THE CEREBRAL HEMISPHERES AND CEREBELLUM: ASSESSMENT OF HIGHER COGNITIVE FUNCTION

chapter 5


The Cerebral Hemispheres and Cerebellum


ASSESSMENT OF HIGHER COGNITIVE FUNCTION


Although subtitled ‘Assessment of higher cognitive function’, this chapter will not deal with the very complex neuropsychological testing that can be performed in patients with impairment of cognitive function. Rather, this chapter discusses a simplistic assessment of language disturbances and some very basic higher cortical functions, in particular of the parietal lobes, which can be used while taking the neurological history and examining the patient to localise the site of the pathology within the nervous system. This involves the usage of terms such as the primary pathways and secondary association areas. These terms are an aid to understanding and there is no evidence that the parietal lobes are ‘hard-wired’ in such a manner. In fact, symptoms from lesions in a part of the nervous system do not necessarily reflect the function of that part; the symptoms may arise from either a loss of certain functions or functional overactivity of the portions of the brain that remain intact [1]. The higher cortical functions represent the parallels of latitude within the cerebral hemisphere and, as already stated, ‘the pathology is ALWAYS at the level of the parallel of latitude’. More comprehensive discussions of higher cortical function can be found in the major textbooks [13]. Figure 5.1 gives a lateral view of the cerebral hemispheres and the cerebellum. The frontal, parietal, occipital and temporal lobes and the various functions pertaining to those lobes are shown.




THE FRONTAL LOBES


Patients should be suspected of having a problem in a frontal lobe if they present with one of the following:




Contralateral weakness


The area just in front of the central sulcus, referred to as the pre-central gyrus (see Figure 5.1), is the origin of the motor fibres that innervate the muscles on the opposite side of the body. This pathway is referred to as the corticospinal tract and it crosses the midline at the level of the foramen magnum, the junction of the lower end of the brainstem and the upper end of the cervical spinal cord. Lesions affecting the pre-central gyrus will result in a contralateral hemiparesis affecting the face, arm and leg although, one limb may be weaker than the other and the face may be affected to a variable degree (partial weakness) or with hemiplegia (total weakness).


The clues that the hemiparesis is derived from a lesion in the frontal lobe are:



Very rarely, an irritative lesion can result in a contralateral hemiparesis with the eyes deviated to the side of the weakness and away from the side of the pathology, mimicking a pontine lesion in the brainstem where there is an area responsible for conjugate deviation of the eyes to the same side. When this area of the brainstem is affected the eyes deviate away from the side of the lesion and towards the side of the paralysis.



Changes to personality or behaviour


The pre-frontal region is that part of the frontal lobes anterior to the motor cortex and the motor speech area. Diseases affecting this region of the frontal lobe result in changes to personality or behaviour best summarised by the phrase ‘he/she is not him/herself’.


These patients can lose spontaneity and initiative and appear apathetic and depressed or, at the other end of the spectrum, patients can appear to ‘have a short fuse’. In this situation patients can become aggressive and violent, even after a trivial incident. This is seen following some head injuries. Patients with frontal lobe pathology can develop an elevated mood and behave inappropriately, such as the vicar’s wife who begins to tell dirty jokes and who was subsequently diagnosed with a meningioma (a benign tumour) in the frontal lobe.


If the olfactory tracts that lie beneath the frontal lobe are also affected, there can be an associated unilateral or bilateral anosmia (loss of the sense of smell).


An inability to perform acts voluntarily or to make decisions is seen in patients with pre-frontal problems and is termed abulia, reflecting pathology in the medial aspect of the frontal lobes. Abulia sometimes occurs with a subarachnoid haemorrhage (bleeding into the subarachnoid space) from vasospasm of the anterior cerebral artery related to a ruptured berry aneurysm on the anterior communicating artery (see Chapter 10, ‘Cerebrovascular disease’).



Difficulty walking


Another presentation of patients with frontal lobe pathology is apraxia of gait. The history is one of progressive difficulty walking in the absence of any weakness, sensory disturbance, ataxia or extrapyramidal dysfunction to account for the difficulty walking. It is as if the patient has forgotten how to walk. This is discussed in Chapter 12, ‘Back pain and common leg problems with or without difficulty walking’.


There are a number of frontal lobe signs, referred to as primitive reflexes, because they are present in normal babies and may or may not be seen in patients with frontal lobe pathology. These include:



Although changes in personality and behaviour may be one of the presenting symptoms of dementia, severe depression can produce a similar clinical picture termed ‘pseudo-dementia’.



THE PARIETAL LOBES


Patients with parietal lobe problems may present in a number of ways:



Visual disturbances can occur in both dominant and non-dominant hemisphere problems; dysphasia is a feature of dominant hemisphere problems; being lost, disoriented and unaware that anything is amiss are very typical features of problems in the non-dominant hemisphere.


In both the dominant and non-dominant hemispheres, there are visual pathways that pass through and sensory pathways that terminate in the parietal lobe. If the so-called ‘primary pathway’ is affected, there will be a contralateral loss of function. If the ‘primary pathway’ is intact but the so-called ‘secondary association areas’ are affected, there will be a contralateral inattention with double simultaneous stimuli where the patient is not aware of the stimulus on one side when the other side is stimulated in the same manner simultaneously.


Speech disturbance (dysphasia) and the term RAAF (an abbreviation for the Royal Australian Air Force) best describe the features of problems in the dominant hemisphere, while the term ‘lost in space’ is the easy way to remember the characteristics of non-dominant parietal lobe problems (see Table 5.1). RAAF refers to the clinical findings seen in dominant parietal lobe problems and, in its pure form, is referred to as Gerstmann’s syndrome (for an explanation of RAAF, see ‘Gerstmann’s syndrome’ below).




Abnormalities of vision




• Contralateral loss of vision. The optic radiation from the contralateral visual field passes through the parietal lobes to the occipital lobe (see Figure 5.1). If the ‘primary pathways’ are affected, there will be a loss of vision on the contralateral side resulting in a lower quadrantanopia if only the parietal lobe is affected, but often the optic radiation fibres in the temporal lobe are also affected and the resulting deficit is a contralateral homonymous (the visual field disturbance is identical when each eye is tested separately) hemianopia.


• Contralateral visual inattention. If the ‘primary visual pathways’ are preserved, but the so-called ‘secondary association areas’ are affected, there will be visual inattention elicited with double simultaneous stimuli. The patient is unable to appreciate the visual stimulus in one visual field, usually a moving finger applied simultaneously to both visual fields, but can if only one visual field is stimulated at a time.



Abnormalities of sensation


Although abnormalities of sensation are commonly detected on examination of patients with parietal lobe problems, they are only occasionally the presenting symptom.



The sensory pathways from the body terminate in the contralateral post-central gyrus of the parietal lobes. If the so-called ‘primary sensory pathways’ in the parietal lobes are affected there will be abnormal sensation on the opposite side of the body, resulting in loss or impairment of vibration and proprioception with either cortical or deep hemisphere parietal lobe problems and/or loss or impairment of pain and temperature sensation with deep hemisphere but not cortical parietal lobe problems.


If the ‘primary pathways’ are not affected, but the area of the parietal lobe that interprets these sensory stimuli (referred to in this chapter as the ‘secondary association areas’) is involved in the pathological process, the resulting abnormality will be a loss of appreciation of sensation elicited with double simultaneous stimulation termed sensory inattention. Here, the examiner applies the same stimulus to both sides of the body simultaneously. This is usually performed by stroking the skin with a finger. Patients with sensory inattention do not notice the sensory stimulus on the contralateral side of the body to the pathology when both sides are stimulated simultaneously but can appreciate the sensory stimulus when only one side is stimulated.


Although abnormalities of vibration and proprioception can occur in cortical lesions, the detection of impaired vibration and proprioception cannot localise the problem to the cortex. If the primary sensory modalities of pain, temperature, vibration and proprioception are normal or only mildly affected, it is possible to perform more detailed sensory testing looking for abnormalities seen with contralateral parietal cortical lesions. The three cortical signs consist of:



1. Impairment of stereognosis: the patient is unable to appreciate the shape and size of an object, for example a pen or a coin, placed in the affected hand. Proprioception must not be affected, otherwise the inability to identify the object would reflect impairment of the proprioceptive pathway and not necessarily the contralateral parietal cortex as the site of abnormality.


2. Impairment of graphaesthesia: the patient is unable to identify a number drawn on the palm of the contralateral affected hand. The drawing must be greater than 4 cm in size and initially a number is drawn on the palm of the hand while the patient watches. The patient is then instructed to close their eyes and identify another number drawn on the palm. It is easier for patients to identify numbers if their palm is facing towards them and not the examiner. If light touch is severely impaired, abnormal graphaesthesia may occur and not indicate a cortical problem.


3. Impairment of 2-point discrimination: an inability to distinguish two points from one. This can be tested on any part of the body but the distance between the two points varies greatly from 1 mm on the tip of the tongue to 20–30 mm on the dorsum of the hands and feet [1]. The most useful site to test 2-point discrimination is on the fingertips where the normal distance is 3–5 mm. It is recommended to test the opposite hand (if it is normal) to determine the distance between the two points in this particular patient as this can vary from patient to patient depending on the age of the patient.

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Jun 19, 2016 | Posted by in NEUROLOGY | Comments Off on THE CEREBRAL HEMISPHERES AND CEREBELLUM: ASSESSMENT OF HIGHER COGNITIVE FUNCTION

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