Phenytoin-related cerebellar degeneration without seizures

Phenytoin and damage to the cerebellum - a systematic review of published cases

INTRODUCTION

The antiseizure medication phenytoin has been associated with changes in the cerebellum, cerebellar signs, and permanent cerebellar damage. We have systematically reviewed the clinical and radiological features, and the correlation between them.

AREAS COVERED

We identified sixty case reports and case series of the effects of phenytoin on the cerebellum by searching Medline and Embase and relevant reference lists. The reports described 92 [median 1, range 1-5] cases, documented median age 28 [2.7-78] years. Eighty-one cases described one or more clinical sign of ataxia (present in 96%), dysarthria (63%), and nystagmus (70%). The neurological outcome (in 76 cases): 10 (13%) recovered by 12 months; 55 (72%) suffered residual disability; and 11 (14%) died. Median serum phenytoin concentration (48 cases) was 50 (interquartile range 31-66) mg/L; only three values were below 20 mg/L. The radiological findings included cerebellar atrophy in 41 of 61 patients (67%) with at least one scan.

EXPERT OPINION

Evidence mainly comes from case reports, and is inevitably biased. Most patients with cerebellar dysfunction have phenytoin concentrations above the reference range. Clinical signs of ataxia can persist without radiological evidence of cerebellar atrophy, and cerebellar atrophy is seen without any clinical evidence of cerebellar dysfunction.

The diagnostic challenge of dizziness: computed tomography and magnetic resonance imaging findings

Dizziness is a prevalent symptom in the general population, accounting for a considerable share of physician office visits, and most causes are clinically treatable. It is also a common indication for neuroimaging studies, in order to identify a specific etiology and exclude surgical causes. Here, we illustrate the main peripheral and central causes of dizziness, discussing their possible differential diagnoses, as well as their most important image aspects.

Can Therapeutic-Range Chronic Phenytoin Administration Cause Cerebellar Ataxia?

Phenytoin (PHT) is a first line antiepileptic drug (AED) used to treat many epilepsy syndromes. As with other AEDs, there are various adverse effects associated with PHT. For this brief review, we searched for evidence of cerebellar ataxia as a chronic adverse effect of therapeutic-range PHT treatment. Many previous studies appeared related to this issue, but many of those studies were designed to resolve questions related to the persistent residual effects of toxic-range PHT therapy, or they were inconclusive due to an absence of critical information such as PHT serum concentration, cerebellar symptoms/signs, and other factors contributing to cerebellar degeneration. Nevertheless, there were a few reports suggesting that cerebellar ataxia may be a chronic adverse effect of therapeutic-range PHT therapy and that a possible pathomechanism for that effect is folate deficiency. Moreover, there is the possibility that there may be patient-specific susceptibility factors affecting ataxia. Further studies are needed to elucidate the incidence, risk factors, and pathomechanism of cerebellar ataxia as a chronic adverse effect of therapeutic-range PHT treatment.

Stereological study of the neuronal number and volume of 38 brain subdivisions of subjects diagnosed with autism reveals significant alterations restricted to the striatum, amygdala and cerebellum

Introduction

A total of 38 brain cytoarchitectonic subdivisions, representing subcortical and cortical structures, cerebellum, and brainstem, were examined in 4- to 60-year-old subjects diagnosed with autism and control subjects (a) to detect a global pattern of developmental abnormalities and (b) to establish whether the function of developmentally modified structures matches the behavioral alterations that are diagnostic for autism. The volume of cytoarchitectonic subdivisions, neuronal numerical density, and total number of neurons per region of interest were determined in 14 subjects with autism and 14 age-matched controls by using unbiased stereological methods.

Results

The study revealed that significant differences between the group of subjects with autism and control groups are limited to a few brain regions, including the cerebellum and some striatum and amygdala subdivisions. In the group of individuals with autism, the total number and numerical density of Purkinje cells in the cerebellum were reduced by 25% and 24%, respectively. In the amygdala, significant reduction of neuronal density was limited to the lateral nucleus (by 12%). Another sign of the topographic selectivity of developmental alterations in the brain of individuals with autism was an increase in the volumes of the caudate nucleus and nucleus accumbens by 22% and 34%, respectively, and the reduced numerical density of neurons in the nucleus accumbens and putamen by 15% and 13%, respectively.

Conclusions

The observed pattern of developmental alterations in the cerebellum, amygdala and striatum is consistent with the results of magnetic resonance imaging studies and their clinical correlations, and of some morphometric studies that indicate that detected abnormalities may contribute to the social and communication deficits, and repetitive and stereotypical behaviors observed in individuals with autism.

The long-term safety of antiepileptic drugs

Antiepileptic drugs (AEDs) are used by millions of people worldwide for the treatment of epilepsy, as well as in many other neurological and psychiatric conditions. They are frequently associated with adverse effects (AEs), which have an impact on the tolerability and success of treatment. Half the people who develop intolerable AEs discontinue treatment early on after initiation, while the majority of people will continue to be exposed to their effects for long periods of time. The long-term safety of AEDs reflects their potential for chronic, cumulative dose effects; rare, but potentially serious late idiosyncratic effects; late, dose-related effects; and delayed, teratogenic or neurodevelopmental effects. These AEs can affect every body system and are usually insidious. With the exception of delayed effects, most other late or chronic AEs are reversible. To date, there is no clear evidence of a carcinogenic effect of AEDs in humans. While physicians are aware of the long-term AEs of old AEDs (the traditional liver enzyme-inducing AEDs and valproate), information about AEs of new AEDs (such as lamotrigine, levetiracetam, oxcarbazepine, topiramate or zonisamide), particularly of their teratogenic effects, has emerged over the years. Sporadic publications have raised issues about AEs of the newer AEDs eslicarbazepine, retigabine, rufinamide, lacosamide and perampanel but their long-term safety profiles may take years to be fully appreciated. Physicians should not only be aware of the late and chronic AEs of AEDs but should systematically enquire and screen for these according to the individual AED AE profile. Care should be taken for individuals with comorbid conditions that may render them more susceptible to specific AEs. Prevention and appropriate management of long-term AED AEs is expected to improve adherence to treatment, quality of life and control of epilepsy.

Cerebellar Purkinje cells are reduced in a subpopulation of autistic brains: a stereological experiment using calbindin-D28k

Although a decreased number of cerebellar Purkinje cells (PCs) in the autistic brain has been widely reported with a variety of qualitative and quantitative methods, the more accurate method of cell counting with modern stereology has not yet been employed. An additional possible problem with prior reports is the use of Nissl staining to identify the PCs, as this can miss cells due to staining irregularities. In the present study, PCs were immunostained for calbindin-D28k (CB), as this has been shown to be a more reliable marker for PCs than the Nissl stain, with more than 99% of the PCs immunopositive (Whitney, Kemper, Rosene, Bauman, Blatt, J Neurosci Methods 168:42-47, 2008). Using stereology and CB immunostaining, the density of PCs was determined in serial sections from a consistently defined area of the cerebellar hemisphere in four control and six autistic brains, with the density of PCs then correlated with the clinical severity of autism. Overall, there was no significant difference in the density of PCs between the autistic and control groups. However, three of six autistic brains had PC numbers that fell within the control range, whereas the remaining three autistic brains revealed a reduction compared with the control brains. These data demonstrate that a reduction in cerebellar PCs was not a consistent feature of these autistic brains and that it occurred without discernible correlation between their density and the clinical features or severity of autism.

Cerebellar atrophy in temporal lobe epilepsy

PURPOSE

The goal of this work was to determine the presence and degree of cerebellar atrophy in chronic temporal lobe epilepsy, its clinical seizure correlates, and its association with general cortical atrophy.

METHODS

Study participants were 78 persons with temporal lobe epilepsy and 63 age- and gender-matched healthy controls. All subjects underwent high-resolution MRI with manual tracing of the cerebellum. Clinical seizure features and history were obtained by structured interview and review of medical records.

RESULTS

The epilepsy group exhibited significant abnormality in cerebellar volume, with mean reductions ranging from 4 to 6.6% depending on adjustments. Significantly more individual subjects with epilepsy exhibited cerebellar atrophy compared with controls across all operational definitions or thresholds of abnormality including z < or = -2.0 (13% TLE, 3.4% controls) and z < or = 1.5 (22% TLE, 3.4% controls). Clinical seizure features reflecting both neurodevelopmental (history of initial precipitating injuries) and severity of course (longer duration, increased number of lifetime generalized tonic-clonic seizures) factors were associated with cerebellar atrophy. Atrophy of the cerebellum could be observed independent of more general (cerebral) atrophic processes.

CONCLUSIONS

The presence of cerebellar atrophy is a reflection of the extratemporal abnormalities that can be observed in localization-related temporal lobe epilepsy, which may be due, at least in part, to factors associated with epilepsy chronicity.

Therapeutic safety monitoring: what to look for and when to look for it

This review focuses on the safety problems associated with antiepileptic drugs (AEDs) as revealed by laboratory testing and clinical examination. There are two classes of side effects: (a) common and mild and (b) rare and severe. Allergic reactions to AEDs are common and usually mild. However, on rare occasions, they can progress to more severe cutaneous disorders, including Stevens-Johnson syndrome and toxic epidermal necrolysis. Severe allergic reactions to AEDs range from immune responses with fever to multiorgan dysfunction. Allergic rashes may be genetically or immunologically determined. Laboratory abnormalities produced by AEDs are common and mild, and include hepatic enzyme elevation associated with phenytoin and mild elevation in ammonia associated with valproate. Serious, although rare, idiosyncratic side effects, such as aplastic anemia, hepatotoxicity, and thrombocytopenia, have also occurred with AEDs. These reactions are largely confined to the "classic" AEDs. With the exception of felbamate, AEDs approved in the past decade have not been plagued by severe idiosyncratic reactions. Subtle endocrine abnormalities, including variations in thyroid function tests and bone metabolism, and the often subclinical effects on peripheral nerve conduction produced by phenytoin and carbamazepine, are also examined.

[Dose-dependent relationship of chronic use of phenytoin and cerebellar atrophy in patients with epilepsy]

The chronic treatment with phenytoin or the acute intoxication by this drug may cause permanent cerebellar injury with atrophy of cerebellum vermis and hemispheres, which can be detected by neuroimaging studies. The aim of the present study was to investigate the correlation between the dosage and duration of treatment with phenytoin and the occurrence of cerebellar atrophy. Sixty-six patients were studied and had their tomographies analyzed for cerebellar atrophy. Of the 66 patients studied, 18 had moderate/severe atrophy, 15 had mild atrophy and 33 were considered to be normal. The patients with moderate/severe atrophy were those with higher exposure to phenytoin (longer duration of treatment and higher total dosage) showing statistically significant difference when compared to patients with mild atrophy or without atrophy (p=0. 02). Further, the patients with moderate/severe atrophy had serum levels of phenytoin statistically higher than those of patients with mild atrophy or without atrophy (p = 0.008). There was no association between other antiepileptic drugs dosage or duration of treatment and degree of cerebellar atrophy. We also found that older patients had cerebellar atrophy more frequently, indicating that age or duration of the seizure disorder may also be important in the determination of cerebellar degeneration in these patients. We conclude that although there is a possibility that repeated seizures contribute to cerebellar damage, long term exposure to phenytoin, particularly in high doses and toxic serum levels, cause cerebellar atrophy.

Ataxia in institutionalized patients with epilepsy

Fifty-four per cent of 41 chronically institutionalized adult patients with epilepsy had ataxia of gait (wide mean stride width). None of the following correlated with stride width: serum phenytoin, previous phenytoin toxicity, seizure frequency, or status epilepticus. Seventeen of the 41 patients had computed tomographic head scans. Patients with radiological evidence of cerebellar atrophy had a wider mean stride width, later age of onset of seizures, greater peak serum concentrations of phenytoin than did those without cerebellar atrophy. Ataxia of gait was inconsistently associated with cerebellar atrophy. Elevated serum/plasma concentrations of phenytoin may be a risk factor for cerebellar atrophy, but seizure frequency or status epilepticus are not independently related to this complication.

Neocerebellar hypoplasia in a neonate following intra-uterine exposure to anticonvulsants

An infant with dysmorphic features was born to an epileptic mother who had taken phenytoin and sodium valproate throughout pregnancy. The infant began to have intractable seizures 10 minutes after delivery, and retrospective reports from the mother suggested they may have occurred in utero. Ultrasound examination of the brain showed a very wide subarachnoid space and CT confirmed cerebral and cerebellar underdevelopment. The infant died at three days of age and autopsy revealed a small brain with neocerebellar hypoplasia. This case might represent an extreme example of anticonvulsant teratogenicity.

Axonal dystrophy in the posterior column nuclei of young adult epileptics with chronic phenytoin intoxication

Axonal spheroids in the posterior column nuclei of phenytoin-intoxicated epileptics were classified according to their predominant subcellular components into six types, and their incidences were compared with those in controls. Spheroids from phenytoin-intoxicated epileptics showed significantly higher proportions of the tubulomembranous (TM) and layered membrane loop (LML) types in the gracile nucleus, appearance of the same types in the cuneate nucleus, and a significant decrease of the neurofilamentous (NF) type in both nuclei. The incidences of the complex body (CB) and granular material types and of the homogeneous dense-body (HDB) type, which appeared only in the gracile nucleus, showed no difference between the intoxicated patients and the controls. The NF, CB and HDB types were therefore considered to be nonspecific. It was thought that chronic phenytoin intoxication might induce dystrophic changes, such as those characterized by the presence of the TM and LML types, in the axon terminals of the gracile and cuneate nuclei, possibly due to some abnormalities of the axoplasmic transport system.

Cerebellar injury due to phenytoin. Identification and evolution of Purkinje cell axonal swellings in deep cerebellar nuclei of mice

The present study describes the identification and the ultrastructural and numerical evolution of Purkinje cell axonal swellings induced by phenytoin. Thirty male C57Bl/6J mice received phenytoin orally in doses up to 100 mg/kg daily and were killed after 3, 6, 10, 14, and 48 days of treatment. Light and electron microscopic investigations as well as morphometric analysis of cut surface area and numerical density of axonal swellings were performed. The swellings appeared as early as 6 days after initiation of treatment and gradually increased in size and frequency. Use of an anti-lymphocyte monoclonal antibody (CD 3), specifically cross-reacting with Purkinje cells, identified the swellings as dystrophic Purkinje cell axons. On grounds of their ultrastructural appearance they were classified into three distinct types occurring at different time intervals after phenytoin exposure. At 6 days, most axonal swellings contained loosely aggregated membranous vesicles and tubules in a finely granulated matrix (type 1). At 14 days, larger axonal swellings appeared characterized by the presence of three-dimensional networks of branched and anastomosing membranous tubules (type 2). At 48 days, even larger axons contained bodies of highly condensed membranous material of sometimes paracrystalline appearance (type 3). It is suggested that phenytoin-induced axonal pathology of Purkinje cells is a dynamic process characterized by the progressive accumulation of proliferating membranous material arranged in an increasingly complex fashion.

Congenital granuloprival hypoplasia of cerebellar and hippocampal cortex

Two siblings with congenital granule cell hypoplasia of the cerebellum and hippocampus are reported. The patients, both male, showed severe psychomotor retardation, microcephaly, hypotonia, athetosis, and seizures; they died at the ages of 3 7/12 years and 5 10/12 years, respectively. Postmortem examinations in both cases revealed nearly complete absence of the granule cells of the cerebellum with relative preservation of the Purkinje cells. Also absent were the granule cells of the fascia dentata of the Ammon's horns of the hippocampus, without any detectable gliosis; this has not previously been reported. Twenty-three autopsy cases of granuloprival cerebellar hypoplasia are reviewed. The present cases illustrate a singularly unique disease process, comparable to the classical cerebellar hypoplasia experimentally induced by parvoviruses, and suggest a granule cell specific insult to the brain during the late second trimester.

Complex partial seizures: cerebellar metabolism

We used positron emission tomography (PET) with [18F]2-deoxyglucose to study cerebellar glucose metabolism (LCMRglu) and the effect of phenytoin (PHT) in 42 patients with complex partial seizures (CPS), and 12 normal controls. Mean +/- SD patient LCMRglu was 6.9 +/- 1.8 mg glucose/100 g/min (left = right), significantly lower than control values of 8.5 +/- 1.8 (left, p less than 0.006), and 8.3 +/- 1.6 (right, p less than 0.02). Only four patients had cerebellar atrophy on CT/MRI; cerebellar LCMRglu in these was 5.5 +/- 1.5 (p = 0.054 vs. total patient sample). Patients with unilateral temporal hypometabolism or EEG foci did not have lateralized cerebellar hypometabolism. Patients receiving phenytoin (PHT) at the time of scan and patients with less than 5 years total PHT exposure had lower LCMRglu, but the differences were not significant. There were weak inverse correlations between PHT level and cerebellar LCMRglu in patients receiving PHT (r = -0.36; 0.05 less than p less than 0.1), as well as between length of illness and LCMRglu (r = -0.22; 0.05 less than p less than 0.1). Patients with complex partial seizures have cerebellar hypometabolism that is bilateral and due only in part to the effect of PHT.

Neurological manifestations and toxicities of the antituberculosis drugs. A review

The neurological manifestations and toxicities of 12 antituberculosis drugs [isoniazid, rifampicin (rifampin), ethambutol, p-aminosalicylic acid, pyrazinamide, streptomycin, kanamycin, ethionamide, cycloserine, capreomycin, viomycin and thiacetazone] are reviewed. Their effects upon the central nervous system, cranial nerves, peripheral nerves and the neuromuscular junction are examined, and drug interactions of neurological concern are briefly discussed. Isoniazid is well known to increase the concentrations of gamma-aminobutyric acid in neural tissues. Although conflicting data have been published, isoniazid may play a limited future role in reducing the degree of adventitious movements noted in certain neurological diseases such as multiple sclerosis, spasmodic torticollis, and other segmental dystonic syndromes. With rifampicin neurological complications have been observed infrequently. Rifampicin penetrates into the CSF and has been shown to have useful activity against various micro-organisms in the CSF, including certain viruses; however, contrary to earlier suggestions, it appears to have no role in the treatment of subacute sclerosing panencephalitis. A number of studies have indicated that isoniazid is associated with a large number of accidental and intentional poisonings. The highest incidence has been observed with Southwestern American Indians in which this agent was involved in 7% of all suicide attempts and 19% of the suicide deaths. Degeneration of the optic chiasma and nerve is a well-known adverse effect of ethambutol; toxicity is manifested by impairment of visual acuity, marked loss of colour discrimination, constricted visual fields, and central and peripheral scotoma. Ototoxicity is a well known problem caused by streptomycin, kanamycin, capreomycin and viomycin. The use of streptomycin in pregnant mothers is associated with congenital deafness in newborns in certain cases. The aminoglycoside antibiotics are also associated with flaccid paralysis following neuromuscular blockade. Adverse reactions to cycloserine are mainly dose-related with neurological and psychiatric syndromes noted in up to 50% of patients. Recent data indicate that isoniazid, rifampicin, ethambutol, pyrazinamide, streptomycin, kanamycin, ethionamide, and cycloserine appear in measurable quantities in the cerebrospinal fluid. Five of these compounds (isoniazid, rifampicin, ethambutol, kanamycin, cycloserine) pass to some degree through non-inflamed meninges. Other than discontinuation of the therapeutic regimen and general supportive measures, very few methods are described in the literature for treatment of acute intoxications with antituberculosis drugs.

Degeneration of granule cells following chronic phenytoin administration: an electron microscopic investigation of the mouse cerebellum

Fifteen male mice (C57/Bl6J) were fed the liquid diet "Stardit" supplemented with vitamins together with phenytoin for 8 weeks; experimental animals and controls were pair-fed. After 8 weeks of treatment, the anesthetized animals were perfused with 3.5% glutaraldehyde. Tissue samples of the cerebral cortex (area 3), cerebellum (vermis), thalamus, hypothalamus, and liver were embedded in Araldite. All phenytoin-treated animals displayed a hepatomegaly. Semithin sections and ultrastructural investigations of the cerebellar vermis showed pyknoses of granule cells and an enlargement and swelling of parallel fibers in presynaptic areas in the molecular layer. The swollen axons showed an accumulation of tubular structures which represented proliferated smooth endoplasmic reticulum. Similar tubular structures were observed in hepatocytes of experimental animals. It is proposed that phenytoin caused an induction of the microsomal system of hepatocytes and granule cells which led to a proliferation of the smooth endoplasmic reticulum. The transport of these organelles to the axon terminals of parallel fibers via the axoplasmic flow is assumed to cause a swelling of the presynaptic area. A dying-back process may then lead to pyknosis of granule cells. Chronic phenytoin administration to mice is a new experimental model of neuroaxonal dystrophy.

Damage of Purkinje cell axons following chronic phenytoin administration: an animal model of distal axonopathy

An animal model of central distal axonopathy following chronic administration of phenytoin is described. Male C57/BL6J mice received diphenylhydantoin (DPH) in the daily diet (liquid diet 'Stardit', supplemented with vitamins) over a period of 8 weeks. Control and experimental animals were pair-fed. Twelve mice of both groups were perfused via the left ventricle with glutaraldehyde. Representative samples of the cerebral cortex (area 3), cerebellum (vermis and deep cerebellar nuclei), thalamus, hypothalamus, and liver were embedded in araldite. Semi-thin sections and electron microscopy of the cerebellar vermis revealed marked dystrophic changes in the Purkinje cell axons. The presynaptic segments of Purkinje cell axons in the deep cerebellar nuclei showed massive enlargement and swelling due to accumulation of spherical particles and tubular structures in the axoplasm. These structures represent a proliferation of the smooth endoplasmic reticulum. Identical changes were found in hepatocytes of treated animals. Because phenytoin induces hepatic microsomal enzymes, we suggest that phenytoin-related Purkinje cell damage may be produced by an induction of Purkinje cell microsomes with proliferation of the smooth endoplasmic reticulum which causes a swelling and enlargement of presynaptic segments of Purkinje cell axons in deep cerebellar nuclei. Chronic phenytoin administration to mice is a new model of phenytoin-induced encephalopathy and of distal axonopathy of cerebellar neurons.

Cerebellar atrophy following phenytoin intoxication

Cerebellar degeneration has been demonstrated in several patients receiving phenytoin therapy. In most cases it has been unclear, however, whether the degeneration was caused by the drug per se or by other mechanisms known to lead to cerebellar damage. We describe a patient who developed a marked cerebellar atrophy, demonstrated on computed tomographic scan, following an episode of acute, severe phenytoin intoxication. The patient received phenytoin prophylactically for 2 1/2 months after an uncomplicated subarachnoid hemorrhage and was in good health when the treatment was started. He has never had seizures, and no other possible cause of chronic cerebellar changes is known. The initially severe clinical signs of cerebellar dysfunction have subsided slowly. We conclude that phenytoin can directly cause cerebellar degeneration.

Truncal ataxia in chronic anticonvulsant treatment. Association with drug-induced folate deficiency

The association of truncal ataxia with a number of different factors has been studied in a group of 95 epileptic outpatients on chronic anticonvulsant treatment. The 28 patients showing truncal ataxia had been epileptic for a longer period of time, received a significantly larger number of drugs, and had higher serum levels of phenobarbital than the non-ataxic group. Serum folate levels were significantly lower in the ataxic group. A role is postulated for anticonvulsant-induced folate deficiency in the appearance of truncal ataxia presenting after prolonged anti-convulsant therapy, either by increasing the serum levels of the anticonvulsants or through other, unknown mechanisms. The presence of tonic-clonic seizures, presumably associated with brain anoxia, was not associated with the appearance of truncal ataxia.

Cerebellar degeneration due to chronic phenytoin therapy

Five patients developed cerebellar degeneration while being treated with phenytoin. All had high plasma levels of the drug, and none was having seizures of a type that could have caused systemic hypoxia at the time the cerebellar syndrome appeared. Cerebellar degeneration was confirmed by the finding of atrophy on CT scan and by persistence of cerebellar signs when plasma phenytoin levels were decreased. We suggest that chronic phenytoin therapy can cause cerebellar degeneration. The question of whether phenytoin or the cumulative effect of hypoxia from repeated convulsions causes cerebellar degeneration should not be posed as one of exclusive alternatives, since hypoxia is a well-known cause of cerebellar atrophy. Instead, the question should be whether or not phenytoin can also be responsible. The cases reported here suggest that it can.

Sensorimotor deficits produced by phenytoin and chlorpromazine in unanesthetized cats

Unanesthetized adult cats were evaluated for suprasegmental reflex activity and motor skills before and after administration of chlorpromazine (0.0625--0.5 mg/kg) alone and in combination with phenytoin (20 mg/kg). The greatest deficits were seen in the tests of balance and corrdination in which half the animals failed to match their control responses after administration of chlorpromazine and phenytoin. The impairment was most noticeable with the most stringent test (walking a 4 cm wide beam), and the effects of the two drugs were additive. Although there was no effect of either drug on muscle strength, the two drugs in combination depressed the animals' motivational state, making them less willing to work against imposed loads. Neither drug, alone or in combination, altered responses to the flexor reflex, blind placing, the hopping response or visually aided placing. It is concluded that the effects of chlorpromazine and phenytoin on motor control are selective for the CNS loci which control balance and coordination. Although the two drugs produce additive responses, the deficits occur only at doses which are well above those needed for clinical efficacy and thus may not pose a problem in their long term clinical use.