Making headway: problem-oriented approaches to neurological disease

Transcription

1 Vet Times The website for the veterinary profession Making headway: problem-oriented approaches to neurological disease Author : Mark Lowrie Categories : Vets Date : July 4, 2011 Mark Lowrie provides information on neurological conditions, describing different presentations as well as specifying principles that can be applied to them PATIENTS can present with an array of neurological diseases involving the head. However, a thorough cranial nerve examination in general practice is often difficult during a short consultation. Therefore, it s helpful to be familiar with the varying importance of these nerves in patients with different neurological presentations of the head. This allows the practitioner to focus on specific tests during a consultation to decide on the most appropriate course of investigation. The aim of this two-part feature is not to give an exhaustive list of tests and diseases, but to provide principles that can be applied to particular neurological presentations involving the head. Anatomy The cranial nerves are numbered from I to XII. They predominantly originate from the brainstem ( Figure 1 ). The brainstem can essentially be considered as a continuation of the spinal cord. For example, the proprioceptive pathways pass through the brainstem conveying information on limb position and are also involved in synaptic connections for numerous cranial nerve reflexes. Clinical signs of cranial nerve dysfunction appear ipsilateral to the side of the lesion with all cranial 1 / 8

2 nerves except cranial nerve IV, which decussates, causing signs to appear contralateral to the side of the lesion. Cranial nerves I and II Cranial nerves I (olfactory) and II (optic) are different to the other cranial nerves in that they originate from the forebrain as opposed to the brainstem and are therefore considered an extension of the brain ( Figure 1 ). The olfactory nerve is small in humans, but in our patients it has evolved to become a lobe (the olfactory lobe) at the rostral aspect of the brain. The optic nerve is an extension of the central nervous system connecting the brain with the eye and is ensheathed by the meninges, unlike all other cranial nerves. Cranial nerves III to XII The remaining cranial nerves all originate from the brainstem. As such, they are generally only affected in patients exhibiting brainstem disease or peripheral nerve diseases involving certain cranial nerves (such as neuropathies; Figure 1 ). Physiology The key to understanding how to evaluate the cranial nerves lies in the basic concept of a reflex arc ( Figure 2 ). The reflex that most people are familiar with is the patellar reflex; a simple monosynaptic reflex arc. The basic components of this reflex arc are a sensory receptor (for example, stretch receptors in the patella tendon) that connects with the sensory (or afferent ) nerve conveying this information to the spinal cord where it synapses with a motor (or efferent ) nerve. The nerve transmits impulses back down the limb into the effector organ (in this case the quadriceps muscle). This basic concept can be applied to reflexes involving the cranial nerves. One cranial nerve acts as the sensory afferent nerve that enters the brainstem, where it synapses with a second efferent cranial nerve, which terminates on the effector organ. In reality, it is often more complicated than this because reflexes are usually more complex and involve more than one synapse, such as the cutaneous trunci reflex. Polysynaptic reflexes still involve an afferent and efferent arm, but may involve one or more interneurons. In the example of the cutaneous trunci reflex, a single interneuron within the white matter of the spinal cord connects the afferent and efferent nerves, thus completing the reflex arc. The majority of reflexes assessed during a cranial nerve examination are polysynaptic reflexes. One example of this is the palpebral reflex ( Figure 3 ) in which the trigeminal (V) nerve is stimulated via palpation of the medial or lateral canthus of the eye. Subsequently, this nerve synapses on an 2 / 8

3 interneuron that connects to the facial (VII) nerve innervating the orbicularis oculi muscle. Another example is the pupillary light reflex ( Figure 4 ). The afferent sensory neuron in this case is the optic (II) nerve and the efferent motor neuron is the oculomotor (III) nerve. These two neurons are linked via an interneuron that originates in the pretectal nucleus of the brainstem and terminates on the parasympathetic nucleus of the oculomotor nerve. The menace response is an important test of neurological function, but it is not considered to be a true reflex ( Figure 5 ). Instead, this is a learned response that develops in cats and dogs by 12 weeks of age (although this varies depending on the species and the breed) and hence it is not a true innate reflex. The menace response is a multisynaptic pathway assessing many components of the brain ( Figure 6 ). It has both an afferent (provided by cranial nerve II) and an efferent (provided by cranial nerve VII) neuron, but the synaptic pathways or interneurons are complex and pass through the forebrain, cerebellum and brainstem. This cortical pathway is the reason for it being considered a response and not a reflex. A list of the cranial nerves is shown in Table 1. Table 2 highlights the reflexes that can be performed during a neurological examination to evaluate these nerves. The basic concept to remember is that when testing any reflex, an assessment is being made of the afferent or sensory neuron, the interneuron (when present), the efferent or motor neuron and the sensory and motor organs. The interneuron(s), when considering cranial nerve reflexes, will be situated within the brain, usually the brainstem, but possibly also the forebrain and/or cerebellum. Therefore, in the situation of an abnormal menace response, this test alone is fairly non-specific as it could relate to a lesion anywhere within this pathway. However, by collating the information obtained by performing two or more cranial nerve reflex tests and the menace response, the lesion can be localised more precisely. For example, an abnormal menace response with a normal pupillary light reflex and normal palpebral reflex suggests that cranial nerves II, III, VII and the brainstem are all unaffected and therefore the lesion must lie within the forebrain or cerebellum. At this point, clinical signs relating specifically to forebrain and cerebellar disease are used for localisation. When to perform a cranial nerve examination A problem-oriented approach would seem the most practical way to approach the cranial nerves. For example, if a patient is presented with a vestibular disorder, it is important to understand which 3 / 8

4 aspects of the neurological examination are important to localise the lesion, formulate a list of potential diseases and determine a diagnostic plan. The presenting problems that will be considered in this two-part feature are: blindness; dropped jaw; trismus; unilateral or bilateral masticatory muscle atrophy; facial nerve paralysis; and head tilt. The remainder of this article will concentrate on blindness, while part two will concentrate on the other conditions. Each condition is discussed with tips on specific aspects of the neurological examination that should be evaluated. Blindness Blindness can often be tricky to identify, and although some owners will have already perceived blindness to be present, others will not be so certain of the situation. Bilaterally dilated (mydriatic) pupils often accompany blindness and whenever this is seen it is very easy to perform a menace response in both eyes to assess vision. Another test that is often described is the negotiation of an obstacle course. However, in reality this is a time-demanding process, it can be difficult within the confines of a short consultation and is unable to assess unilateral blindness. Visual placing can also be used to Figure assess for blindness ( 7). Central blindness is due to a lesion in the occipital cortex (absent menace response in the presence of a normal PLR). Peripheral blindness is due to a lesion within cranial nerve II or the eye (absent menace response in the presence of an absent PLR). A negative or reduced menace response unilaterally or bilaterally should lead to a complete ophthalmological and neurological assessment. However, two specific tests that are essential are a fundic examination and the pupillary light reflex (PLR). 4 / 8

5 Fundic examination The fundic examination allows the clinician to evaluate the appearance of the retina and also allows visualisation of part of the optic nerve. This is the only cranial nerve that can be seen in the live animal, so this part of the examination is an opportunity not to be missed. Pupillary light reflex An absent or reduced PLR with an absent or reduced menace response suggests the lesion is within the pathway that is shared by both these tests. This would mean the lesion lies in the eye, such as the retina, the optic disc, the optic nerve, optic chiasm or optic tract. An absent or reduced menace response in the presence of an intact PLR suggests the lesion lies within the forebrain, brainstem, cerebellum or facial nerve (VII). However, if the animal has no cerebellar, brainstem or cranial nerve (VII) signs then the lesion must lie within the forebrain. Note: a cerebellar or brainstem lesion will only affect the efferent components of this pathway and as such will not affect conscious perception of sight. It will only affect the motor response to the menace test. Diseases to consider The diseases to consider in a patient presenting with blindness and a normal ophthalmic examination depend on the neurological localisation. Bilateral blindness and PLR deficit. These findings are consistent with a lesion somewhere within the pathway shared by the menace response test and PLR. The most likely explanation would be an ophthalmic disease (with no overt ophthalmic abnormalities seen on examination) or an optic chiasm lesion. Retinal disease with minimal retinal changes: Sudden Acqui red Retinal Degeneration Syndrome (SARDS). Optic chiasm/bilateral optic nerve lesion: neoplasia. For example, a pituitary mass compressing the optic chiasm; inflammatory disease of the optic tract/nerves, such as granulomatous meningoencephalitis; traumatic damage to the optic nerves; 5 / 8

6 optic nerve hypoplasia; ischaemic necrosis of the optic chiasm. Unilateral blindness and PLR deficit. This may be due to an optic nerve lesion on the affected (ipsilateral) side. Or, in rare cases, may be due to a lesion of the optic tract on the opposite (contralateral) side to the clinical signs (within the pathway linking cranial nerve II with the pretectal nucleus Figure 4 ). However, the most common reason for this presentation would be an ophthalmic disease and this should be ruled out before considering the diseases below. Optic nerve lesion: retrobulbar abscess or cellulitis; neoplasia affecting the optic nerve; inflammatory disease of the optic nerve; optic nerve hypoplasia; trauma. Contralateral optic tract lesion: neoplasia in the region of the optic tract (such as the hypothalamus); cerebrovascular accident. Bilateral blindness with an intact PLR. This suggests a multifocal or diffuse forebrain lesion is present the part of the menace test pathway that is not shared by the PLR pathway. congenital malformation, such as hydrocephalus or cystic lesions; metabolic encephalopathy, such as hypoglycaemia or hepatic encephalopathy; inflammatory disease of the brain, such as granulomatous meningoencephalitis; infectious intracranial disease, such as neosporosis, toxoplasmosis, bacterial abscessation; toxin ingestion, such as lead poisoning, rodenticide toxicity; 6 / 8

7 cerebrovascular accident; head trauma; intracranial neoplasia, primary or metastatic disease. Unilateral blindness with an intact PLR. This would localise the problem to the contralateral forebrain: intracranial neoplasia; inflammatory disease of the brain, such as granulomatous meningoencephalitis; infectious intracranial disease, such as neosporosis, toxoplasmosis, bacterial abscessation; cerebrovascular accident, such as haemorrhagic or ischaemic stroke; head trauma. Investigation of blindness The next diagnostic step depends on whether a central or a peripheral blindness is suspected. If this cannot easily be established then the full list of investigations should be performed starting with the least invasive. The tests are written in the order they should be performed: Peripheral blindness (PLR and menace deficits) fundic examination; electroretinogram: to assess the retinal response to light. An abnormal response, in the presence of a normal ophthalmic examination, may suggest SARDS, but is not specific for this disease; advanced imaging: magnetic resonance imaging (MRI) is preferred, but computed tomography (CT) is suitable; cerebrospinal fluid (CSF) collection: even in the presence of a normal MRI scan, CSF analysis may reveal evidence for an inflammatory disease process, such as optic neuritis; Central blindness (menace deficit with normal PLR) blood profile: to include haematology, biochemistry, electrolytes, and bile acid stimulation test; 7 / 8

8 Powered by TCPDF ( advanced imaging; and cerebrospinal fluid collection. Summary Cranial nerves predominantly originate within the brain stem and form reflexes with one another that the clinician can use to localise a neurological lesion. Blindness can be due to a central or peripheral lesion. Having determined the likely location of the lesion, a shortlist of likely diseases can be formed and this then leads to a diagnostic plan of action. Figure 8 summarises this process. The next article will focus on some of the other common presentations involving the cranial nerves and give tips on how to recognise and diagnose them. 8 / 8