal sclerosis, the expected findings on EEG include anterior or mid temporal interictal spikes, either ipsilateral or predominantly over the pathologic temporal lobe. A characteristic rhythmic 5-7 Hz ictal discharge is typically seen accompanying seizures.


In familial temporal lobe epilepsy, focal IED is only seen in about 20% of cases. However, when present, these are a defining feature of familial focal epilepsy with variable foci, usually congruent with the seizure type in individual cases.


Interictal EEG is not useful in cases of well controlled, chronic epilepsy.

Carbamazepine-induced Hyponatremia

Carbamazepine is an important cause of iatrogenic hyponatremia via its antidiuretic effects.


On confirmation of a serum sodium of less than 135 mEq/L, the type of hyponatremia should be identified as hypovolemic, hypervolemic or euvolemic. This can be achieved via a clinical assessment along with measuring the plasma osmolality and urinary excretion of sodium.


Hyponatremia can be quite severe if left untreated; acutely low sodium levels can lead to cerebral edema and other complications.


When hyponatremia is acute and severe (defined as less than 48 hours in duration of a serum sodium less than 125 mEq/L), urgent treatment should be commenced with hypertonic saline at a rate of no more than 1-2 mmol per liter per hour. The patient should not reach normo- or hypernatremia within the first 48 hours of correction.


Chronic, asymptomatic hyponatremia can be treated by removing the offending cause, or if this is unsuccessful, fluid restriction to less than 1-1.5L per day.

References

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