Neurological Glossary

Neurological terms are often complicated and difficult to understand. The following glossary defines a variety of terms to help you understand neurological terminology:

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Obstructive sleep apnea

Repetitive cessation of breathing during sleep for 10 seconds or more due to complete closure of the throat; characterized by snoring


    Mild to moderate reduction in alertness with slow response to stimulation

      Occupational Therapy

      Therapy involving evaluation and treatment of physical and psychiatric conditions Occupational Therapists also advise on equipment needs

        Ocular migraine

        Form of migraine characterized by monocular visual loss typically followed by an ipsilateral periorbital (or less commonly diffuse) headache

          Oculocephalic response (OCR/Dolls phenomenon)

          Reflex eye movements assessed in an uncooperative or unresponsive patient to determine whether brainstem extraocular pathways are intact; performed by quickly turning the head

            Oculogyric crisis

            Acute dystonic reaction to neuroleptic drugs manifested by sudden sustained twisting of the head and upward deviation of the eyes; responsive to anticholinergic or antihistamine medications

              Oculomotor nerve (cranial nerve III)

              Motor nerve innervating the superior

                Oculovestibular response (OVR/cold calorics/vestibulo-ocular response/VOR)

                Reflex eye movements that keep vision stable upon head rotation and involve connections between the ipsilateral 8th and 3rd and the contralateral 6th nerve nuclei; horizontal movements are tested by instilling ice water into one ear

                  Odontoid fracture

                  fracture through the odontoid process of C2 due to horizontal shearing and axial loading; type I usually stable and type II usually unstable Oedema Swelling due to excessive accumulation of fluid in the body tissues.

                    Olfactory nerve (cranial nerve I)

                    Sensory nerve mediating smell


                      Reduction in blood flow associated with an increase in oxygen extraction fraction but maintenance of normal metabolism

                        Oligoclonal bands

                        Two to five bands of immunoglobulins on protein electrophoresis of cerebrospinal fluid seen in multiple sclerosis and other central nervous system disorders


                          Form of CNS glia responsible for the synthesis and maintenance of myelin and the formation of perineuronal satellites


                            WHO grade II primary CNS tumour typically involving the cerebral hemispheres; characterized histologically by uniform round nuclei with a perinuclear halo (fried egg artifact) and perineuronal satellitosis

                              Olivopontocerebellar atrophy (OPCA)

                              Older term for a variant of multisystems atrophy characterized by preferential degeneration of the cerebellum and brainstem producing symmetric parkinsonism and ataxia

                                Ondine's curse (central alveolar hypoventilation)

                                Condition in which the respiratory centre in the brain is unable to stimulate breathing in response to an increased amount of carbon dioxide in the blood; typically worsens during sleep

                                  One-and-a-half syndrome

                                  Combination of an abducens nerve nuclear lesion and an ipsilateral medial longitudinal fasciculus lesion resulting in an ipsilateral gaze paralysis and loss of adduction of the ipsilateral eye

                                    On-off phenomenon


                                      Onset seizures

                                      Seizures which happen on the day of a stroke.

                                        Opening pressure

                                        Cerebrospinal fluid pressure obtained upon entering the subarachnoid space by spinal puncture


                                          Paralysis or weakness of one or more of the muscles that control eye movement

                                            Ophthalmoplegic migraine

                                            Form of migraine characterized by periorbital pain and ocular cranial nerve palsy (usually the third nerve and including ptosis and a dilated pupil)


                                              Abnormal posturing of the body with severe arching of the back and the head thrown backwards; may occur with meningeal irritation

                                                Optic atrophy

                                                Wasting of the optic disc that results from partial or complete degeneration of optic nerve fibres and is associated with a loss of visual acuity

                                                  Optic chiasm

                                                  Partial crossing of about half the axons in each optic nerve to enter the contralateral optic tract; lesion results in loss of vision in the temporal hemifield of both eyes

                                                    Optic disc (optic nerve head)

                                                    Location where all the ganglion cells exit the retina to form the optic nerve; known as the blind spot since there are no receptors in this part of the retina

                                                      Optic nerve

                                                      Extent of the visual system pathway from the retina to the optic chiasm; contains axons of ganglion cells in the retina of the ipsilateral eye; lesion results in loss of vision in one eye

                                                        Optic neuritis

                                                        Inflammation or demyelination of the optic nerve with transient or permanent loss of vision +/- pain

                                                          Optic radiations

                                                          Extent of the visual system pathway from the lateral geniculate nucleus of the thalamus to primary visual cortex in the occipital lobe; involves temporal (Meyers loop) and parietal lobes

                                                            Optic tract

                                                            Extent of the visual system pathway from the optic chiasm to the lateral geniculate nucleus of the thalamus; contains axons from ganglion cells in the retinas of both the left and right eyes

                                                              Optokinetic nystagmus

                                                              Reflexive eye movement that consists of the intermittent rotation of the eye when a subject looks at moving objects; designed to keep an entire moving scene stable on the retina so that it doesnt blur


                                                                Professional working with people who have visual problems and abnormal eye movements

                                                                  Orthostatic hypotension

                                                                  Sudden drop in blood pressure experienced upon standing up


                                                                    Mechanical appliance such as a leg brace or splint that is designed to control


                                                                      Professional who specialises in the supply and fitting of aids and equipment to help improve mobility


                                                                        Visual disturbance characterized by continuous


                                                                          Enhanced sensitivity to odors


                                                                            Premature fusion of all cranial sutures producing an elongated head