Saturday, 19 January 2013

The Neurological Exam

Level 1: the neurological examination

I found a very good site that is well indexed with embedded videos of the neurological examination. You may want to use this as a refresher. 

Neuroexam

Knowing how to perform the neurological examination and knowing the relevant functional neuroanatomy that underpins the examination is vital if you want to graduate from medical school a neurophile. The alternative is neurophobia and a life of professional hell every time you have to see a patient with a neurological complaint. 

Sunday, 13 January 2013

Blink reflex or optical reflex

Level 1: optical reflex & Level 3: blindsight

Optical reflex: In addition to the corneal or sensory blink reflex, blinking can be elicited by visual or auditory input; i.e. bright lights, central and peripheral stimuli and loud noises. The evolutionary purpose of this reflex is again to protect the eyes from foreign bodies and bright lights. Blinking in response to a threatening visual stimulus is known as the optical reflex. 

Neuroanatomy and neurophysiology: The optical reflex is subcortical and is controlled via a pathway that bypasses the lateral geniculate body, optic radiations and visual cortex and goes directly to the superior colliculi. This pathway then relays to several brain stem and spinal nuclei via the tectobulbar and tectospinal pathways that initiate the motor response; i.e. blinking, flexing your arms upwards to protect your eyes, flexion and lateral rotation of the head away from the stimulus and finally flexion of the trunk and lower limbs to duck under the stimulus. Most of us will have experienced this reflex, for example when an insect flies suddenly into our field of vision, or when we duck or dive to prevent being hit in the face by a tennis or football, or the reflex ducking of the head to prevent hitting an overhanging branch when walking. This neuronal pathway is responsible for reflex movements in response to visual stimuli and is the pathway that allows us to play ball sports that require very rapid movements, for example tennis, cricket, baseball etc. A cricketer or tennis player hits the ball before they become consciously aware of the ball. 


Clinical Utility: The optical reflexes can be used to test visual function in patients who are semi-concious or unconscious and indicate that the retina and brain stem are functioning. This reflex is particularly useful in young children. It is also used in patient who present with blindness; if present it indicates that the lesion is very posterior, i.e. cortical, or the patient has functional or hysterical blindness. In these situations the pupillary reflexes are also present , but not the optokinetic response or reflex that is a more complex visual reflex that relies cortical functioning to activate it. 

The optical reflex is usually tested creating  a threatening lateral visual stimulus, typically your finger or hand that is brought into the lateral visual field very rapidly. There is one caveat to the so called hand method is that it may cause a gust of air over the cornea that can stimulate the corneal reflex. To prevent this from happening it can be done from behind a glass screen. This is rarely necessary in a clinical situation. 


This science experiment shows the optical blink reflex very well!


The optical reflex is responsible for the clinical phenomenon of blindsight, i.e. patients responding to visual stimuli without being of aware of it. The majority of these patients are conciously awarr of being blind, but rarely  they may deny being blind. The latter is referred to as the Anton-Babinski syndrome

The following is a short clip from the two-part documentary "Phantoms in the Brain" in which neurologist V S Ramachandran describes how the study of patients with certain types of brain damage can give us clues about the nature of consicousness and perception. For those of you who are interested in this can see both documentaries for free online. 


"Phantoms in the Brain" V S Ramachandran 

For those of you who are interested you may find this case study of interest: 

Hamm et al. Affective blindsight: intact fear conditioning to a visual cue in a cortically blind patient. Brain. 2003 Feb;126(Pt 2):267-75.

Blindsight refers to remarkable residual visual abilities of patients with damage to the primary visual cortex (V1). Recent studies revealed that such residual abilities do not apply only to relatively simple object discriminations, but that these patients can also differentially categorize and respond to emotionally salient stimuli. The current study reports on a case of intact fear conditioning to a visual cue in a male patient with complete bilateral cortical blindness. The patient was admitted to the stroke unit of the neurological department because of complete loss of vision. Both CT and structural MRI scans confirmed lesions in both territories of the posterior cerebral artery. No visual evoked potentials could be detected confirming complete cortical blindness. During fear conditioning, a visual cue predicted the occurrence of an aversive electric shock. Acoustic startle probes were presented during and between the conditioned stimuli. Relative to the control condition, startle reflexes were substantially potentiated when elicited in the presence of the conditioned stimuli. No such potentiation was observed prior to conditioning. These data suggest that fear learning to visual cues does not require a cortical representation of the conditioned stimulus in the primary sensory cortex and that subcortical pathways are sufficient to activate the fear module in humans.


Further reading: blindsight

Friday, 11 January 2013

Multiple sclerosis lecture - Brain and Behaviour 2

Level 1:  Year-2 Brain & Behaviour Lecture on 17th Jan 2013





Saltatory axonal conduction!

Conduction via a demyelinated axonal segment!

The corneal or blink reflex

Level 1



Description: The corneal is one of the blink reflexes, is an involuntary blinking of the eyelids elicited by stimulation of the cornea. Stimulation should elicit both a direct and indirect or consensual response (opposite eye). The reflex consumes a rapid rate of 0.1 second. The evolutionary purpose of this reflex is to protect the eyes from foreign bodies. 

Neuroanatomy: As will all reflexes it has an afferent (sensory) and efferent (motor) arm. The reflex is mediated by the nasociliary branch of the ophthalmic branch (Vi) of the trigeminal or 5th cranial nerve that senses the stimulus on the cornea, lid, or conjunctiva. The temporal and zygomatic branches of the facial or 7th cranial nerve initiates the motor response. The reflex is driven via interneurones in the medulla. 


Interpretation: An absent corneal reflex can be due to sensory loss in Vi (e.g. neuropathy or ganglionpathy), weakness or paralysis of the facial muscles (myopathy) or facial nerve (facial palsy, for example Bell's palsy) or brain stem disease. For a myopathy to cause a loss of the blink reflex the weakness has to be very severe, for example a chronic progressive external ophthalmoplegia (CPEO)

Contact lenses may diminish or abolish the testing of this reflex; therefore an absent corneal reflex is not necessarily abnormal. The examination of the corneal reflex is useful in unconscious patients and if present indicates that the lower brain stem is functioning. It is used as part of the assessment for determining if someone is brain dead; if the corneal reflex is present the person can't be diagnosed with brain death.

Clinical demonstration: The following YouTube video shows you how to do a corneal reflex:


Neurophysiology: The blink reflex can be tested electrophysiologically by stimulating the supra-orbital nerve and measuring the blink in both eyes. The ipsilateral blink occurs quicker (R1 component) compared to the contralateral blink that occurs a few milliseconds later with the R2 component. In the figure below you will notice that the R2 component affects both eyes, i.e. the ipsilateral eye has a double input. The figure below demonstrates the hypothesized wiring diagram of the blink reflex. 


James Parkinson's London

Level 1

It should be compulsory for all medical students at Barts and The London to watch this video:


"Professor Gerald Stern, Emeritus Professor of Neurology UCL, who narrates this short documentary is an alumnus of The London Medical School. He also grew up in Whitechapel; his grandfather ran a general store on the Whitechapel Road next to the Whitechapel Bell Foundry on the site that is now the East London Mosque! Professor Stern is one of my mentors and a great neurologist." 

Friday, 4 January 2013

4th-yr lecture notes on multiple sclerosis

Level 1: the following are my lecture notes and presentation from my lecture on the 17th December 2012. These can be downloaded.