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(lē″zhŏn-ek′tŏ-mē) [lesion + -ectomy] Surgical removal of a lesion. In neurosurgery, it is the removal of a structural abnormality in which epileptic seizures are not controlled by medications.


(lē′zhŏ-nĭng) The injuring of tissue, esp. neurologic tissue, for therapeutic purposes, e.g., with radiofrequency or radioactive energy.

lesion network mapping

The identification of neuropsychiatric syndromes based on the functional connections between regions of the brain, i.e., on studies of the human connectome. VAR: lesion network localization.


linear energy transfer. A measure of the rate of energy transfer from ionizing radiation to soft tissue.


[Gr. lethe, oblivion] Pert. to or that which causes death.


(lē-thal′ĭt-ē) The propensity for a disease or injury to cause death, rather than disability, illness, or pain.

synthetic l. The killing of cancer cells by exploiting vulnerable mutations in their genomes that are not present in normal cells. It involves introducing a second abnormality into the malignant cell that kills it when neither the primary vulnerability nor the introduced anomaly would normally result in cell death.


(lĕth′ăr-jē) [Gr. lethargos, drowsiness] Sleepiness, drowsiness, somnolence, or mental sluggishness.

induced l. A hypnotic trance. lethargic (lă-thahr′jĭk), adj.


(lē′thē) [Gr., oblivion] Amnesia.

Letterer-Siwe disease

(lĕt′ĕr-ĕr-sī′wē) [Erich Letterer, Ger. physician, b. 1895; S. August Siwe, Ger. physician, 1897–1966] The most common of three distinct histiocytosis syndromes collectively known as Langerhans cell histiocytosis, marked by proliferation of histiocytes in the viscera, bones, and skin. It is believed that this disease and the other two forms—eosinophilic granuloma of bone and Hand-Schüller-Christian syndrome—share a common pattern of granulomatous lesions with histiocyte proliferation.

 The cutaneous lesions often develop during infancy or early childhood and in some cases are present at birth. These lesions include papulovesicular eruptions; inflamed, pruritic diaper area rashes; and scaly scalp lesions, all of which can be misdiagnosed as “cradle cap” (seborrheic dermatitis of the scalp) or severe diaper rash. When the disease is confined to the skin, spontaneous resolution in infancy may occur. In systemic presentations, the spleen and liver are enlarged, pulmonary infiltration is widespread, and bone marrow failure is accompanied by fever and infections. The cause of the disease is unknown.

 DIAGNOSIS: Diagnosis is based on results of a skin biopsy performed with special staining techniques.

 TREATMENT: No specific treatment exists. Corticosteroids and antineoplastic drugs are used in the more severe forms of the disease, but many children die of pulmonary failure or overwhelming infections despite treatment. SEE: histiocytosis, Langerhans ...

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