Editing Cranial nerves (section)

==Clinical significance==

<!-- This section refers only to conditions which damage the nerves themselves, not symptoms of damage (see 'Function')-->

===Examination===
{{Main|Cranial nerve examination}}
Doctors, [[neurologist]]s and other medical professionals may conduct a [[cranial nerve examination]] as part of a [[neurological examination]] to examine the cranial nerves. This is a highly formalised series of steps involving specific tests for each nerve.<ref name=TC2018 >{{cite book |last1=Talley |first1=Nicholas J. |last2=O'Connor |first2=Simon |title=Clinical examination|year=2018 |publisher=Elsevier |location=Chatswood |isbn=978-0-7295-4286-9 |edition=8th|chapter=Chapter 32. The neurological examination: general signs and the cranial nerves|pages=500–539}}</ref> Dysfunction of a nerve identified during testing may point to a problem with the nerve or of a part of the brain.<ref name=TC2018 />

A cranial nerve exam starts with observation of the patient, as some cranial nerve lesions may affect the symmetry of the eyes or face.<ref name=TC2018 /> Vision may be tested by examining the [[visual field]]s, or by examining the [[retina]] with an [[ophthalmoscope]], using a process known as [[funduscopy]]. Visual field testing may be used to pin-point structural lesions in the optic nerve, or further along the visual pathways.<ref name="Kandel Appendix B" /> Eye movement is tested and abnormalities such as [[nystagmus]] are observed for. The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks. Hearing is checked by voice and [[tuning fork]]s. The patient's [[uvula]] is examined. After performing a shrug and head turn, the patient's tongue function is assessed by various tongue movements.<ref name=TC2018 />

Smell is not routinely tested, but if there is suspicion of a change in the sense of smell, each nostril is tested with substances of known odors such as coffee or soap.  Intensely smelling substances, for example [[ammonia]], may lead to the activation of [[Nociceptor|pain receptors]] of the trigeminal nerve (V) located in the nasal cavity and this can confound olfactory testing.<ref name="TC2018" /><ref name="Kandel Appendix B" />

===Damage===

====Compression====
Nerves may be compressed because of increased [[intracranial pressure]], a [[Mass effect (medicine)|mass effect]] of an [[intracerebral haemorrhage]], or tumour that presses against the nerves and interferes with the transmission of impulses along the nerve.<ref name=DAVIDSONS2010C>{{cite book |editor=Nicki R. Colledge |editor2=Brian R. Walker |editor3=Stuart H. Ralston |title=Davidson's principles and practice of medicine.|year=2010|publisher=Churchill Livingstone/Elsevier|location=Edinburgh|isbn=978-0-7020-3085-7|pages=787, 1215–1217|edition=21st}}</ref>  Loss of function of a cranial nerve may sometimes be the first symptom of an [[cranium|intracranial]] or [[skull base]] cancer.<ref name="Robbins">{{cite book|last=Kumar ()|first=Vinay|title=Robbins and Cotran pathologic basis of disease|year=2010|publisher=Saunders/Elsevier|location=Philadelphia, PA|isbn=978-1-4160-3121-5|page=1266|edition=8th|display-authors=etal }}</ref>

An increase in intracranial pressure may lead to impairment of the optic nerves (II) due to compression of the surrounding veins and capillaries, causing swelling of the eyeball ([[papilloedema]]).<ref>{{cite book |editor=Nicki R. Colledge |editor2=Brian R. Walker |editor3=Stuart H. Ralston |title=Davidson's principles and practice of medicine|year=2010|publisher=Churchill Livingstone/Elsevier|location=Edinburgh|isbn=978-0-7020-3085-7|page=1166|edition=21st}}</ref> A cancer, such as an [[optic nerve glioma]], may also impact the optic nerve (II). A [[pituitary tumour]] may compress the optic tracts or the [[optic chiasm]] of the optic nerve (II), leading to visual field loss. A pituitary tumour may also extend into the cavernous sinus, compressing the oculomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI), leading to double-vision and [[strabismus]]. These nerves may also be affected by herniation of the [[temporal lobe]]s of the brain through the [[falx cerebri]].<ref name=DAVIDSONS2010C />

The cause of [[trigeminal neuralgia]], in which one side of the face is exquisitely painful, is thought to be compression of the nerve by an artery as the nerve emerges from the brain stem.<ref name=DAVIDSONS2010C/> An [[acoustic neuroma]], particularly at the junction between the pons and medulla, may compress the facial nerve (VII) and vestibulocochlear nerve (VIII), leading to hearing and sensory loss on the affected side.<ref name=DAVIDSONS2010C /><ref name=HARRISONS2008CN />

====Stroke====
Occlusion of blood vessels that supply the nerves or their nuclei, an [[ischemia|ischemic]] [[stroke]], may cause specific signs and symptoms relating to the damaged area. If there is a stroke of the [[midbrain]], [[pons]] or [[Medulla oblongata|medulla]], various cranial nerves may be damaged, resulting in dysfunction and symptoms of [[:Template:Lesions of spinal cord and brain|a number of different syndromes]].<ref>{{cite book|editor1=Anthony S. Fauci |editor2=T. R. Harrison |display-editors=etal|title=Harrison's principles of internal medicine|url=https://archive.org/details/harrisonsprincip00asfa |url-access=limited |year=2008|publisher=McGraw-Hill Medical|location=New York [etc.]|isbn=978-0-07-147693-5|pages=[https://archive.org/details/harrisonsprincip00asfa/page/n2564 2526]–2531|edition=17th}}</ref> [[Thrombosis]], such as a [[cavernous sinus thrombosis]], refers to a clot ([[thrombus]]) affecting the venous drainage from the [[cavernous sinus]], affects the optic (II), oculomotor (III), trochlear (IV), ophthalmic branch of the trigeminal nerve (V1) and the abducens nerve (VI).<ref name=HARRISONS2008CN>{{cite book|editor1=Anthony S. Fauci |editor2=T. R. Harrison |display-editors=etal |title=Harrison's principles of internal medicine|url=https://archive.org/details/harrisonsprincip00asfa |url-access=limited |year=2008|publisher=McGraw-Hill Medical|location=New York [etc.]|isbn=978-0-07-147693-5|pages=[https://archive.org/details/harrisonsprincip00asfa/page/n2621 2583]–2587|edition=17th}}</ref>

====Inflammation====
Inflammation of a cranial nerve can occur as a result of infection, such as viral causes like reactivated [[herpes simplex virus]], or can occur spontaneously. Inflammation of the facial nerve (VII) may result in [[Bell's palsy]].<ref name=DAVIDSONS2010>{{cite book |editor=Nicki R. Colledge |editor2=Brian R. Walker |editor3=Stuart H. Ralston |title=Davidson's principles and practice of medicine|year=2010|publisher=Churchill Livingstone/Elsevier|location=Edinburgh|isbn=978-0-7020-3085-7|pages=1164–1170, 1192–1193|edition=21st}}</ref>

[[Multiple sclerosis]], an inflammatory process resulting in a loss of the [[myelin]] sheathes which surround the cranial nerves, may cause a variety of shifting symptoms affecting multiple cranial nerves. Inflammation may also affect other cranial nerves.<ref name=DAVIDSONS2010/> Other rarer inflammatory causes affecting the function of multiple cranial nerves include [[sarcoidosis]], [[miliary tuberculosis]], and [[vasculitis|inflammation of arteries]], such as [[granulomatosis with polyangiitis]].<ref name=HARRISONS2008CN />

====Other====
Trauma to the skull, disease of bone, such as [[Paget's disease of bone|Paget's disease]], and injury to nerves during surgery are other causes of nerve damage.<ref name=HARRISONS2008CN />