Parkinson’s disease (PD) is one of the most common neurodegenerative disorders.^1,2 PD frequently occurs with no obvious underlying cause.³ Environmental toxins such as pesticides and MPTP may increase the risk of developing PD.⁴ Familial forms of PD also exist.⁵ Neuropathological hallmarks of PD include the presence of Lewy bodies and neuronal cell loss in the pars compacta of the substantia nigra.⁴ The prevalence of PD may be difficult to ascertain; however, recent estimates place the figure at 384/100,000 in North America.¹ Clinical features include motor, autonomic and psychiatric complications. This review will focus on the motor complications of PD.

Bradykinesia

Bradykinesia is a decrease in the speed and amplitude of complex movements.⁶ It is the most characteristic symptom of basal ganglia dysfunction in PD.¹ It may be manifested by a delay in the initiation and slowness of execution of a movement.¹ Other aspects of bradykinesia include a delay in arresting movement, decrementing amplitude and speed of repetitive movement, and an inability to execute simultaneous or sequential actions.¹ Bradykinesia correlates well with striatal dopamine deficiency, more than any other cardinal sign of PD.¹

Like other symptoms, bradykinesia is dependent on the emotional state of the patient (kinesia paradoxica).¹ With a sudden surge of emotional energy, the immobile patient may become capable of making fast movements.¹ This phenomenon demonstrates that the motor programs are intact in PD, but that patients have difficulty in utilising or accessing them without the help of an external trigger.¹

Tremor

Tremor, although less specific than bradykinesia, is one of the most recognisable symptoms of PD.¹ However, contrary to popular belief, it is not a universal symptom – approximately 15-25% of patients do not have tremor.^1,7 Resting tremor (4-6Hz) is the typical Parkinsonian tremor.¹ The characteristic movement is described as ‘pill-rolling’.⁶ This is a tremor affecting the thumb and first finger.⁷

The tremor of PD is referred to as a ‘resting tremor’ because it is present at rest, and usually subsides when the affected limb performs a motor task.² Patients may also have tremor during activity (postural tremor), and this postural tremor (ranging from 5-8Hz) may be more disabling than the resting tremor.¹ It has been postulated that the typical resting tremor in PD is a consequence of nigrostriatal degeneration, and resulting disinhibition of tremorigenic pacemaker cells in the thalamus.^1,2

However, the fact that tremor can be affected and influenced by emotion, motor activity and general health implies that the relevant circuitry is linked to (and modified by) a wide array of pathways and structures within the nervous system.²

Rigidity

Rigidity is a less variable motor complication of PD than tremor, and is probably a better reflection of the affected patient’s functional disability.¹ The rigidity is caused by an involuntary increase in muscle tone, which may affect all skeletal muscle groups.² A recent study has revealed that axial rigidity impacts significantly on quality of life and functional status in patients with PD.⁸

‘Lead-pipe’ rigidity is a sustained resistance to passive movement of a limb segment; this resistance to movement remains reasonably constant throughout the entire range of motion.² ‘Cogwheel’ rigidity presents as a jerky resistance to passive movement of a limb segment, and is believed to reflect lead-pipe rigidity with superimposed tremor.² An interesting study published in Clinical Neurophysiology demonstrates that contralateral hand activation manoeuvres (in this case, a concurrent gripping action in the contralateral hand) substantially increase rigidity in patients with PD.⁹ The pathophysiological basis of rigidity in PD is not completely understood.²

Rigidity in PD may be associated with postural abnormalities, which are frequent and disabling complications of PD.^1,10 The most recognised postural deformity is the characteristic stooped appearance, with flexion of the hips and knees and rounding of the shoulders.¹⁰ Some patients display more severe postural abnormalities, and these may lead to significant disability and motor impairment. 

Postural instability

Postural instability (with associated gait disorder) is generally the last of the cardinal signs of PD to become apparent, but it frequently proves to be the most disabling and least manageable manifestation of the disease.² Loss of postural reflexes normally occurs in more advanced stages of PD and, along with freezing, is the most common cause of falls in patients with PD.¹ Hip fractures are a common consequence of this loss of postural reflexes.¹ 

As an interesting aside, while rigidity, bradykinesia and postural instability predict falls, fractures also correlate with bone mineral density, which is normally lowered in PD patients when compared with controls.¹¹ Patients with postural instability frequently have festination (especially when associated with flexed truncal posture).^1,2 Festination is manifested as progressively faster and faster walking to prevent falling, as the legs attempt to catch up with the body’s forward momentum.^1,2

Freezing

Freezing is a disabling manifestation of PD, and is a form of akinesia.¹ It consists of an abrupt, transient inability to move, which occurs especially when starting to walk, when turning, or when approaching a narrow or crowded space (typically a doorway).^1,2 It has been suggested that bradykinesia and freezing have different pathophysiologies.¹ The observation that the former responds well to levodopa treatment, while the latter does not, additionally suggests that freezing may be a manifestation of a non-dopaminergic disturbance.^1,2

A pathophysiological model of freezing of gait has been proposed by Lewis and Barker, which suggests that this phenomenon is linked to motor, limbic and cognitive brain circuits.^12,13 Frontal-basal ganglia circuits, vital to gait, are hypothesised to play a significant role in the executive control of movement.¹⁴ The disruption of frontostriatal pathways is hypothesised to play an important role in the mechanism underlying freezing, as it is related to progression of the disease and cognitive impairment.^15,16

Facial akinesia

Patients with PD frequently display hypomimia (a reduced facial expression).¹ Facial akinesia can lead to a mask-like, staring expression.² PD may also be associated with orofacial dysfunction and impaired mastication.¹⁷ A study demonstrated that orofacial function is reduced, and mastication and jaw opening are poorer, in patients with PD than in controls.¹⁷ Rigidity, reduced mobility, and tremor all impair the formation and the placement of the food bolus, and the chewing process.^18,19

Bulbar symptoms in PD include dysarthria, hypophonia, dysphagia and sialorrhoea^.1 These may result from orofacial-laryngeal bradykinesia and rigidity.¹ One of the characteristic features of PD is changes of voice and speech.²⁰ The motor deficits associated with PD adversely affect the systems that play a role in speech motor control.²⁰

Speech deficits related to PD include dysarthria and hypophonia, and can be characterised by monotony, imprecise consonants and inappropriate silences.²⁰ Patients may have a tendency to repeat the first syllable (palilalia).⁴ Motor complications of PD may lead to respiratory difficulties.¹ These may result from a variety of mechanisms, including restricted breathing due to rigidity of the respiratory muscles.¹

Oculomotor problems

Various oculomotor problems are characteristically seen in PD.¹ These include limitation of upward gaze, limitation of convergence, impaired saccades and smooth pursuit and spontaneous/reflex blepharospasm.¹ The neuropathological background of oculomotor deficits in PD is thought to be beyond alterations in dopamine-mediated circuits alone.²¹

Primitive reflex and handwriting impairment

Some patients with PD display the re-emergence of primitive reflexes, attributed to the breakdown of the frontal lobe inhibitory mechanisms normally present.¹ For example, the glabellar tap reflex (Meyerson’s sign) is a typical feature of PD.² This presents as persistent eye blinking when the forehead is repeatedly tapped.² Patients with PD often display handwriting impairment, including micrographia and tremulous handwriting.¹ Interestingly, it has been demonstrated that withdrawal of visual feedback (by eye closure) improves micrographia in PD.²²

Treatment-related motor problems

There are two main types of treatment-related motor problems in PD: peak-dose choreathetoid dyskinesias and motor fluctuations (‘on-off effects’).^2-4 In this case, choreathetoid dyskinesias are involuntary writhing movements induced by levodopa treatment.^2,3 These are not apparent initially, but will develop in the majority of patients within two years of starting treatment with levodopa.³ The movements usually affect the face and limbs.³ On-off effects are rapid, extreme fluctuations in clinical state, where bradykinesia and rigidity may suddenly worsen.^2,3 The ‘off effect’ can be so sudden that the patient stops while walking and is effectively rooted to the spot.³ This problem seems to be related to fluctuating plasma levodopa concentrations.³   

References

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