Bma Illustrated Medical Dictionary

the central nervous system (CNS) or be

part of the peripheral nervous system.

Patches of demyelination are visible

on MRIof the brain in multiple sclerosis.

The cause of the demyelination is not

known. In many cases, demyelination

attacks alternate with periods of partial

or complete recovery of nerve function.

In encephalomyelitis, there is inflamma-

tion of nerve cells within the CNS and

sometimes areas of demyelination.

dendritic ulcerA type of corneal ulcer

commonly caused by infection with her-

pes simplexvirus.

dengueA tropical disease caused by a

virus spread by the mosquito AEDES

AEGYPTI. Symptoms include fever, head-

ache, rash, and joint and muscle pains,

which often subside after about 3 days.

There is no specific treatment for dengue.

Prevention involves protection against

mosquito bites (see insect bites).

densitometryAn imaging technique that

uses low-dose X-raysto measure bone

density, as determined by the concen-

tration of calcified material. It is used to

diagnose and assess the severity of

osteoporosis, especially in the spine and

femur, and to assess its response to

treatment. During the procedure, X-rays

are passed through the body. A computer

assesses the amount of X-rays absorbed

by the body and uses this information

to calculate the bone density.

densityThe “compactness” of a subs-

tance, defined as its mass per unit

volume. In radiology, the term relates to

the amount of radiation absorbed by the

structure being X-rayed.Bone, which ab-

sorbs radiation well, appears white on

X-ray film. A lung, which contains mostly

air, absorbs little radiation and is dark

on film. The same holds true in CT scan-

ningand MRI. (See also specific gravity.)

dental emergenciesInjuries or disor-

ders of the teeth and gums that require

immediate treatment because of severe

pain and/or because delay could lead to

poor healing or complications. A dis-

lodged tooth can be reimplanted (see

reimplantation, dental) successfully if it is

done without delay. A partly dislodged

tooth should be manipulated back into

the socket straight away. Other dental

emergencies include a broken tooth

(see fracture, dental), severe toothache,

which may be caused by an abscess

(see abscess, dental), and Vincent’s dis-

ease (see gingivitis,acute ulcerative).

dental examinationAn examination of

the mouth, gums, and teeth by a dentist

as a routine check or as part of the

assessment a person complaining of a

symptom. Routine examinations enable

tooth decay and gum disease to be

detected and treated at an early stage.

During a routine dental examination,

the dentist uses a metal instrument to

DENDRITIC ULCER DENTAL EXAMINATION

D

X-ray detector picks

up unabsorbed

X-rays

Monitor displays X-ray

image of spine

X-ray

generator

Foam support

for legs

DENSITOMETRY